[REDBIRD]

I have a hood tach on my 67 Firebird. I bought from Ames a few years ago.it has been disconnected for a while and tried to reconnect it and it does not move. There are three wires on the pigtail. I ran 1 to a good ground and the other to 12 v. Power. Light comes on but no movement. Is there a way to bench test the tach? Any ideas appreciated

[salem1912]

Got one going to the coil?

[REDBIRD]

Forgot to mention that. Yes one to distributor. Stock GM Hei

[Simple Man]

I have an aftermarket hood tach on the 68. The car sat for a lot of years, when I got the car going again, the tach worked sporadically. But the more I drove it, the problem seemed to work itself out.

[69Farburd]

I have read somewhere on this site that to use the stock hood tach with HEI some sort of modification is necessary. When I switched from points to Pertronics, my aftermarket hood tach quit working. Hopefully someone will chime in with a fix.

[REDBIRD]

Mine is aftermarket and worked with this dist. Before

[GT182]

Old Goat 67 will know the fix. Ask him. He loves working with electronics.

[goatmanmitch]

I have to rap on my PY hood tach sometimes and the needle jumps to life. Sometimes if I don't get out of the car to do that, it will start working after I get up to 3000 rpms or thereabouts.

[Tambo]

I asked this question a few years back and this is the answer I got ...

Firstly:
If the tach is the original GM unit and has only 3 wires to it - Brown, Black, Gray - then yes.
The stock circuit used from 67 - 72 on all GM hood tachs will only work off points ignition.

When you run any aftermarket electrical conversion you need a T.I. compatible tach board which uses 4 wires. One wire feeds the tach 12 volts of plus + battery power with the key on, one is attached to the negative side of your coil (RPM), one is ground (black) and the gray wire is used to light up the tach at night.

The old points tachs are not designed to de-code the RPM signature signal from Modern Pertronix and do not work well, or for very long, wired to same.

Secondly:
The tach's circuit board may have gone bad, those use surface printed resistors on a tiny Ivory colored disc shaped board. Those, starting in 1968. For GM the new disc boards were very inexpensive and quick to manufacture however, the down-side is that they tend to corrode from the flux and have a shorter life span than a traditional copper board.

The old way, they used to make those circuits; each electrical device was soldered in place on the board & the pathways were on one side made out of thick copper. The 68-and up disc boards were designed to be much smaller and lighter so they take up less space in behind the dash. The disc board was designed as non-serviceable, so you have to replace the entire disc when one goes bad. There are at least 2 versions of the disc, one for a car with a points ignition and one for the Delco transistorized ignition. If you get the right board you don't need an external adapter.

[69Farburd]

Thanks for the info Tambo. I think my aftermarket hood tach is a Dixco. It has a yellow wire going to the light circuit so the tach only lights up when the lights are on, red to ignition, black to ground, and brown to coil. Is there a place where you can still buy the circuit boards?

[Tambo]

I enquired at the time and nobody seemed to know anything about the boards, so I phoned Pertronix and spoke with a "technician" who told be to wire in a diode (can't remember type) into the brown wire. I did this but it didn't solve the problem.

[Stuart]

This may be a stupid question, but is the movement stuck? Can you shake the tach and make the needle move? I'm thinking it's possible that some debris or corrosion may have gotten into the works, jamming it up.

[REDBIRD]

If I tap my hood tach the needle moves. Mine is a 3 wire. Power,ground and coil