[Tech] Alt charging light probs after MFA diesel install

[drew at scirocco.cs.uoguelph.ca (Drew MacPherson)]

As if I didn't have enough to do, I decided to reinstall MFA in the 84 
Wolfsburg.  I'd removed it when I installed the A2 TD instrument cluster, 
and find myself really missing the trip/time functions, along with the 
outdoor temp sensor.  Fuel mileage is NA for the time being, but I have 
some ideas on that...

I took a donor cluster (from an 85 or 86 GTI probably) and modified the A2 
tach to fit (required drilling two new holes in the faceplate and using 
the small adapter piece from the gas tach.)  I had to wire in a newer 
style MFA stalk switch, as the 84 had the old style MFA where the memory 
selctor switch is integral with the cluster, but that was pretty 
straightforward stuff.

The problem I'm having is that the alt no longer self-excites.  This was 
pretty much instantaneous with the old instrument cluster, however now I 
have to blip the foot feed to get the light to go out (and the tach to 
start working, as it operates on the W+ terminal of the alternator.)

I have pored over the wiring diagrams and the printed circuits for the
diesel and gas clusters and can see no differences in the exciter circuit.  
The D+ terminal on the alternator is wired to T7a-5 on the cluster (blue
wire) - this is common for both clusters.  There's a resistor inline
between terminal 15 (switched 12V) circuit on the cluster (61 ohm IIRC)  
and the LED, then in the wiring diagram there appears to be an additional
semiconductor (not sure if it's a diode or a resistor) parallel to this
circuit.

Does anyone have any experience with troubleshooting ALT exciter circuit 
problems inside the cluster?  One theory I have is that the second 
semiconductor's value is different on the diesel clusters VS the gas 
cluster - although the only reasoning I could account for this is that the 
diesel's rpm's are typically lower at startup than the gas rpm's,  There's 
no difference in the alternator (other than the presence of a W+ terminal 
in some cases.)  Does anyone know what the mystery semiconductor is/what 
its value is?  I need to pull the cluster back out and try again to locate 
it (tracing this printed circuit foil gives me a headache!)

Lastly, I seem to recall running across a picture on the web of a printed 
circuit that someone had colourized in photshop to make following the 
traces easier - does anyone have a link to this?

Thanks,

Drew

-- 
/=============================================\
|  84 Wolfsburg Edition TurboDiesel Scirocco  |
|    http://scirocco.cs.uoguelph.ca/gtd       |
\=============================================/