Got my engine running, sort of, today. I did notice that the car tacho (original sierra) was reading zero at tickover, and was 1000rpm below what my gunson was showing at other rpm's.

Are they adjustable? or just replaceable? or is there something in the car wiring that could be causing this?

Thanks
Mark

Hi Mark

Doesn't the sierra's rev counter connect to one side of the coil? If so and the fact you've said your engines only running on 3 cylinders in the engine section are you sure its not an ignition problem?

Adrian

Yes I'm sure, I did think of that initially, but the problem migrated to number 2 cylinder when I swapped the injectors over. Bit odd though

Hi Mark

Just sounded like they might be connected

I did have a sierra once with a dodge ign module, that didn't run very good, but it was only a base model so no rev counter

Adrian

Mine has the full monty, ecu the size of a small suitcase, I was wondering if the dwell on the injector might be feeding back

Its still a pinto though? If so their should be a seperate ign module, my donor was an efi pinto, i'm sure a choped one off. also according to the wiring diagram in the haynes manual there should be a suppressor between the coil and the rev counter, maybe thats faulty?

Adrian

It does have a separate module, currently bolted to my scuttle, I will have to check out the suppressor

if it's a 1000 revs out over the whole range then the needle's in the wrong place! It may have been moved when dismantling or the wee spring might have sustained some damage.
The scaling is set by a potentiometer on the circuit board, but it doesn't sound like that's your problem.
Cheers
Bob
PS well played - it's going to be a long time before my engine makes a noise!

Bob's answer is spot on. A scaling error such as incorrect number of cylinders does not provide a fixed offset, but one that is proportional to rpm.

This frequently happens when people fit white dial kits that involve removing the needle, which is then put back on in the wrong place.

Bob, a lot of tacho's I've calibrated have not had a potentiometer, but instead a fixed SOL (select on test) resistor.

I have a similiar problem with the Smiths tacho fitted to my car.It is the same as an early MG or Triumph type. What happens is it seems to read correctlywhen the car is cold and intermitently undereads when the engine or maybe tacho is hot . It reads under by 2 or 3000 revs and wavers around then occasionally reads correctly. After the car has coled down for a short while it works correctly fo a while.
I think it is more likely to be the instrument than the engine but cant be certain. I am using the bosch electronic dizzy and the lead from coil to module passes through the loop on the back of the tacho.
Any ideas?
Paul.

I have used an aftermarket coil, new as the old one was looking very used! Do you think this would have any bearing?

I've got one on my V8 that intermittently overreads since putting electronic ignition on, one day I'll fix that with a filter in the way to the instrument - just needs a bit of time & I'm spending my spare time on the lo(hi)cost when I can. Intermittent underreading sounds like an instrument problem, i'd suspect the adjustment pot (if it has one - see above) as the most likely dodgy thing, & that might be quite easy to replace. But remember it could be any of a thousand dry joints/old components/electric breakdown type problems
Cheers
Bob

It might be a trip to the breakers, if just for one as a diagnostic aid