Tachometer: Stopped working, still runs fine

So over the past few weekends my tachometer has decided to stop intermittently, then magically start working again. This happens seemingly independent of going over bumps or heavy cornering.

I have two green wires that connect to the back of the tach, the two white wires that connect to the inductive pickup leads, black ground wire and the red/white wire for the bulb.

So the bulb works fine, the ground wire is a sole ground going to the body. If the car is running and I pull one of the white wires the engine dies - so is the inductive pickup working?

The green wires should supply 12 volts so I can check that with a DVOM to ground?

Also one of the white wires should have continuity back to the coil? The other one to the ignition switch?

Thanks again for the help,

Brett

Brett, most of the ground wires that go to the body are located under the dash in the area of the windshield wiper motor. Based on what you have said in your post it sounds like a wire is broken in the sheath at the connection to the tach. I would check this out first.

Fixed it...The problem was one of two things: the voltage stabilizer circuit was not grounded properly - the circuit is not a stock one but one of the Ray Lemke ones (for future searches).

Also the ground loop that goes from thumbnut on the Fuel gauge to thumbnut on the tach was also loose. Connected both of those grounds and magic! Tach is working.

Tom thanks again for the input!

-Brett

The problem is back. Tach worked for two minutes this morning then quit.

Can anyone add some insight on how to check the tach itself to see if it is working fine and also verify what voltage I should see at the green wires that go in the back of the tach?

Am I correct in assuming the ground loop connects to the thumbnut? I also have a ground that goes from the back tab on the tach to the firewall...

Thanks

Brett

Brett,
I've seen the grounding to the tab and the knurled nut on different cars. Shouldn't make a difference as long as one or the other is grounded. I don't have a schematic with me but I'm fairly sure the tach's circuit power (green) doesn't involve the voltage stabilizer.

Be VERY CAREFUL with the white wires these can (and will) fry your wiring as they are not fused.

It is "possible" that you have a second source of power to your coil. This can cause strange tach problems like killing the tach when you turn the lights on, etc...

We had several post over the last six months concerning repairs to tachs, you might try a search.

Thanks Clay,

I do have a Pertronix ignition...not sure what other power sources could be providing power to my coil...the connections are rather simple...Do you have any insight to that?

I'm beginning to think my tach has given up the ghost, but I sure would like to know that the wiring is not the suspect.

I have searched with about 10 different combinations and have read everything I can...for future searches here is a great resource:

http://www.classictiger.com/techtips/motach.html