Tacometer 1980 spider

[sigkadetm]

Well I'm back to square one. My tachometer is still not responding correctly. I thought the ignition switch my have been the problem but doing the ignition switch fix did not help. The tachometer only responds when I rev up the engine and shows zero at idle. What to do next???? Maybe 0.68uF 400V capacitor fix?

[spider2081]

Wow you have tried so many things to get your Tachometer working properly.
Just curious how did the contacts in the ignition switch look when you took it apart. I have taken a number apart and usually the start contacts are the most burned next is the "INt" for the lights.
Anyway the reason you looked at he ignition switch was: pushing up on the wires near the switch made the tach work at idle. After you finished with the ignition switch can you still get the tach to work by moving wires near the ignition switch.
If you can, that is still the area I would focus on. The ground for the instruments is in that area.

[sigkadetm]

But why does to work when I rev the engine??? I'll check all the grounds again....The contacts on the ignition switch were a little dirty but not too bad

[sigkadetm]

Also after the engine warms up the tachometer starts to run fine......crazy

[spider2081]

The part about warming up has me wondering about the coil pack, actually the electronic control module mounted underneath the coil. They have been known to be temperature sensitive. However usually they fail when hot. Its the ECU that makes momentary ground mimicking ignition points that triggers the coil and the fuel injection computer. It is also the signal that drives the tachometer.

[sigkadetm]

Do you think I should try replacing the ECM and coat with new heat sink ???

[ORFORD2004]

I think I have a replacement if you want to switch.

[spider2081]

I don't know how to measure the pulse at the D or - post on the coil other than with an oscilloscope. I doubt you will have access to one. So if you could borrow someones coil pack that is working in their car if nothing else you could eliminate that as a possible problem. Changine a coil pack is easier than replacing the ECM and you are changing the ECM and coil if you change the pack. Again just a thought

[sigkadetm]

It has to be the wiring from the coil to the tachometer. I took the tachometer out and wired it directly to the coil and wow it worked. Going to rewire .........finally!!! Thanks for all the input. Next adjusting AFM with exhaust gas analyzer.....

[spider2081]

Great news thanks for sharing. I know I have stated this before but I think its great you are pursuing this issue and sharing it with us.

[chrisg]

the symptoms sound extremely similar to the behavior of tachometers I've experienced with late electronic ignitions on (some) early cars, "cured" using the capacitor that's spec'd documented probably a few places (like https://www.mirafiori.com/faq/content/t ... achfix.htm) on the 'net. In my experience, prior to the 'fix' the tach would be mostly dead, with periodic registering on high rev's or randomly jumping about, until the engine was up to operating temp. Un-nerving since that's the usual symptom of failing wiring in the magnetic pickup also.

Don't know if you've taken the tach apart & tried adjusting the potentiometer...in 20 years, I've never had to or done that, but a couple of months ago, someone suggested it for a rather lazy tach on my '85 X1/9 and it did the trick. I would not get into that until addressing the wiring and/or capacitor issue. The fact that you are reporting it works correctly once everything is up to temperature would lead me to believe you do NOT have a calibration issue.