Tach not working

[Lfino]

A little help please. So I've just installed the new engine in my 67 500. Cam was broke in on a stand by the builder. I've run it a few times (3 or 4 minutes each) just to check for leaks and whatever else I might have missed on the build. Idles about 8 or 900.
Yesterday I went to start it and the battery was low. It's an old battery while waiting for the new one. Connected the battery charger and fired her up. Checked on RPM, and the tach is not working. I've spoken with several people who say it shouldn't hurt to start it with the charger connected. I've done this many times in the past and hasn't been an issue. Any ideas? Thanks

[rmarble57]

The Tach in your car works off of a "drop" in voltage from the wire going to the coil.  If you do not use the proper resistor wire to the coil which drops the input voltage down to around 9v, the tach will never see a drop big enough to count that pulse.

This generally happens if you are using aftermarket ignition parts etc.

Hope this helps.

[shelbydoug]

I recently had issues with the tach in my '68.

It was working fine and all of a sudden it wouldn't read over 1,200 rpm.

I immediately blamed the Petronix in the car. I replaced that with points. a new pink resistance wire and a new yellow top coil. No change.

Logic would say that the only thing left that it could be was the tach. I sent the tach out to someone who I trusted.


The results of testing it were unexpected.

Electronically it was fine. The issue was that it had accumulated magnetic dust on the "jewel movement" which was restricting the needles response.

The Tech cleaned it, apparently lubricated the jewel and retested and it was fine.

It appearantly can't even be around an atmosphere where there is any kind of an airbound metal. Even microscopically.

You learn something new every day.

[Lfino]

Tach is newly rebuilt by a trusted man known to this forum.
Distributor is a pertronix. Engine feed wires are new. ??

[shelbydoug]

Tach is newly rebuilt by a trusted man known to this forum.
Distributor is a pertronix. Engine feed wires are new. ??

What is the quote of Sherlock Holmes?

"When you have eliminated the impossible, whatever remains, however improbable, must be the truth".

The jewel movement is rarely looked at. The tachs usually are just looked at electronically and the needle rarely paid attention too?

I'm just trying to help, not add confusion to the fire.

[67 GT350]

I recently had issues with the tack in my '68.

It was working fine and all of a sudden it wouldn't read over 1,200 rpm.

I immediately blamed the Petronix in the car. I replaced that with points. a new pink resistance wire and a new yellow top coil. No change.

Logic would say that the only thing left that it could be was the tach. I sent the tach out to someone who I trusted.


The results of testing it were unexpected.

Electronically it was fine. The issue was that it had accumulated magnetic dust on the "jewel movement" which was restricting the needles response.

The Tech cleaned it, apparently lubricated the jewel and retested and it was fine.

It appearantly can't even be around an atmosphere where there is any kind of an airbound metal. Even microscopically.

You learn something new every day.

This is the trusted "tach tech" Mine will not go over 2500 to 3000...I guess I need to send it to someone like that??

[shelbydoug]

I recently had issues with the tack in my '68.

It was working fine and all of a sudden it wouldn't read over 1,200 rpm.

I immediately blamed the Petronix in the car. I replaced that with points. a new pink resistance wire and a new yellow top coil. No change.

Logic would say that the only thing left that it could be was the tach. I sent the tach out to someone who I trusted.


The results of testing it were unexpected.

Electronically it was fine. The issue was that it had accumulated magnetic dust on the "jewel movement" which was restricting the needles response.

The Tech cleaned it, apparently lubricated the jewel and retested and it was fine.

It appearantly can't even be around an atmosphere where there is any kind of an airbound metal. Even microscopically.

You learn something new every day.

This is the trusted "tach tech" Mine will not go over 2500 to 3000...I guess I need to send it to someone like that??

Sounds like the dreaded "needle schpeedle"?

[tonys_shelby]

Lfino, from what I've heard the only Petronius system that works properly with factory tach and wiring is Gen 1.  I'm not sure what system the distributor from the Petronius company uses. Probably a later generation. I have a Gen 1 and it works perfectly.

[67 GT350]

I have Petronix in my cars and have never had any issues, since 2016, 70 Boss, 2017 66 Mustang, 2017 67 GT K code, still working, but my 67 Shelby with points will not rev over 3000....the needle stops.
I can believe there is metallic dust within it, so I will entertain that idea, thus who is the tach tech with the open mind to have found that?

[Lfino]

Just can't understand why it would work perfectly for 4 or 5 starts, then quit after a jump start.   A little confusing to say the least.
Thanks for your efforts.

[shelbydoug]

I have Petronix in my cars and have never had any issues, since 2016, 70 Boss, 2017 66 Mustang, 2017 67 GT K code, still working, but my 67 Shelby with points will not rev over 3000....the needle stops.
I can believe there is metallic dust within it, so I will entertain that idea, thus who is the tach tech with the open mind to have found that?

Jon Haas at Pantera-electronics.com.

He's done four tachs for me. Two Shelbys and two Panteras.

You can email him over there. See how busy he is. He's considerably more then just a Tech but he's easy to work with and very reliable.

[Lfino]

Thanks, I'll check it out.

[Rocket]

Philodon did my tach for 550. Great turnaround and price. Ron

[shelbydoug]

It's not going to cost $550 to clean and calibrate a tach in good shape.

Not every tach needs a rebuild.

[Rocket]

My post should have said for 6S550. It did not cost $550. Ron