Well, that wasn’t helpful...

9 hours ago, David F said:

I think the voltage sensing circuitry in the tach might be the culprit.

Hey David F I think that you might not be understanding what we're telling you here, and maybe you're not clear on how the early tachometers work.  They don't sense voltage, they sense current.  A blast of current rushes through the circuit for every spark.  The tachometer "senses" it and "counts" it.  Current, generally, causes heat.  Too much current can cause too much heat. 

Lower resistance allows more current to flow.  That's why the coil's are fairly high resistance on the primary circuit, and why there is a ballast resistor to lower it even further.  And that's why the primary circuit resistance is so important.  

So, the "help" is - measure the primary circuit resistance of both coils and post them up.  If they're correct then maybe your tach just died.  If they're incorrect, then making them right might get the tach working.

Good luck.  The part about super conductors was a joke.  Your coils do have some resistance, I'm positive.  You just need a decent meter and some time to get it.

I know the coils have resistance, I simply meant it was the correct coil that requires a ballast resistor and thus does not have built in “additional” resistance to protect the points.

the coil that has been in car for past year or more and several thousand miles with working tach...and the one the tach died on measures;

1.3 ohms (or up to 1.8 ohms...reading is difficult to get stable and seemed to settle on 1.3 ohms) on primary coil and 8.23 k ohms secondary coil.

Edited November 29, 20186 yr by David F

Update:  Last night, tach was not working.  This morning, tach is working seemingly normal (not sure if/how accurate).  So, cracked solder joints at tach?  Poor connection somewhere.  I guess I am going to have to find the time to troubleshoot in detail...I just hate getting under the dash.

My old original 76 tach would stop working, it would only go so high if I remember right, after a drive on hot days, then come back when it cooled off.  Consider heat.  It seemed like something binding.

I had similar symptoms and ended up sending it to Speedometer Plus in Los Angeles.  Was repaired and has been rock solid since.

27 minutes ago, 2377kN said:

I had similar symptoms and ended up sending it to Speedometer Plus in Los Angeles.  Was repaired and has been rock solid since.

Could you "ball park" the cost to have that done? 

http://www.speedometerplus.com/

I just checked my receipt. Was 155 plus shipping. 

I first brought it to North Hollywood Speedometer & Clock Co to see if they could diagnose and repair.  They could not but they could rebuild with modern components that would require a re-wire.  I then looked and ebay and other stores.  Did some more looking and found Speedometer Plus.  Wasn't much more for a rebuild I trust vs gambling on something else. 

I mailed it to them on a Monday and it was back to me by Friday the same week.  It's been great ever since.