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

Marc Boucher mboucher70 at
Mon Feb 2 22:56:32 EST 2004

I unplugged the lead to the potentiometer during cranking.

It was just one way I thought of ensuring that the plate was lifting as a
result of intake vacuum.

I'd already performed a test for vacuum leaks, a test that the plate lifted
with manual effort, and a test that the potentiometer resistance increased
with its deflection.

This seemed a good additional check.

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?  Since
its mounted inside Aluminum on top of plastic I can't see it getting ground

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?


----- Original Message ----- 
From: "Huw Powell" <audi at>
To: "Marc Boucher" <mboucher70 at>
Cc: "SJ" <syljay at>; <quattro at>
Sent: Monday, February 02, 2004 10:37 PM
Subject: Re: SJ Audi 100 Won't Start (Vacuum Leak?)

> Previously I'd tested the potentiometer resistance with the bonnet open.
> I'd found that The resistance between the bottom and the middle contact is
> about 2 kOhms at rest and rises to a max of about 13kOhms near full open
> falling slightly to 11kOhms at full open.  Today I decided to measure this
> resistance during an attempted start just to ensure that the plunger is
> actually moving.  The resistance moved from about 2 kOhms up to about 4 or
> kOhms. when it appeared that the engine was about to start and then as it
> sputtered the resistance fell back to 2kOhms.

Something about this has been bothering me since I read it... mostly
because the pot. acts as a voltage divider on a 5v/G feed from the ECU.

Of course, measuring the varying resistance should be a fine way to see
if it is clean and tracking properly, with the power off.

But while cranking/running etc., how can you measure a resistance that
has voltage across it?

Huw Powell

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