Vanagon Bucking, Stalling, Hesitation, (Not Starting) Type Problems - The Fixes
From The Archives
Compiled and Edited by Sam Walters -„84 Vanagon, novice auto mechanic,
Who Has Learned All He Knows About This Subject From Reading This Material and the
Trials and Tribulations of Bucking From His Van (now absent - knock on wood), Has
Never Done Any of These Repairs Yet, But Was In Need of Personal Education and
Thought He‟d Pass Along What He Found In An Easier To Find Format.
Please Point Out Any Errors So That I Can Correct Them and Repost.
[Material Inside This Type of Bracket or In Contrasting Font Has Been Added By SW For
Completion, To Avoid Confusion, etc.]
1. Problem Defined - terms frequently used to describe it: Bucking, Stumbling, Hesitation,
Loss of Power, Cutting Out, etc. At its worse, car will drive about half a mile, buck a time
or two and then cut out, then restart just fine (after some period of time from a immediately
to 20 or 30 minutes), only to repeat the process, over and over. Someone reported taking
a long trip with this happening every half mile or so.
2. A Total Analysis of The Potential Sources of the Problem - Covers Several Major Systems
of the Vanagon, While Most of What Follows Focuses on The Fuel Injection System.
1. A very small group of problems can cause the type of hesitation/poor performance
you speak of. It seems that the remainder of your message was clipped, so we don't
have a lot to go on, so I will explain the obvious things to me...you may have one or
many of the following problems. It has to be one or a combination of three things.
1. the fuel injection system, or
2. the ignition system,
3. a bad ground.
2. Most likely problem is the fuel system. Bucking here is often caused by a dirty wiper
on the Air flow meter (AFM), but never have I seen this fault cause such a marked
decrease in performance. There is a lot of recent information in the archives (a post
in the past 24 hours) on cleaning the AFM wiper. Follow the info and clean it. This may
3. To diagnose the ignition system, look for an old condenser, ugly cap/rotor, or bum
ignition wires/a high voltage spark leak. Change your plugs, rotor, points, and
condenser if this hasn't been done recently. If your plug wires are old, replace these.
If the ignition is the problem, this will probably fix it. The only thing that would remain
questionable is the coil, which probably wouldn't be the problem, but also 'could'
cause your problem, though it is less likely.
4. Finally, (III) check all the grounds. There is a group of grounds under the ERG valve,
and MANY of the fuel related components rely on a solid ground. If, for example, the
dual relay isn't screwed tightly to the firewall, it may not feed a clear signal, and cause
'bucking'. Check all of these.
5. Also, the "dual relay" (on the driver's side of the firewall) may be to blame (small black
box with lots of wires). This is a semi-costly part, ($50?) and I am in favor of opening
it up and cleaning the contacts thoroughly prior to out-and-out replacement. It is time
consuming and requires patience, but may be worth the effort. Finally, your fuel filter
'could' be the cause, but that usually doesn't cause bucking when it is clogged, just
horrible performance (like not being able to get above 40 mph).
1. [SW - the fuel pump relay is available for $5 from Bus Depot, when a mechanic
swapped an old one from a dead van in his lot, my performance increased
markedly, I say a post somewhere saying that if this relay is bad, that it could
gradually fry the fuel pump. The same mechanic had also found poor output from
my fuel pump and replacing it had also improved things substantially.]
6. Also, the battery MUST be firmly attached to the circuit in the L-Jetronic system (which
you have). I had a 1976 bus that bucked wildly, but intermittently from NC to TX. At
2:00 a.m, after looking at everything else on the van for two days, and fed up with the
bucking, we wearily stopped in the parking lot of a place called "Clown Around" in the
middle of nowhere, and figured out that the negative lead to the battery was loose.
7. In the broad analysis, ignition related bucking is usually more "sharp" or severe than
fuel related bucking, if than makes any sense. The cylinder will usually fire a little bit on
remaining fuel vapor if the fuel cuts out for a microsecond, but a lack of ignition means
a total failure to fire for that instant. Comprendez vous?
Please let us know what the problem turns out to be...
Best o' luck,
G. Matthew Bulley, Bulley-Hewlett & Associates www.bulley-hewlett.com Cary, NC
3. Summary descriptions of problem & alternative fixes:
1. This is a common problem...most likely. If you check the archives....other people refer
to it as stumbling, bucking, etc. For some, it only happens at higher elevation. [For
me, it happened much more frequently when the weather was hot. But was not
completely restricted to hot weather.]
2. The cures vary....they are:
1. [Replacing the Air Flow Meter (AFM)];
2. VW makes a special wiring harness that replaces the old one at the air flow
meter that is designed to cure this... takes about 10 minutes to install....about
$90, I think, but that is not always the problem;
3. [The cheap capacitor fix - duplicates the special wiring harness from VW.]
4. Cleaning the carbon track in the sealed Air flow meter (AFM) has worked for
many and is free....don't ask the dealer to do this, he won't and will want you to
buy a new one for hundreds instead...see the archive for the procedure...fairly
5. Replace your 02 sensor if it hasn't been for years (details in the archive)...make
sure to get one from either the Old Volks Home or bus depot...or possibly volks
motorsports cafe....for much less than 1/2 dealer price....about $25;
6. There is a small temp sensor, called the TempII Sensor, near your thermostat that
sends signals about the coolant temperature to the fuel injection...some have
found this to be the cause...about $23 from a dealer or any of the list suppliers;
(1) [This is a recent post about the proper functioning of the Temp II sensor:
The ECU does not actually change fuel mixture in any relationship to temp
except for cold start. The ECU basically uses the temp sensor to replace multiple
temperature switches. During cold start, a range is set for cold start enrichment
and operation. When the coolant reaches about 100'F, the ECU goes into "Closed
Loop" operation. At this point, the final fuel mixture is determined by the oxygen
sensor. This is where modifications can be made to alter the mixture. Even
modifying the airflow meter will have little effect unless things are so far out of
range, the O2 sensor can't compensate. Dennis 10/26/00]
7. [Sam addition: On my 84 Vanagon, the problem never occurred when the two
leads into the Idle Stabilizer Unit were disconnected from the unit and plugged
into one another. With this unit replaced, so far so good. It is less than $30 from
8. It never hurts to replace an old fuel filter, but probably not the problem.
3. Another summary:
1. Marshall Ruskin writes: I cannot help you with the head gasket problem, but I
may be able to help with the bucking. It is an extremely common problem, often,
but not always caused by electrical noise in the Air Flow Meter (AFM). There are
several similar ways to fix this:
(1) Most expensive - replace AFM and pray that was the problem; [$150 list
(2) Next most expensive - Obtain $150 US AFM harness from VW, and pray
that it fixes the problem; [$120 list vendor]
(3) Cheap solution - Attach (solder) a 22 microfarad 35V tantalum capacitor
across pins 2 and 4 (somebody correct me if I got the pins wrong) of the
connector to the AFM ($0.25 - $3.00) - no need to pray;[See below for the
(4) Carefully pry the black plastic lid off of the AFM, (may need to cut the silicon
seal), spray electrical contact cleaner liberally, move wiper a few times, let
dry. Avoid touching contacting surfaces with your fingers, or getting dirt,
dust or oil inside. This is my personal favorite. I pray to MTNGAL who
discovered this trick - works great, solves the problem, and is very
easy.[More on HOWTO below.]
(5) Cheapest solution: Open AFM, move the potentiometer's wiper contact
surface so the wiper contacts a fresh surface. Some serious skill required.
Cost $0.00. [HOWTO below.] Hope this helps,
Marshall Ruskin 84 Westy Iron Igloo Winnipeg, Manitoba, Canada
4. An alternative view before you go out and start doing all of the technical do it yourself
fixes. Uses $$ instead. Not always a bad idea.
I'd like to add my two cents to the recent discussions on cleaning the AFM wiper track.
The point I'd like to make is that a brand new AFM costs about $125 which isn't too awfully bad
for a part that is so critical to the performance of the Vanagon's fuel injection system. My
engine was running pretty well but I recently tested the AFM wiper output with an ohmmet er
and noticed there were some gaps or dead spots in the output. In other words, the resistance
did not increase smoothly as I moved the flap through its range. I pulled out my spare AFM
(brand new) and tested it and it's output was smooth. After installin g the new AFM I noticed
some definate improvements driveability, especially in the lower RPM range. Note that I had
already done the repositioned wiper trick on the old AFM about two years ago. I bet there are
many high-mileage Vanagons out there that would benefit from a new AFM.
5. From the VW Technical Bulletins:
1. The first VW Service Bulletin, # 92-01, 2/29/92, applied only to Digifant, but was
soon superceded by Service Bulletin 92-02, issued 4/30/92, to include all Digijet
AND Digifant models. No aircools were involved, as they had the non-digital
(analog) L-Jetronic fuel injection system. This is the first technical bulletin, as they
were essectially the same, and some comments by list members over the years:
====== from Service Bulletins, Model Year 1992
Technical Bulletin Group: 24 Number: 92-01 Date: February 29, 1992 Subject: Vanagon
Intermittent Misfire Model(s): All Vanagon Digifant (and Digijet )Fuel Injection Engines
CONDITION After driving for 1/2 to 2 hours at a constant speed, vehicle experiences lack of
power, cuts-out or stalls, often accompanied by bucking or jerking of the vehicle. In ALL cases, the
vehicle will restart immediately. This symptom may not re-occur for several weeks/months.
Inspection by the dealer results in nothing conclusive.
Some air-flow meters may experience a vibration resonance of the metering potentiometer
wiper during extended constant driving. This resonance causes the air -flow meter to supply an
intermittent signal to the Digifant ECU. The ECU will then default to a "no -load" condition and
reduce the injection time.
SERVICE Harness, Part No. 025 906 302A, is available to resolve this condition. * remove
electrical connector from air flow sensor (see Figure 24-174) * install converter assembly, Part No.
025 906 302A, between harness and air flow sensor.
NOTE If customer complaints persist after performing the above repair, contact your Zone Product
Support Specialist (TSM in Canada).
** CAUTION** Part numbers are for reference only. Always check with your Parts Department for
2. From Tech Bulletin #24-92-02 (the only thing unique except for adding
NOTE: When troubleshooting this system and performing continuity checks, the following should
be noted. Due to active components in the wire harness, Part No. 025 906 302A, there is not
continuity between pin two of the connectors. The following chart on page 2 lists the resistence of
each circuit. -------
Terminal # Approx. Resistance
1 Zero Ohms
2 Greater than 1 Meg. Ohms
3 Zero Ohms
4 Zero Ohms
5 Zero Ohms
Comment to both bulletins from the archive: Harness costs about $100! very simple to
install. Takes about three minutes maximum. the new harness fits between the old harness and
the silver air-flow meter box attached to the air cleaner box. remove the silver clip, wiggle out the
big wiring connector (from the side of the air-flow meter), connect the proper end of the new
harness to the big big wiring connector you just wiggled out, put the new clip into the new
connection; then take the other end of the new harness and plug it into the air -flow meter
connection. Put the silver clip back and you are done.
6. A Reasonably Detailed Description of “Driveability” Problems and Causes and
1. There are lots of things that can cause engine misbehavior, including a bad
AFM. The VW add-on harness device is “intended” to cure a problem with a
supposedly good AFM, where in a long period of driving at a steady speed the
the ECU gets confused by (supposedly normal) oscillations from the AFM and
panics, resulting in loss of power (but not bucking) which is cured by turning the
ignition off and on again (thus resetting the ECU).
(1) The VW harness is an active IC low-pass filter that limits how rapidly the
signal from the AFM can change. Anyway, this electronic device
(estimated around $80 - $100) inserts between existing harness and the
AFM. They were $75 when introduced -- now they are about $175.
2. The VW harness may also help situations (common by now) where the AFM is
worn and causing spikes in the signal.
3. Many people have had success by installing a 22 uF 35v tantalum capacitor
across the AFM active leads, cost about $2.50 plus your time and solder. (This
fix, how to do it, etc. detailed below.)
4. In the case of a worn AFM, cleaning the carbon track is helpful, as is slightly
relocating the board so that the wiper tracks in a different area. You may have
to open out the mounting holes a little to move the board, but it only has to move
a little -- 1mm maybe? The wiper has two contacts, so you want the new tracks
to straddle one of the old ones.
5. A worn AFM can definitely cause poor idle and no doubt highway bucking as
well. If replacing it cures the problem, then it was “not” the problem that VW
designed the special harness for -- although that harness or the capacitor may
Cheers David Beierl - Providence, RI http://pws.prserv.net/synergy/Vanagon/
'84 Westy "Dutiful Passage" '85 GL "Poor Relation"
7. The Most Technical Description of the Bucking Problem, If It Is Caused by the Air Flow
1. What goes wrong inside the black box of the AFM! (? Move this >and the
(1) This (the bucking, stalling, etc.) happens because the AFM is, according to
the Bosch literature I read, the master sensor for the fuel injection. The AFM
tells the computer (ECU) how much air is being "breathed" and this
information is used to roughly tell the ecu how much fuel to meter into the
(2) This is further fine-tuned by the O2 sensor, to more precisely adjust the
amount of fuel injected. Now, when dirt or a defect in the AFM exists at a
particular setting of the AFM, the signal from the AFM will "jump around",
which should not occur in a properly functioning unit.
(3) The ECU sees this "jumping around", and thinks that something is
dramatically wrong with the system - and begins to shut down the fuel, as it
thinks the system is flawed. When that happens, the AFM voltage will drop,
because less air is being forced through it. When that happens, the wiper
or vane inside the AFM moves to a new position - usually past the area
where there is a defect (dirt or discontinuity).
(4) At this new position - the AFM does not present a noisy signal, so the ecu
thinks the system is back to normal again, and begins to properly supply
fuel - and you suddenly get power again. You have just experienced a single
(5) [Added by Sam and from other people‟s reports - sometimes there are
multiple bucks and then the engine shuts down.]
2. Now, here are several solutions and “why these fixes work” are for the following
(1) Replacement - Obvious;
(2) Harness: An electronic filter that screens the AFM output signal from quick
changes - (like what a noisy signal has);
(3) Capacitor: accomplishes the same thing - except homebrewed and much
(4) Cleaning AFM: Removes cause of noisy signal in the first place, hence, no
need for a filter. I suspect, although I have no proof, that this will extend the
life of an afm, by removing abrasive elements.
(5) [Added by Sam, w. apologies to Marshall - adjustment of the plate in the
AFM. See below.]
Hope this helps, Marshall Ruskin
8. A Somewhat Technical, Historical View of the Problem and the Electrical Solution
1. The Problem, Personal and Historical:
(1) My first experience related to the problem was when I drove on the highway
on a constant speed and after an hour or so my engine just suddenly lose
all power, and after I stop and start again it work fine , the same problem
that Derek has told about in the Technical bulletin TB#12 and TB#13 he has
put together from VW information he has. I phone some friends in Sweden
and quite soon I understood that the problem was known as well as very
special. This was back in 93.
(2) It seems like that the fault first was found in Sweden but when reported first
didn't the German company Bosch believe that it was a problem with the air
flow meter, but a local VW service person use a oscilloscope to monitor the
output voltage from the resistor wiper that measure the air flow value during
a long measuring time. And his work paid off. He found out that if the engine
runs for a long time very constant and specially also when the weather type
is little frosty then the wiper could start to lose the electrical contact on the
resistor trace and the voltage starts to oscillate instead of have a relative
(3) The DIGIJET and DIGIFANT electronics box accumulate this oscillator
voltage value, but after a while it thinks it is a malfunction and shut of the
electronic pulses to that control the gasoline flow to the engine: the engine
suddenly lose all power !! When the engine is restarted all memory in the
control box is reset and the engine runs fine until the same situation
happens again, that can take days, weeks or may not happen again.
2. The Electrical Solution - VW v. Resourceful and Cheap, Again From Sweden.
(1) The solution ( (if the guarantee is passed and you don't want to try the $100
version first )at least for a while ( if the air flow wiper resistance trace is totally
unconducting(worn out) then the whole air flow meter has to be exchanged)) is to
stabilize the output voltage round its oscillating average value.
(2) The air flow box connector has four pins. Between pin 1 and 4 is the NTC-1
resistor that measures the temperature inside the air flow meter. Between
pin 4 and 3 is the Potentiometer endpoints and pin 2 is the wiper of the
potentiometer that moves by the air flow and also is the output voltage signal
that must not oscillate.
(3) The easiest way is to add a $0.5 25 V 10 uF tantal capacitor with its +side
connected to pin 4 and its common to pin 2 (who is the air flow voltage
output) to stabilise the voltage. Later Bosch came out with the $100 box
that I think more or less make the same thing.
(4) Practically I mount the capacitor on a two wire 15 cm cable that I solder on
the pin 4 and 2 (note the polarity) in the connector that mounts on the air flow
meter box on the top. and then taped so it was completely water resistant
and dust resistant.
(5) I can only say that I have not have the problem ever since. But you never
know if this fix was the whole solution for the engine power lose. Good
Luck!! If it not works, don't blame me, if it works use your other $ 99 to
something more fun!
Lars Herrnsdorf - Gottenborg Sweden
9. From: IMPORT SERVICE MAGAZINE July '93 Vol. 6 No. 8
Subject: more 'Ghost harness' info
After driving for an extended period of time at a constant speed, some water cooled
Vanagon engines may loose performance or hesitate and surge. In many cases, the
symptoms will disappear after turning off the engine and restarting. After that, the symptoms
may not reoccur for several weeks or months. A new wiring harness (P/N 025 906 302A) is
available to correct this problem. The new harness provides better signal voltage stabilization
than the old harness it is designed to replace. To install the new harness, remove the air flow
sensor harness connector. Install the new harness between the main harness connector and
the air flow sensor connector.
There are active components in the new wiring harness. For this reason, there is no
continuity between terminal number 2 at one of the harness connectors and terminal 2 in the
harness connector at the other end of the new harness. This is normal and should not be
considered a fault.
All other connector terminals should have 0 ohms resistance from one harness
connector to its mate in the connector at the other end of the harness. Keep this in mind when
troubleshooting this system and performing continuity checks.
From: "Dan Houg" <HOUGD@mdh-bemidji.health.state.mn.us>
10. Another comment on general success in eliminating bucking:
1. When I had the problem, cleaning the AFM carbon track and replacing my 02
sensor cured the problem....for about $25 and 1 1/2 hour in my time. I would do
those 2 first....then the temp sensor and then the wiring harness. If you have the
problem I think you do, it may not appear again for months. Mine did it
once....then not again for a year....then a couple times in a year....then it started
getting more frequent and I fixed it. Usually, when it does appear, it will go away
if you just pull over and shut off the motor for a few minutes. I've never heard of
this stranding anyone. steve
4. WHY NOT TO ONLY RELY UPON THE CAP FIX OR THE VW EXTRA HARNESS, or
HOW TWO AIRFLOW METER FIXES TACKLE THE ROOT PROBLEM, or
YOU MIGHT WANT TO COVER ALL OF YOUR BASES AND TRY ALL THREE.
The problem is extremely common, and is often (but not always) caused by electrical
noise originating in the Air Flow Meter, which confuses the Electronic Control Unit (ECU).
You can "mask it" with the well-known capacitor fix, or you can clean the AFM with
electronic spray cleaner (the Debi fix - really easy, and worked perfectly for me and the
others that have tried it) and eliminate it. The main thing is, IMHO, to remedy the problem,
not the symptom - which is all the cap fix does. Marshall
1. Cleaning the Air Flow Meter and/or Adjusting the Air Flow Meter (AFM)
1. In many cases, solving the actual problem is easier than cooking up a cap fix
anyways. Basically, the idea is to clean the inside of the AFM with electronic
spray cleaner. Any flaps will have it - the brand doesn't matter, as all you want is
that highly volatile spray stuff for cleaning circuit boards etc. Marshall Ruskin
84 Westy "Leader of the Pack"
2. How To Clean the Air Flow Meter
1. Products to Use
(1) Today I cleaned the inside of my AFM with Motomaster electronic spray
cleaner. Anyhow, tracks in the resistive surface were clearly visible. I
sprayed itwith a liberal amount of electronic cleaner spray, and moved the
wiper across the track several times while it was still wet. Marshall
(2) Said cleaner is available at any FLAPS - just make sure is is electronic
spray cleaner - not electrical cleaner. You want a highly volatile cleaner that
evaporates really quickly. Marshall Ruskin 84 Westy Buck-free since the
spray fix (about a year)
(3) A can spray of QD Electronic cleaner with a straw (like WD-40 uses)
applies so easy without touching any parts, which means you don't leave
any body oils on electronic parts... I don't know it's your van, your parts, your
2. Various Takes on How to Clean the AFM (some editing, combining of various
versions, especially those from Marshall, who has been kind enough to explain
this several times through the years)
(1) Marshall On Cleaning the AFM (compilation)
(1) First remove lid of AFM, the hardest part of this job, as it was held on
with a silicone seal, obviously never opened before. I ran a utility knife
around the seal to cut it. Then I slowly and carefully pried it open using
a screwdriver. I think if you rush this you risk breaking the lid. Start with
a corner and don't force it - you could break the cover if you worked too
fast. You'll struggle for a while, then the seal will give, and the thing will
essentially peel off.
(2) Never having seen the internals, I was surprised at how delicate it is.
Like the inside of a watch!
(3) Anyhow, tracks in the resistive surface were clearly visible. I sprayed it
with a liberal amount of electronic cleaner spray, and moved the wiper
across the track several times while it was still wet. (Today I cleaned
the inside of my AFM with Motomaster electronic spray cleaner.)
(4) I also ensured that the central contact pivot, which rides above the
assembly on a post, was also clean.
(5) When dry, I closed the lid, and sealed it with electrical tape to keep it
clean and closed.
(6) I am a believer and you will be too. Thanks to Debbi (MtnGal) for this
Marshall Ruskin 84 Westy "Iron Igloo" Founding Member of
(2) The responses you've gotten are right - this is typical of Air Flow Meter
problems which VW tried to fix with the harness cable. While the cable help
some on both my cars, it didn't eliminate the problem. I finally cured it by
pulling glued-on cover off the AFM and cleaning the contacts with a contact
cleaner/lubricant. Since then I've had no hint of the problem for 3 years - with
or without the cable. I still think the lubricant they used is too
moisture-retentive & gums up in higher humidity. Air coming in at high
velocity cools the whole intake system and moisture condenses on all parts
(especially OUT of the airstream) and messes up the lube on the contacts.
1. Digifant-Diagnosis-Checks Air flow Sensor Modification.>The addition of
a capacitor to the air flow sensor wiring may help with intermitent miss. (The
mechanic at the dealership says this will also cause stalling after being driven
awhile and will not restart for about 30 minutes. Runs fine when restarted.) A
10uF/25 Volt capacitor can be installed.) Remove the connector from the air flow
sensor.) Pull back the boot on the connector for the air flow sensor.) Remove
connectors 2 and 4 from the hardshell connector for the air flow sensor.) Cut the
connectors of the ends of the wires, strip the ends of the wire and connect the '+'
side of the capacitor to the #2 wire and the '-' side of the capacitor to the #4
wire.Crimp on new power timer connectors on the #2 and #4 wires and reinsert
them back into the hardshell connector. 5) Reconnect the airflow sensor
connector) Connect a ground from the metal part of the air box to the empty 6mm
ground connection point on the right cylinder head.
(1) Solder the + side to wire #4 and the negative side to #2. wrap the bare wires
in electrical tape, then slide the boot back up over everything, this seals
everything in well. This information was made available to me through the
many helpful Vanagon people on the web. Also note the added ground
2. Another variant of the installation which begins with a testimonial:
This does work, cleared my probs immediately, so far for 14months now.
My only recommendation is that when installing simply pull back the rubber boot
exposing the 4 wires that go to the connector. Find #s 2 and 4 and simply nick
them for a 1/2" or so to expose the bare copper. Pry the wire out of the insulation
by pushing a nail/knife under. Then solder the capacitor right across the bared
loops of copper. No 15cm leads, no fighting with connector lugs etc. Use some
electrical tape over the solder sites then tuck, everything neatly back inside the
(1) The capacitor I used was a 25Volt 22microfarad tantalum type. I read of
mention of the stuttering problems not being completely solved with 10ufd
size, quoted from a LIMBO article (via email) maybe? NOTE: it says to use
#2 as positive, #4 as negative !!! I used it this way. Any electrical geeks
wanna tell if it makes a diff in this application? Prolly. Tim Smith '87
(2) Getting the polarity right is certainly key, as a backwards one will get leakier
and leakier (electrically) and finally short. If there's enough power in the
circuit, an aluminum capacitor will blow its can off (until the spoilsports put
a crease in the top so now it just peels open) and a tantalum will squirt a tiny
jet of molten tantalum in some random direction. David Beierl - Providence,
RI http://pws.prserv.net/synergy/Vanagon/ '84 Westy "Dutiful Passage" '85
GL "Poor Relation"
3. Details on the capacitor for this fix.
(1) I bought a tantalum 22MFD 35VDC capacitor cost me $4.50, the tantalum
types cost more other types. The capacitor needed is a 25 volt 22
microfarad tantalum type (cost me .41 cents).
(2) Tantalum is just another type of electrolytic capacitor, it uses different
material. Most electrolytic capacitors use aluminum. The difference is
tantalum in place of aluminum. The higher the voltage the better off you will
be also. Go with the 35Vdc rating. This will handle any transient spikes. Also
because of the various temperatures located there, there will also be
variations in the capacitance. Bill
(3) Response to someone who said polarity didn‟t matter. I beg to differ.
Tantalum capacitors are polarized. They have a marking to show the
positive connection. The actual symbol for identification can vary but it is
there. Have a look at any comprehensive parts catalog and it will identify the
positive connection. Regards, Indulis
(4) Responses to a person whose fix had stopped working:
(1) The problem with your 22mF/35V capacitor is that it's failed, not that
it's 35 volts. My guess is that the heat in the engine bay took it out. Use
the highest temperature spec available. There is a special
"automotive" spec, with reason.
(2) Capacitors have limits as to the amount of voltage it can handle across
its plates. Thus, the rating givin' specifies the MAX dc voltage that it
can withstand before failure. As a rule of thumb, you should always use
a higher rating when designing circuits.
(3) I suspect that the electrolytic (tantalum) capacitor was breaking down.
Heat has a definite reaction to capacitors. This depends on the
temperature coefficients. An example would be 100 ppm/C for a 1
micro farad capacitor calculates to 1 degree Celsius rise in
temperature, the capacitor will decrease by 100 Pico farads. Yes,
temperature vs capacitance is inversely proportional
4. Adjusting the Circuit Board Inside the AFM or Several Fixes Are Better Than One or
1. Try my trick. (This one assumes that you have already gotten inside the AFM.
See previous instructions on how to do this, CAREFULLY.
(1) Loosen, don't remove, the screws that hold down the circuit board.
(2) Put the tip of a flat screwdriver between the circuit board and the edge of
the AFM body and gently twist. You'll move the circuit board a millimeter or
(3) Tighten the screws, and the pickup will be on a slightly different track. This
should be good for a while, and you can start putting aside $s for a new
(4) If this doesn't fix your problem, most likely the problem is elsewhere, i.e., not
in the AFM. Karl Wolz
2. Another twist and more resourcefulness from a recent convert. As Matthew and
Karl had suggested, I was able to move the plate which supports the "carbon rainbow"
under the screws about a mm. This seems to have solved my woes.
(1) The only problem I had was loosening the Phillips head screws which had
been loc-tited with some kind of pink glue.
(2) After almost giving up on the one screw, I took my dremel tool, and cut a
deep slot in the screw head allowing me to get purchase on it with a straight
(1) I hated to get all the filings in there, but what alternative did I have, save
to buy a new AFM?
(3) After thorough cleaning (again) and reassembly (silicone latex caulk
dispensed with a glue syringe worked nicely) My bus runs like it did with a
mere 100,000 miles on it! Thanks fellows for your suggestions!
Brian Doss '88 Weekender, Queequeg
5. Another Vanagonic Gets Self Educated & Tackles the AFM but with less than perfect
1. Volks - [Details of driving woes due to bucking omitted.]
When I got back, I dug out some articles I have been saving, and decided to
check out the AFM. I bought a decent multimeter and some electronic spray
cleaner, and carefully removed the plastic top. The interior looked clean and
unworn, although I could see a pair of thin lines in the black semicircular contact
surface where the spring loaded brass piece rubbed along it.
2. According to Kay Fremgen, Bosch says that if you measure the voltage between
pins 2 and 3 while you are moving the air vane, it should smoothly change from
.2-.3 volts to 4.5 volts. On mine the rest position was 4.6 volts, and as I rotated
the wheel that controlled the air vane, the contact swept smoothly across the
black strip and the voltage dropped evenly to .4 volts. Looked good, although it
would have been nice to see some evidence of the problem.
3. I sprayed it with some Radio Shack electronics cleaner, and while I was waiting
for that to dry, I unscrewed the ground wires from under the air conditioner
compressor, cleaned the connections, even though they looked pretty good, and
tightened it back down. The other end of the strap looked as if it has recently
been cleaned, so I just loosened it, worked the wires against each other a bit,
and re-tightened it.
4. Re-checking the AFM returned the same values, so I reassembled everything
and went for a test drive. There is still a little bit of stumbling while under mild
acceleration, but not full throttle. A ten mile drive revealed almost no pulsing, just
a very little bit a couple of times. I wish I hadn't touched the ground wire, because
I don't know which of them had an effect!
5. What should I do to eliminate the pulsing completley? Steve recommended
doing everything I have done plus replacing the coolant temp sensor, 02 sensor
and the fuel filter. Shoule I just spend the money as preventive maintenance?
Mike Finkbiner '87 Westy
6. More ambitious cleaning and adjustment of the AFM:
I to have had success cleaning my AFM. Let me add something to Marshall
1. You can easily remove the "sweep arm" to bend the contacts. The arm is held
onto the shaft by a screw clamp. There is another screw that will allow adjustment
of the arm on the shaft, don't loosen that one! You don't have to bend the copper
2. The 4 prong connector can be removed from the AFM (4 screws on the outside)
and the contact points that connect to the circuit board can be cleaned.
3. The carbon track can be cleaned with 95% iso propanol or 95% ethanol. Both
work as well as any proprietary cleaners in this case. Use a cotton tip applicator,
and don't scrub a way too much of the carbon!
4. Metal to metal electrical contacts can be cleaned with a pencil eraser.
5. I used a product called "Jiff-Action" to protect the contacts after cleaning. Its a
cleaner/protectant for electronics, does a great job on removing "noise" from
carbon track type potentiometers(ie volume adjustment).
6. Just recently, and just for laughs, I have been playing around with the AFM spring
tension. The tension can be adjusted very easily with only a phillips screwdriver.
The tip of the screwdriver engages the cogs on the adjusting wheel and by
rotating the driver, the adjusting wheel will turn. The adjusting wheel is held by a
spring wire that engages the cogs. The tension on this wire such that it allows
Alistair 84 Westy
Malcom; Mount the capacitor inside i.e. under the cover. One lead goes to the wire
corresponding to the "wiper arm". The other lead goes to the wire that is at ground potential.
Drive Safely & Good Luck Ken Lewis<Kernersville,NC>
The tantalums are much preferable due to better temp coefficient and better stability. You can
make a non-polarized cap by connecting two caps (preferably of the same capacitance and
voltage range) "back to back" (e.g. connect the two "+" sides together and connect the "-" wires
to the points on the AFM. You can also use nonpolarized caps if you check the voltage at the
AFM connections and install the cap with the "+" side to the AFM connection that measured
You may find it much easier to just tack solder the capacitor inside the AFM box. Pop off the top
black cover, solder the capacitor onto the leads inside the box, silicone the cover back on. No
poking, prying or messing around with any wires. The mod only took me 5 min.
Dana E Hager
Sorry Tom -- take them back. You want a tantalum cap, they can order it for you under p/n
While trouble shooting mine I got so desperate I put a booster cable on it
and grounded it to the grounding point on the left head. Turned out to be
the AFM. To test it, meter between pins 15 & 19 of ecm plug, look for smooth
rise in resistance as you move the wiper thing from rest to full open. Don't
push the arm, use the finger on the opposite side. You might see it jump
wildly at a certain point and then settle back down. That's your bucking. My
84 started at about 500 ohms at rest. I cleaned mine 3 times before I
checked it this way and then cleaned it real good, now it runs perfectly.