Citroen XM Forum

Hi Russ

Sorry no idea where the Hydractive ECU's got put after I pulled them to bits while taking record photos.
Do not think I reassembled them as I had no active use for them or any idea of their condition or history.
I was going to investigate programme and fault storage devices but did not get round to that.
You need a known good fault cleared ECU for effective fault clearing on your car.
Just plugging another unknown state and history one may just add new strange results.
Getting your ECU cleared or trying it in a known good car would be my next steps.

Hi All

There is a reason why most XM major ECU's and radios have at least two fused positive feeds.
One feed is direct off the battery and keeps the microprocessor alive but sleeping when the ignition is off.
A second feed via the ignition switch runs the microprocessor start up programme.
An example of this set up is seen on the ABS ECU.
Switching the second feed from on to off starts the static start up test with the blinking light.
The ignition switch is wired so that when switched to starter the ECU second feed is turned off and the static test then runs.
If while the car is running you gently nudge the ignition switch towards start you will see the ABS light go though the blink test.
If trying this be very careful not to move the switch enough to engage the starter.
Also do not try it too often as each time it is done the ECU stores a power supply fault in its memory.
Get enough of those stored faults and the ECU shuts the ABS down and puts the ABS light on.

John

Another fascinating and useful piece of information John, how do you know all this!

Ciarán

Hi Ciaran

Credit where credit is due.
Paul told me about the effect of flicking the ignition switch on the ABS.
I tried it and then wanted to know why it caused the ABS reset.
So all down to something getting my interest and then chasing it up.
Note that the original thought does not usually come from me.

One thing I forgot to mention above.
An ECU missing a single power supply line cannot be read by a LEXIA.
Typical Citroen logic: If it is not working the test device cannot tell you why.

There is a feature of the Citroen workshop manual that also sparks my interest.
They draw all ECU's (however minor) as white boxes with just wire numbers going into them and just an item code number.
That really annoys me on two levels.
Having to turn to an index page each time you want to know what the item actually is is a nuisance in reading a circuit diagram.
Particularly as I am usually balancing the book on my knee. Table space is hard to come by in this house.
Not having any details of the internal circuit components makes testing and repair real hard work.
So I try to find out what is inside each box, try to understand what it should do and record the details for later use.

John

xmexclusive wrote:Hi Russ

Sorry no idea where the Hydractive ECU's got put after I pulled them to bits while taking record photos.
Do not think I reassembled them as I had no active use for them or any idea of their condition or history.
I was going to investigate programme and fault storage devices but did not get round to that.
You need a known good fault cleared ECU for effective fault clearing on your car.
Just plugging another unknown state and history one may just add new strange results.
Getting your ECU cleared or trying it in a known good car would be my next steps.

Thanks John. I will try and do that. The case for a second XM of the same age suddenly becomes a good idea!!

Interesting info regarding the ABS.

xmexclusive wrote:The ignition switch is wired so that when switched to starter the ECU second feed is turned off and the static test then runs.

. . . and there are some folk on here who laugh and snigger at me because of my complete lack of understanding of electrics.
. . . is it any wonder when the basis of getting something to work is to switch it off!!!

It's beyond me as well Eddie, I think I must go start reading it all again. Bit too much for me today I'm afraid but highly interesting nevertheless!

Eddie, we all learn slowly over time, the XM's teach us unless we lose patience and cube them first.

It cost me £20 to learn about the Lexia not reading with missing power feeds.
A good while ago I had a bronze V6 XM car with an ABS fault that I needed to clear for its MOT.
While on holiday in Cromer I decided seek help from a well respected Citroen Indy.
They duly set too and after trying both their Lexias confirmed that the ECU could not be read.
They could not say why but the result was they could not tell me what was wrong with the ABS but £20 for their time please.
It took me another month to find and replace the blown fuse as I too did not then realise about multiple feeds to ECU's.
That information came to light when reading the wiring diagrams from the workshop manual copy that Noz sold me.

John

John that would explain perfectly why Jim's Lexia couldn't read Russ' ECU, even though it's obviously partially active (operating the valves on door open)...hmmmmm!

Ciarán

I too have come to that conclusion. I have no idea if it blew as it was being reconnected, or had done ages ago, but certainly strongly points to why the ECU couldn't be read.

Can I confirm something: from reading back on this thread, Jim has put his ELIT to Russ's car. John said the LEXIA won't read if 1 power feed is cut. So, does what apply with the LEXIA apply with the ELIT too John?

Second: is this valid for all ecu types (ABS, engine, Hydractive)?

Many thanks
George