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Capacitor Fix

Model/Type:  Digifant
Symptom(s):  Bucking cut outs till ignition is reset


From: "Dan Houg"
=========================================================================
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 togeteher from VW information he has. I phone some friends in Sweden and quite soon I understod that the problem was known as well as very special.
This was back in 93.

It seems like that the fault first was found in Sweden but when reported first
didn't the german compane Boch belive 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 pays 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 (It is not to easy to find the word in english, but I think you understand what I mean) and the voltage starts to oscillate instead of have a relative stable value.

The DIGIJET (and probely DIGIFANT) electronically 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 reseted and the engine runs fine until the same
situation happens again , that can take days, weeks or may not happen again

The solution ( (if the gurantee 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., 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. The easiest way is to add a $0.5 25 V
20 uF tantalum capacitor with its +side connected to pin 4 and its common to
pin 2(which 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.

Practically I mount the capasitor 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.

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





From: smitht@jupiter.sun.csd.unb.ca (Tim Smith)
Unsolicited endorsement, from a sample size of one vanagon: This does
work, cleared my probs immediately, so far for 14months now. My only rec. 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 rubber boot. 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.

Tim Smith
'87 non-stuttering Syncro






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