Citroen XM Forum

Looking at the above document it looks like the TPS is fed 5v on the left pin, grounded on the right pin and signal is the centre pin, so check you are seeing 5v on the plug to the TPS on the left pin using the battery post earth, if you have 5v then check resistance between the far right pin to the battery post to confirm if the earth is good or not.

If substitute sensors are maxing out at 3.? volts I would think its a grounding problem or supply voltage from the ECU.

D

I have already carried out the checks on the harness connector Dean, with readings of 4.85v and 0.8v as mentioned earlier, but somehow the voltage does not appear to be getting through to the sensor! With regards, Matthew T.

MTXM wrote: In response to Mike I had expected a inlet manifold leak that apparently does cause a lean mix and then found the split pressure sensor pipe, which if resulting in a rich mix may have been the cause of the high oxygen sensor readings possibly? The disconnection was extremely brief so I think the cat will be fine.

An air leak into the manifold does not cause a lean mix, as the MAP sensor detects it and adds more fuel to compensate (the MAP sensor and ECU don't care if the air came through the throttle of via a leak. If the leak is on one of the cylinder runners, then obviously that cylinder will run lean, but the others will all be rich, as the ECU fuels according to the average pressure.

It would be useful to get a proper gas analyser on the car, to determine if the engine really is running rich or lean.

It would also be useful to measure, either directly or with Lexia, the manifold pressure at idle and at high revs, both on this engine and one that is running normally.

It is now possible to report that I have checked continuity and voltage to the TPS socket contacts and all appears to be in order. The strange thing is that the range of measurements for the sensor even after adjustment is 2.0-3.3v, which is some way from the norm. In addition, with the engine running and limited revs possible, the reading barely changes from 2.0v! The TPS has to be the problem, surely, but why should it behave in this way and what can be done to fix? I have also checked wiring back to the ecu that included the introduction of direct connections and also tried other known good ecu with no change. Regards, Matthew T.

Just to refer back to the sheet posted earlier by John and to confirm, my readings are exactly 1.99v at rest and 3.22v full throttle, as compared to 0.5v +/- 0.1v and 4.5v!! Regards, Matthew T.

Thinking further about this problem with the TPS, it is like something has upset the parameters, which is apparently not within the sensor or ecu. The range of values now given (1.99-3.22v) is much reduced from the norm (0.5-4.5v), rather than just having higher base and upper figures, both are squashed together oddly! The difference is actually 1.23 against 4. If as John states less than 4.5v on full throttle from the sensor is 'limp mode', it is no wonder the car is not running right. Regards, Matthew T.

Im starting to think this TPS is a red herring, i think the earlier suggestion of checking manifold pressure at idle and 2k against a known good car would be a good way to go now.
To save wasting time it may be worth waiting untill the Lexia turns up.

Something just doesnt seem right here.

D

Has the resistance (end to end, and wiper to one end and then the other - sensor disconnected) of the TPS been checked, if book value is known?
Is the mechanical sweep of the sensor unduly limited? Looks like there's an adjustment for it in the document above. Does sound as if the wiper is only travelling part of the resistance track.
What is the voltage (down to good ground) at each end of the sensor (shd be 0v and 5v?) ?

Perhaps its something daft, like Dean reported here, with the wrong components being connected up (throttle & isc valve), - here viewtopic.php?f=4&t=3497&hilit=throttle ... s&start=30" onclick="window.open(this.href);return false;" onclick="window.open(this.href);return false;" onclick="window.open(this.href);return false;" onclick="window.open(this.href);return false;

Somewhat similar symptoms - only revs to 2-3K.

I remain pretty convinced myself that the TPS is the problem Dean, given the very unacceptable range of readings obtained and distinct sense of 'limp mode'! It is also my plan to check all parameters against other cars, but unfortunately the dedicated s/h XP laptop does not arrive until next Thursday. In the meantime I can now report that I have inspected the TPS on another XM and that has a range of 0.46v - 4.22v, which is where it needs to be. I have additionally found that when I connect a loose sensor to the problem car I can obtain a range of 0 - 4.6v, but this cannot be achieved insitu. In response to Chris the sweep of the TPS is limited with only slight adjustment that I have explored as mentioned above. Please be assured that there are no wrong connections Steve and the TPS connector plug is quite unlike any other! Regards, Matthew T.