factory tach issue - wiring all good

Today I was helping a buddy of mine work on his 69 GTX with a factory tach. He got all new wiring from M&H which was ok and checked to the old factory stuff and everything was good. He hooked it up and the engine would then crank but not fire up. Disconnecting the tach, the engine would fire up. Is there a known issue with when tach's go bad, they can cause the engine to not fire? Seems like a really weird issue and we checked that the gray wire is on the neg side of the ignition coil.

is there a chance the wire to the tach is pinched anywhere around the dash or dash frame ? that wire being grounded will cause your symptom. if you have a multi-meter, check end of wire to ground for continuity. if it has, then trace wire back into car until problem area is found. if nothing is found, the tach may have internal issues.

If you do a web search for "No spark with tachometer connected" you'll find it is a more common problem than one would think....one example:

https://www.mgexp.com/phorum/read.php?1,3595849

( might even try adding in Mopar to the search)

MikeR

no apark with tach connected link

Turns out the tach is shot. We pulled the wiring harness that went through the firewall and there was a repair with a solderless connector but still had good continuity through it. We ended up also installing the dash side of the repro tach harness and still had the issue with no spark. So, the only thing left for him to do is to send the tach out over the winter and have it rebuilt.

Weird how that a bad tach would cause no spark.

just read where someone swapped the wires at the tach terminals and had spark and tach worked?? May be worth a try???

hmmm interesting. I'll give that a try the next time I'm over to his house.

A shorted tach line (like the wiring you already checked and is good) or a shorted tach will pull the negative part of the coil to ground continuously and not allow the coil to fire. Mopar tachs are pretty simple with a couple of transistors and a few resistors. A shorted transistor (base to emitter short, ie) will shunt the points or electronic ignition signal to the coil to ground and not allow it to discharge to the secondary and allow a spark.
Craig

had this happen to my old 69 RR.


A shorted tach line (like the wiring you already checked and is good) or a shorted tach will pull the negative part of the coil to ground continuously and not allow the coil to fire.