This one has me a little stumped.
After reassembling the dash on my '70 S4, my tach will no longer go above 3k RPM. It’s like it hits a ceiling and stops there. It seems to indicate correctly up until then. It worked fine prior to being taken down, and was very recently rebuilt, so I don’t suspect any mechanical failure of the tach. It feels like the signal to the tach is getting foiled, somehow. Never had any problem before I pulled the dash apart, and I didn’t mess with any of the tach interconnections.
Pretty strange. Any thoughts? The tach gets pulses from the coil, yes?
Running a pertronix.
Chris
Chris
You don’t say when you put the Petronix Ignitor in, in relation to the tach working, as this could be the source of your trouble. Given you have a number of other vehicles. Could you not jump the tach onto one of these and see if it will work above 3k, which would then show if it was a tach problem or a vehicle problem.
Andy
It’s a good idea, but this is the only electronic tach of the bunch.
The Pertronix went in last year, and I drove it around for about 1k miles since.
It’s pretty strange, as I it worked quite perfectly before I pulled the dash…
I’m going to check voltages, etc, this morning.
Chris;
Is the motor reving past 3K? If not, Then I suspect the Pertronix is acting as a rev limiter. Magnets weakening? If it’s reving to redline then I suspect the tach signal.
I have an optical Mallory ignition with a rotating chopper. It was limiting the motor to 5K until I widened the slots in the chopper to make the spark duration longer. I know Pertronix has magnets which is why I’m suspecting the magnets may be factor.
Phil
Be careful.
I fitted an Aldon, which I think is the same principle as the petronix, a few years ago. The tacho worked fine for about 2K miles and then started indicating erratically. Then one night smoke started billowing from the dash.
Toasted tacho!
Ended up replacing tacho internals with one that could handle the aldon
Tim