SJ Audi 100 Won't Start (Vacuum Leak?)

Huw Powell audi at
Mon Feb 2 23:14:54 EST 2004

> I unplugged the lead to the potentiometer during cranking.

Ah... ok.

> One thing that was curious was I had read to 'ensure the potentiometer is
> properly grounded'.  Does it get its ground from the 3 pronged plug?

Yes, as its name implies, it's just a potentiometer - ground on one 
side, 5v on the other, and an output varying with plate position riding 
in the middle.

> It was also curious why the resistance actually dropped towards the end of
> its deflection.  This doesn't seem unique to mine.  Someone else in the
> archives had done the same test and got a similar result.  I'd bet its a
> valid engineering design (eg for full deflection you'd need to be
> decelerating...although i know this is also handled by the DPR) ...but it
> could also be a worn potentiometer?

Could be, yes.  I'm not impressed by that drop at the end, though it 
probably isn't affecting your problem.  Nate and I rigged up a harness 
once and played with a few, key on, and measured the voltage.  The 2 or 
3 good cars smoothly rose over the plate range.  A bad one had some 
dropouts in the middle (wiper losing contact).  So we broke it open to 
look inside.

At full deflection you'd be getting maximum airflow - why would you need 
to be decelerating?

Too bad these aren't replaceable - I think they should be.

Huw Powell

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