Erratic Tacho

[Archie@]

Hi guys, just wanna run it by anyone who might know, just before i go forking out for parts.

For the last two days when I start up the car, the tacho starts behaving erratically and jumping all over the place, well up into the 7k territory, although the engine is revving normally at around 1k rpm.

It only does this for a short while, whilst the car is still cold, and once the temp rises it goes back to normal.

I'm thinking its the ignition coil that might need replacing, just want to make sure I’m doing the right thing, so if anyone has suggestions/thoughts/ideas, please let me know.

Thanks

Achilles

[Archie@]

Oh well, I guess it is the ignition coil as it just died so a new one has to go in anyway.... I'll find out tomorrow if it's fixed the eratic behaviour of the tachometer.

[PORTS]

So, did that fix the problem?

[Archie@]

Sorta. I replaced the ignition coil. The car starts fine, but the tacho is still erratic... I'm thinking possibly the start motor might be the culprit as It dies yesterday as well... but I revived it with a hammer.

So a new starter motor will go in probably this weekend and if the tacho is still acting up, then I'll probably check the wiring... don't want to pull the dash if I don't have to cause it's a real bitch of a job on a 280....

good thing is that at least the tacho isn't interfering with the engine running.... and since it's an auto sl*t, the tacho isn't all that important as long as the car knows when to shift... which it seems to have no problem doing...

[SteveL]

Sorta. I replaced the ignition coil. The car starts fine, but the tacho is still erratic... I'm thinking possibly the start motor might be the culprit as It dies yesterday as well... but I revived it with a hammer.

Can't see how the starter is related to the tacho problem. Tacho takes it's signal from the negative terminal on the coil, plus there's a dropping resistor in the 'feed' to the tacho which is plugged into the loom nearby. Check terminals on the coil and anything plugged in nearby for corrosion or just simply loose. Another possibility is the dash assembly itself....loose connections, etc, but that's a bit more work.