[spiritwalker]

I need to try and diagnose a non working tach. I understand there will be a hot, ground, and coil negative side or hei tach lead.. Possibly a light lead.

Is there a signal or something coming out of my hei I can test for with my multimeter? Also is there a test I can do on my tach?

Thanks


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[CoolBlueGlow]

Search the forum. Many threads on here about this topic.

Yes, your HEI puts out a rather sloppy pulse halfway between a square wave and a sawtooth, from the terminal called "tach". It modulates between 12 and 0 volts at a frequency of four pulses per crankshaft revolution, or roughly 2.5kHz at idle.

Yes, your other wiring presumptions are correct.

Re: multimeter "test". Use a 'scope.

Keith

 

[spiritwalker]

Thanks for the info Keith.. I got in and took a look at the wiring last night. What a mess.. Tach has a brown, black/red, and white lead coming out of it.. It appears to be the original SS dash mounted tach. It plugs into what appears to be the factory original harness. The tach lead off the ignition has been tapped into the system somewhere.. It's going to take some tracing to figure out what is going on with the tach lead. what I see looks like a major mess.

out of the tach:
Black/red wire - I got 12v with key in run position
White wire - I couldn't figure out what it did (will have to do some searching)
Brown wire.. I'm assuming is the tach lead from coil.

 

[64tempestwagon]

Thanks for the info Keith.. I got in and took a look at the wiring last night. What a mess.. Tach has a brown, black/red, and white lead coming out of it.. It appears to be the original SS dash mounted tach. It plugs into what appears to be the factory original harness. The tach lead off the ignition has been tapped into the system somewhere.. It's going to take some tracing to figure out what is going on with the tach lead. what I see looks like a major mess.

out of the tach:
Black/red wire - I got 12v with key in run position
White wire - I couldn't figure out what it did (will have to do some searching)
Brown wire.. I'm assuming is the tach lead from coil.

Im thinking that "white" wire is really grey and it is the lights for driving at night (plugs into grey on the harness). Where is the ground?

 

[Rich-L79]

The tach is probably grounded by the case and where it bolts to the dash assembly. At least that's how it works on my 65 factory tach. OP should make sure the case is well grounded to the dash structure and that the dash structure is well grounded. Check with an ohm meter between the tach case and a known good body ground.

I will also add that it is highly likely the tach itself is just broken. The factory tach mechanisms do tend to wear out and stop working. The good news is that there are a number of places that can replace the works and get it working again.

 

[spiritwalker]

Thanks guys. I'll keep messin with it. The wiring looks way off and I'm going to need to spend some significant time figuring it out.

Anyone know why someone would splice a wire into the tach lead about 10 inches from the ignition? Gotta love figuring out someone/s else's crappy work.

I did see the tach needle move a tad when starting the car today.


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[Rich-L79]

Anyone know why someone would splice a wire into the tach lead about 10 inches from the ignition? Gotta love figuring out someone/s else's crappy work.

Maybe someone previously had an additional aftermarket tach in the car.

 

[1966_L78]

I did see the tach needle move a tad when starting the car today.

Before you send the tach out, trying adjusting it. On the back of the mechanism (you have to remove it from the chrome housing), there should be a hole with a screw inside (maybe covered by tape).

I had one of my '66 "kneeKnocker" tachs that didn't work (but the needle did jump just a little), but after adjusting that screw, it worked great. It worked fine with an HEI module too.