Citroen XM Forum

Hi Matt,

If all of your sensors are checking out at about 3.7k ohm, then it's unlikely that one of them is duff, rheostat-wise.
If all of them show up the same limited voltage output figures (you say 2-3v) with an approx. 0-5v supply, then the conclusion must be that the problem is the mechanical link from the TPS butterfly to the sensor.
This - I think you have already said - is confirmed by connecting up a sensor, and rotating it by hand off while not linked to the TPS, when you get a much larger range of output.

So, either the butterfly is itself not rotating sufficiently, or the shaft-link into the sensor is at fault (sloppy, bit missing, etc).
I can't think of any other explanation.

It's as if you take a volume control, and only exercise it between 30 and 70% . . . you'll never see either min or max volume.

____________

Just a couple of afterthoughts...

I take it the supply to the Gnd and +5v connections on the sensor(s) remains stable at that voltage while the engine is running?
Am just wondering whether the +5v regulated supply might be faulty...unable to deliver... If all of your available sensors are producing more or less the same result (but work aok on other cars), I'm obviously left wondering about the sensor supply voltage.

If nothing mechanical can be found amiss with the TPS, one way to see whether the reduced voltage output of the sensor is causing the engine revving/power problem would be to provide the sensor with a known-good +5v (or +6v from a battery*) and see what happens then. A pre-test of this could be done static, sensor mechanically connected, but not plugged into ECU. Output voltage range then?

Another test, to be done with care, would be to increase the sensor supply voltage to say 9v (battery again) and see whether the required 4.5v could be achieved. Again, best done with the sensor disconnected from the ECU - you don't want to squirt 9v into the ECU; a diode could be added to prevent that, but you might not be happy to try that.

* If that 3k7 is correct, current drain will be less than 2mA, plus a tad into the ECU maybe. So a 6v torch battery would be ok, but don't use any old plug-in power supply, as the output will probably not be smooth.

Hi Matthew

Back at 19 feb you connected the TPS wires direct to the ECU.
Did you run the engine like this and if so what was the result?
There is no reason why you should not connect a loose TPS direct to the ECU plug and try using it to rev the engine.

John

John/Matt,
Sorry for my ignorance about V6 - lack of V6 mechanical data here - but is that the throttle mechanism: mechanical link direct to and control of the throttle butterfly, with TPS providing the fly-by-wire fuel injection?
C.

Hi Chris

Yes, simple pot sensor sending voltage signal to the ECU for it then to set fuel injection.

John

Ok John, so you must be right: manually open up a loose TPS, and simultaneously open the mechanical throttle . . . engine should rev.
Apologies - not used to the idea of throttles! Too long on the other stuff.

Many thanks Chris and John for the further comments and I have now explored some of the kind suggestions made. Firstly, I can confirm that there is a perfectly stable 5v (4.86v actually) supply to the TPS while running, so there did not seem to be much purpose in connecting a separate battery pack as proposed by Chris.

In response to John, I did tee in wires alongside the ecu plug direct to the TPS and I have also tried connecting a loose sensor and operating while revving, both with no effect. It has since been noted interestingly that removing the TPS altogether actually appears to make no difference to the revving of the engine!! On reflection, I am beginning to think like Dean that it may be a wider and more general issue that just happens to be impacting on the TPS readings.

I think for the moment I will pause and await use of the Lexia, which hopefully can provide a few more clues and especially through comparing with other cars. The running also seems to be getting worse and now barely reaching 2,000rpm!

Any other thoughts would of course be gratefully received.

With regards,

Matthew T.

Faulty ECU? Hopefully Lexia will come up with something.

I have tried several other ecu with no change Chris and I am more inclined to think engine wiring harness, which is about the only thing I have not changed! The wiring has all been checked and found good but it must surely be an electrical issue! Regards, Matthew T.

I don't think it is an electrical issue, as electricity would not normally show the gradual degradation that you are reporting. Can you get a vacuum gauge onto it? A gas analyser reading would also be useful.

Surely it must be electrical Mike with the odd behaviour of the TPS and O2 sensors? I am also fairly confident that there are no issues with the fuel and air supplies including leaks, so that only leaves electrics. The ignition components seem to be behaving, so has to be some degradation on the fuel injection wiring or sensors, despite my rigorous checks not uncovering the cause thus far. Regards, Matthew T.