Citroen XM V6 engine management issues

[Deanxm]

Hello

Now the best of the weather is behind us i have decided to strip the wiring loom out of the car and try and solve my jerky throttle response problem out on the PRV V6 engine in my XM which i believe is related to the ECU not correctly interpreting the TPS signal.

The other issue is no reference voltage for the front knock sensor leaving the ECU, i get zilch at the knock sensor plug.

I went into this thinking i had a short in the loom, hopefully with some part of the TPS wiring which was dragging down the reference voltage to nothing and upsetting the TPS signal, i can now confirm this is not the case and the loom and all my wiring is fine, I even powered the Auto ECU direct from the battery by making a small power supply loom and i can confirm it is outputting no reference voltage from the board with nothing connected to knock sensor (or any other sensor) pinouts, to me this means the ECU is a wrong'un.

I then fitted the Manual ECU John gave me and the car started and ran great, BUT I still have no reference voltage to the front knock sensor and this is not correct in my mind, the ECU cannot see the output of this sensor without a reference voltage surely? other problems this ECU has is a poor hot start, I need to crank a good couple of seconds before the engine fires and it doesn't catch cleanly when it does if that makes sense. The final issue is two-fold, although the jerky transition between trailing throttle and accelerating and vice-versa i have with the auto ECU has gone, which is good, but with this manual ECU, i have another issue.
when pulling away the engine idle drops, so the car is in D and idling happily, i apply a little throttle and the revs drop and the dash warning lights come on for a second as the engine nearly stalls then it recovers and the car lurches forward and then nearly stalls again before pulling itself together and I can drive off normally, when I do get going its ok until I want to accelerate briskly (50%+ throttle) and then the engine coughs and hesitates like mad, the more throttle is applied the worse it gets. To me, this is a bad TPS sensor issue but the sensor is working fine, it is the ECU that is not interpreting the signal correctly.

This evening i found Johns thread on Fenix ECU mods on clubxm and one of the recalls was for exactly what the Auto ECU was doing, jerky transition between trailing and applied throttle, the fix was to solder a capacitor inline on the TPS signal wire to the ECU or something like that, i need to look properly later.

Unless anyone has some pearls of wisdom i am going to source a 3rd ECU before going any further, there is always the chance these ECU's are incorrect for my loom/engine combination or both knackered, if i had problems outside of the ECU i would not expect an ECU swap to solve one set and introduce a new set of problems.

D

[MikeT]

Dean, I could be wrong but I think it's more likely the knock sensor generates it's own signal (pull up type) so there's no need for a reference voltage (to pull down).
You can test this by probing the sensor pins (not the plug) and hitting the engine near the sensor with a hammer or large spanner etc. You should see slight bursts of millivolts (if your meter can refresh quick enough otherwise it'll require a 'scope).

[Deanxm]

Well that is what i always thought Mike but the rear sensor has 5v on pin 2 as the wiring diagram suggests, the diagrams show the front sensor is wired the same way but there is just no 5v there, they are 3 pin plugs on both the front and rear bank sensors, ground on pin3, 5v reference on pin 2 and signal to ecu on pin 1.

D

[MikeT]

Well that is what i always thought Mike but the rear sensor has 5v on pin 2 as the wiring diagram suggests, the diagrams show the front sensor is wired the same way but there is just no 5v there, they are 3 pin plugs on both the front and rear bank sensors, ground on pin3, 5v reference on pin 2 and signal to ecu on pin 1.

D

Then I'm probably wrong; Knock sensors weren't my main focus when watching/learning how to test these voodoo boxes. Sorry.

ETA, just checked and yes, they can have a reference voltage too.

[Deanxm]

I don't understand what the reference voltage is for either, i would guess the sensor generates a varying small voltage depending on the frequency it is exposed to and the ECU is programmed to associate a certain voltage as 'Knock'.


D

[MikeT]

I don't understand what the reference voltage is for either, i would guess the sensor generates a varying small voltage depending on the frequency it is exposed to and the ECU is programmed to associate a certain voltage as 'Knock'.


D

I think it's known as a bias voltage, so the ECU can detect a non-signal from the sensor?
Also, all the examples I've seen are one wire or two, not much help.

[Deanxm]

I dont know, i can see in the case of say a TPS you would feed it 5v but the signal range would be .5v-4.5v, if the ecu saw more or less than this voltage range it could flag a fault but i dont understand the knock sensor, it doesn't use the 5v reference for anything i can see, i have no EML light on and if i unplug the front knock it still doesn't come on, if i unplug the rear knock sensor still no EML so i have no idea.

D

[xantia_v6]

I think that it is likely that the knock sensor neither needs nor uses a reference voltage. A knock sensor is a piezoelectric voltage generator, and usually contains no electronic devices that could use a reference. Perhaps the ECU output is an artefact of some earlier test/diagnostics system?

[Deanxm]

I think that it is likely that the knock sensor neither needs nor uses a reference voltage. A knock sensor is a piezoelectric voltage generator, and usually contains no electronic devices that could use a reference. Perhaps the ECU output is an artefact of some earlier test/diagnostics system?

Maybe, but i would think both sensors would be fed 5v or neither, not one with voltage and one without, having said that i have two ECU's that do the same thing in this regard.

I would assume both ECU's are fine if it wasn't for the fact the Auto ECU gives a very poor and jerky fuel cut off/on with sluggish performance below 50% throttle but is very perky and performs well at wide throttle opening and the manual ECU is very responsive at smaller throttle openings but runs terribly above half throttle, i can't understand why two different ECU's types that other people have swapped with no issue both cause two oposite problems on my car, i am sure my issue is TPS related but the TPS is fine and i have checked and double checked my loom and have found no problems.

D