Linear circuit like this without a schematic is very difficult
to troubleshoot. Unlike digital circuit, a bad component that
causes a signal (voltage) to drip off will be propagated to
everywhere. That makes all signals all over the circuit board to
goo wrong. It then becomes extremely difficult to locate the
component that causes everything to go wrong. Digital circuit
on the other hand is extremely easy to isolate the faulty device.

I would take a dumb approach to troubleshoot such a circuit board.
Check all cable connection first. Check the power transistors
by checking the P-N or N-P junctions to see if any of them becomes
short or open. Check all resistors, especially power resistors, to
see if any of them is burnt out or changed value because of heat
over time. Look for any other components for sign of damage.
This approach usually can be faster and sufficient to locate the
bad component and have it replaced.

If you want to check out the active components then use an oscilloscope,
preferably a digital scope that can memorize the measured signals.
Sync and hold the signal with traditional scope is almost impossible
for this task.

LM2902 can be configured as amplifier, filter, analog buffer, etc. Single
supply voltage makes their circuit more complex than those with dual
differential supply. But LM2902 and LM 2903 operating at such low power
level makes them extremely durable. Troubleshooting with live voltage on
can be more damaging than actual use on the Vanagon.

Your effort in troubleshooting it is admirable. Good luck.

David

--- tinker man <tinkerman007@GMAIL.COM> wrote:

> On 9/17/07, Bob Donalds <donalds1@verizon.net> wrote:
> > TM
> > I did not mean to exclude the list
> > I'm kind of out of my league with the tech part of this
>
> Maybe, but you have lots of important info backed up by lots of experience...
> And yes, those microprocessors ARE my league...:-)
> (but engines, distributors, dynos and mechanics aren't...).
> If my Garage mechanics (not only VW) were competent at fixing fuel
> injection systems I wouldn't get into it, but it seems when it gets to
> electronics they get a dumb look on their faces and start replacing
> very expensive parts unnecessarily...
> So I've learned to diagnose before I let them get near. They do the
> mechanical stuff, I do the thinking...:-) (With the help of the list
> and Bentley).
>
> > so share anything I write you with the list
>
> Great. This will stir much interest, I'm sure.
>
> > the CPU is again not something I looked at I was getting this looked at with
> > SRS in Walpole MA Dick Shine
>
> What's "SRS in Walpole MA Dick Shine"?
>
> > He was taking out the EPROM's and reading them for me that's a far as I got
> > so that's why I have what's on the EPROM's
>
> Like I said, good for backup and verification.
>
> > and as I said Daryl told me the Euro has no hookup for 02s
>
> That's interesting, because in the US Digijet (I think you told me) do
> support O2 sensors, and someone on this thread reported that US
> Digijets use a completely different CPU (Z80). So maybe US and Euro
> Digijets are completely different animals. I'd like to get an internal
> picture of all those. Currently I have just a 1990 Euro Digijet and
> Jon Kenneke has posted on his website
> (http://kenneke.com/vwfi/fi.html) a US Digifant.
> I need more internal pictures of all models available (1983.5-1991, US
> and EURO) to clarify these issues.
>
> > can you tell me what dist you have is it the 2.1 or the 1.9 with the vac can
>
> Already did in previous thread, it seems it does have a vac can.
> TM
>

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