ZX diesel tacho oddity...

[RussellF97]

Hi All,

For a reason best known to itself, on starting, the tacho occasionally goes up to a tad over 2000rpm and sits there, no matter what the engine revs are. Eventually (usually after only several minutes), it decides to join the team and work as Citroen intended.
It doesn't happen very often so I'll just put up with it, but does anyone have any idea why it would do that?

Phil

[citroenxm]

It sounds like the innards of the sensor is breaking or playing up...

Its ONE OF THESE even though its down for an XM its the same as a Xantia, or ZX etc etc...

[RussellF97]

citroenxm: thanks for that. Since it only does it occasionally, I'll probably leave it until it packs up completely. That way, I'll know what the problem was when I fix it!

[spider]

One thing to try:

Hot (or at least warm) engine.

Ignition off. Unplug the glowplug relay (yes really) just the multiplug leave the heavy leads alone.

Start the car, maybe take a couple seconds longer than usual due to no preheat (see why I said hot engine)

Although rare, sometimes interference from the glowplug relay can cause the tacho to overload / stop for a while. Tis unlikely to be this, I've only seen it once that I can recall on a 406 but its a 'free' test if nothing else.

[Peter.N.]

Could be as simple as a poor connection on the TDC sensor plug, worth a try anyway.

Peter

[RussellF97]

Thanks for the replies chaps. It's only done it a couple of times in the three and a half years I've had the car; it's only because it did it 'tother day that I thought I'd ask the question.

Having changed the head gasket, clutch, and injectors this year, I think I'll keep my hands clean for the foreseeable future and leave well alone although I did try your tip, spider. Of course, the tacho worked as advertised...

[RussellF97]

Finally found the answer; it was caused by damp! The car sits under a carport, but given the weather we've been having, the atmosphere hasn't exactly been tinder dry.
The car had been standing over Christmas, and when I went out in it today, zap! needle straight up to 2100 rpm. After several miles - and the speedo cluster getting fogged up until the heat from the backlighting cleared it, the needle started to waver about until it eventually gave up its bid for independence and started reading correctly.
At least I don't have to budget for a new Tacho, just use the car a bit more often...

[citroenxm]

Wont be the Tacho, it will be the Sensor on the Gearbox, its hidden down the back, just above the Drivers drive shaft where it comes out the Diff.. It will play up again..

[RussellF97]

Hi citroenxm: yeh, the sensors are cheap enough so I think I'll replace it - but when it's a bit warmer and drier!