Citroen XM Forum

"The tacho flicks about as well with the ignition on."
That's IGN on, engine not running, Russ?
Has to be a power supply issue to tacho, then . . . but from where????? Instrument panel pcb?
Can you goad it into flicking about by gently thumping the dash?
I know you've sorted the IGN switch, but does a gentle/small rotary movement of the key (without actually causing it to switch) produce any tacho movement?

Yeah Chris. No engine running and it was flicking. But I changed the dash clocks for my spare and the same thing happened.
So it's not the clocks. It's something to the clocks and dot matrix.

Sounds like instrument pcb/connections on it. Dry joint or connector actually on the board?
Working a bit blind here, as I don't have one of the pcb's to look at. Remember BX ones, though.

You've obviously had the instrument pcb exposed before, in order to swap over the tachos.
Could you open up the dash again, switch on the IGN, and see whether some gentle poking/wiggling can throw up where the fault is?

Sounds like a bad live feed to the dashboard area. The dot matrix is displaying the suspension mode each time the display wakes up, as there are no other messages to display as they have all been displayed and cancelled by the system. but the suspension operation stays live continuously.
If it does it again, try opening a door straight away, I suspect it will display that.

Have you tried wiggling the ignition key when it is at the IGN setting, or while driving?

Russ,

Tacho:
I may have misread your post above.
When you say you "changed the dash clocks", did you swap out the entire dash unit carrying tacho and everything else, or just the tacho etc 'clocks' themselves?
If the whole pcb was swapped over, then the fault isn't on the pcb itself, but a feed to it.
Agree with you both on this.

Yes, I plugged in the other set of dash clocks. It repeated what has been going on. So it can't be the PCB.

It has to be what Will is suggesting, it makes sense to me. It must be a duff live feed.
I'll try the ignition key and I have done the door thing, and from memory it did do the door message. But I have replaced the ignition and it was playing up before I replaced.

Hypothetical idea here, and I will check tomorrow,.... not a duff battery is it?

No. A wobbly connection somewhere, or I'll eat my hat.

It is virtually certain that the tacho is not receiving a steady and dependable battery supply.
Nothing else seems to be misbehaving on the instrument panel.

The panel will be receiving a +12v supply, via the IGN switch, and this will arrive at the pcb to which the tacho instrument is connected.

Russ, if you can find this supply cable to the instrument pcb (it should power on and off with the IGN sw), you could check it with a voltmeter (connected between the cable and a guaranteed good ground point), to see whether the supply voltage wavers. Then check the corresponding +12v track on the pcb. Tap the board, wiggle wiring and manoeuvre the IGN key as part of the check.

Similarly, there will be a ground track running round the pcb. Also worth checking that this track (and any other ground connections on the instrument board - particularly the tacho) are making good connection with ground/chassis. Use test meter set to Ohms, and you should get zero.

John has said that your tacho should have 5 wired connections into the pcb. We think these will be:
+12v supply
GND (for supply)
sensor connection 1
sensor connection 2
sensor cable screening*

* This last may also be connected to GND, and possibly also to one of the two sensor wires.

If you can identify the +12v supply connection here, and check it for voltage/stability, this will be an important step forward.


I do realise that some of these tests overlap, and are checking the same thing from several directions.

I agree Chris.

I have started today on tackling the fault. I found and laid out the old wiring harness from the old car. This has been so helpful as I have been able to target exactly which connectors and which wires are potentially at fault.




This grey connector on the drivers side is part of the wiring.

On the passenger side the brown connector connects the engine bay wiring to the dash harness.


And these four connectors in the engine bay feed the power and info to the brown plug.


All the engine bay connectors are clean and connected! I have checked the grey plug and brown plugs and they are clean. So it's down to wiring. Issue is getting to the wiring to the brown plug. It's tucked tight up against the car body and the fuse board. I have disconnected the brown plug ready to test with a multi metre. I'm sort of hoping I might be able to reach and check the wires by taking the glovebox out. So more to follow.

Oh....my....gosh.
I just do not know what to say. For which you are probably grateful.

Some of us use a circuit diagram, while others just lay it all out on the deck!

I needed to see it in reality!!! I get wiring diagrams to a certain degree, but I have a brain that needs to see it how it actually is, if that makes any sense.