My Citroen XM 3.0 PR-Vestige

[Dean]

Here is what i think is the defective component the 5v supply doesn't make it past, all i get out the other side is 1.75v
IMG_3390 by Deanxm, on Flickr
Here on the front of the board is the 2nd to last component on the track (resistor?) before it leaves the board at the multi plug, 1.75v goes in, nothing comes out.
IMG_3387 by Deanxm, on Flickr

I may attempt a fix later but this ecu does still works and is the only way i can move the car currently.

D

[Dieselman]

It's a simple device and one would think a resitor, but it could be a diode.

Any device on the engine will be fed with 5v and the return line will be monitored by the ECU, so look for a common 5v feed to both knock sensors and then the return. What you are monitoring for readings amy be the return line.

[Dean]

Yes all sensors are 5v, they are fed from a substantial common track on the ecu, each sensor has its own individual supply with a series of identical components running along the track to the final output pin. This one is missing power, the knock sensors are fed 5v on pin 2, ground on pin 3 and signal back to ecu on pin 1.
no mistake, the ECU is not outputting the 5v reference voltage on this channel, that and i've also come to the conclusion the ECU as a whole is partially loosing power on the 5v circuit every few seconds which is causing the engine to hickup and fault codes to not stay in the ecu memory when they are no longer existing.

D

[Ciaran]

You could try a small fly wire to manually pipe out 5v from the common track to the suspect pin and see if your symptoms disappear.

That said, it wont fix the intermittent stutters, but it will tell you whether soldering in a replacement diode is a worthwhile task or not.

BTW, epic work Dean, I'm in awe at the level of detail and 'there's nothing that can't be done', everytime I read this thread!

Ciarán

[Dean]

Thank you Ciaran for your kind words, with a big enough hammer anything is possible! although i wish i hadn't bothered pulling the front loom apart again to find the issue before checking the ECU

Do you know what that orange/brown thing is in the first picture?

D

[Dieselman]

I doubt it's a diode as there is no blocking line. It could be an ICP (fuse).

[White Exec]

Not a good idea to guess.
Maybe someone has a good one, which can be compared, to identify that component.

[Ciaran]

It does look very like an IC fuse as Will says, I've come across many like it on laptop motherboards and the like, and it would explain the lack of power, but like Chris says, wouldn't want to completely guess.

Are there any markings or numbers on it?

Ciarán

[Dean]

A new/replacement ECU has been sourced and should be with me next week some time, i may also get another if this is actually the problem because the market seems to be swimming with V6 ECU's at the moment and if they are weak i want a spare for the future.

Ciaran, there are no numbers of markings at all, just the colour. It is being fed 5.3v and outputting just 1.75 which vanishes as soon as it goes through the next resistor in the second picture at the front of the board.

D

[Dieselman]

They are SMD capacitors.