117 Potentiometer problems? 3.0 Litre Cars
Citroen XM Internet Reference Version 1.0 179
3/3/99
117 Potentiometer problems?
Hi all,
I have investigated my angina management problem further and I think the problem may lie with the
throttle potentiometer. Manuel (my Citroen guru) has changed the throttle pot with another 24v and the
behaviour of = both were unchanged: ie mine was still running erratically, and the other was fine.
The problem is, I’m not fully aquatinted with how a throttle pot works. I assume that it is like any other
potentiometer in that its resistance increases from 0 ohms to a maximum in either a linear or a
logarithmic manner. =20
According to the specs, when the throttle is closed, the voltage across pins 1 and 2 (presumably earth
and voltage take-off) should be 0.4 - 0.6V. This should change to 4.5V as soon as the throttle is
opened. The book does not say how the voltage should change as the throttle is opened further.
This sounds a bit odd to me as the resistance (if the specs are correct) should change from high to low
with a very small movement. I have disconnected (electrically) the pot and checked its resistance
throughout its movement range. It changes smoothly apart from a spot in the centre of the movement
where the resistance suddenly rises. My intuition tells me that this is not correct.
Questions:
1) What should the voltage feed to the pot be? In my case it is 4.88V with the engine not running.
Should it be 5V?
2) What should the resistance characteristics of a throttle pot be?
3) Is there a separate voltage regulator for the pot, or does it come straight from the management
computer?
4) What further tests could I do on it?
Regards - (sorry for the length of this!)
___________________________________________________________
Richard Williamson
Email: --- richwill@innotts.co.uk
Home page: --- http://www.innotts.co.uk/~richwill
William Crane School homepage: ---
http://www.campus.bt.com/CampusWorld/orgs/org2823
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