I did install a brand new ignition switch, the electrical part. No difference. When I was under the dash the wiring looked ok, nothing burnt looking, loose, etc. The car has an old phantom alarm in there somewhere, but nothing was changed on the car for over a year. Just started happening out of the blue.

I don't know if it is the gauge itself, but from all the reading I have done, possible culprits I have checked or replaced. Ground wires in engine compartment have been cleaned, fuses up front have been cleaned and re-installed, new ignition switch, and replaced faulty autoheat relay with working used one. The brown wire under the seat from the DME looked ok there. Next I will be pulling the gauges. Odometer needs to go out for repair anyways. Would love to have a working take before my next DE event in two weeks though.

Send me your tach.. mine is totally dead.. I'll take one that bounces with power off LOL.. just kidding.. I'll stand by what I already suggested to you.. if you see power at the thing (which you will Im sure) disconnect the signal wire first and see if it goes away.. from what has been said here one comes from your DME.. brown maybe? No clue on that but should be a signal, a ground and power.. I do not believe that has anything to do with ground myself... but...

Three pages and still not solved? Someone has got to have the answer. I actually saw the tach jumping around when the owner came by my place for a visit. I never would have believed it had I not seen the jumping needle with my own two eyes.

This is the oddest thing you will ever see. I mean, it really jumps.

Maybe I overlooked it in the prior posts, but is it possible, and did you try removing the tach signal wire (only) from the back of the tach, while leaving the ground and power connected? The point is to isolate whether it's the tachometer itself causing the problem, or whether the signal coming to the tach is (for some reason) pulsing even when the engine is not running. Of course, hanging an oscilloscope on the tach signal would also be useful if you have access to one.

Unfortunately, don't have access to an oscilloscope. I will try pulling the tach and just removing the signal wire and see what it does. Is there enough slack on the wires to do so? Cause they certainly don't leave much for the alternator!

yes.

"Anytime the OVERCHARGED battery is connected to the CDI there is a possibility that the elevated battery voltage will cause the SCR to "self-fire"."

Do what??? SCR???

I would ignore the last post if I were you guys.. its a setup for a loooong debate..

What we need is this fellow to follow the instructions so we can proceed...

"You should have a ground, a 12v and a signal I think"

- JP -

Really, great knowledge!

"Im going with the flux capacitor on this one.. "

- JP -

Great post! Learned that in the high school night electric class, right JP?

"what has capacitance in it on your car, Im not sure."

- JP -

Please, and what is the logic here.

The simple test is to remove the tach and connect it to an external 12 volt battery.
If the tach still jumps under this test then it's the tach itself.

What's really sad Lorenfb is you probably have the wisdom to solve this problem already with the data that has been presented. However, you choose to ridicule and poke at people to get a rise for your own personal sick enjoyment. You are just a waste of time on this forum.. why not troll somewhere else and leave people alone here even if you believe they are all lost bumbling fools? By your own assertion we cannot be saved by your vast amount of knowledge.

Grow up brother.. would suit you well.

EDIT: Since the troll joined this conversation, Im unsubscribed.. good luck gentlemen..

"why not troll"

Troll? Like who's the troll? See you had a lot of technical contributions on the MSD
thread, right? Other than attacks and hyperbole, what did you contribute?

This seems highly improbable, but.....

What's happening with the alternator charge light on the instrument panel when the tach is jumping? Certainly should be on but maybe cycling bright/dim...?

About 3 years ago when my '78 was doing this (but engine running) I concluded at the time that an overcharged battery, THE overcharged battery, was resulting in the CDI's SCR self-firing. Fixed the overcharging problem, new alternator with internal VR, and the tach no longer jumped, "twitched".

Just for curosities sake I disassembled the old alternator and was amazed at what I found, it should not have been functional. It had metal standoff here and there for the stator wiring and the insulation on the wires was completed burned away. I assumed due to HEAT.

A second thing I recently noticed is the the tach input comes from the CDI but is sourced from the distributor's internal magnetic inductive sensor. So the self-firing of the SCR would likely have had nothing to do with the tach twitching.

Another thing I noticed at the time but seemed to make no sense so was ignored was that the engine stoppage due to rev-limiter latch-up seemed to always result from a bump in the roadbed.

So, what I now believe...

Hitting the bumps resulted in an momentary short in the stators "uninsulated" windings. With the VR having failed "on", maximum current flowing through the rotor windings, the RFI resulting from the shorted stator windings would have been substantial.

Enough to couple into the nearby distributor magnetic inductiive sensor....?

My theory at the time was that the "twitching" of the tach would often result in "hitting" the rev-limiter, which would then latch up, opening the fuel pump relay.

With the ignition key on but the engine not running the VR is pumping the absolute maximum current into the rotor, an intermittently shorting rotor winding with then generate a fair amount of RFI....

I grant you this is a stretch...but...

Didn't get to removing the Tach yet, but I will try test 1, just removing the signal wire in both key on / engine off and engine running. Test 2, external 12v source key on/ engine off and engine running. Thanks for the help everyone will report back shortly.

"With the ignition key on but the engine not running the VR is pumping the absolute maximum current into the rotor, an intermittently shorting rotor winding with then generate a fair amount of RFI...."

"RFI" - Don't think so!
"engine not running the VR is pumping the absolute maximum current into the rotor,"
- Not really! Only about 100ma from the light (no alt feedback voltage).

Next guess, please.

Never updated this thread. Didn't get to try the separate power to the tach. However, I did get a spare 3.2 motronic Tach and replaced mine. Problem solved. Looks like the tach will need to be sent out to be rebuilt. Wonder how that happened.....