My tach isn’t working and Ive been told to check it for 12 volts. There are three wires connected to the rear of the tach, black, pink, and brown. The wiring diagram shows the black wire to be ground. The pink wire is a constant 12-volt supply and the brown wire goes to a filter on the manifold and from the filter to the "tach" location on the distributor.

With the ignition turned to on and the engine not running I checked the wires. The black is a good ground. The pink is 12 volts and the brown is also 12 volts. Should there be 12 volts at the brown wire? I can't find anything in the wiring diagram showing the voltage of the brown wire and/or when it should have that voltage.

Can anyone help me decipher this problem?
Thanks in advance.

 

Greetings,

The brown wire is the signal coming in from the distributor. (thru the tach filter) It's actually a pulsed signal when the engine is running. You may be able to get some relative voltage reading if you use the AC scale of your voltmeter. However this in itself won't tell you much except if there is nothing there at all.

Dead C3 electronic tachs are often caused by a bad tach board. However to eliminate everything else first I'd suggest the following:

1) Check if the tach filter is OK. To do this simply bypass it and see if the tach starts working. It's located at the drivers side rear of the intake manifold and looks like a typical metal tubular filter cap. (it's not) A wire will be coming from the distributor to it and another going thru a connector, thru the firewall and is attached to the tach input. Unplug the connector after the tach filter and plug the line coming from the tach directly into the distributor. If the tach starts to work, then you need a filter. BTW, don't just leave it bypassed long term as it is there to offer the tach board some protection from electrical spikes.

2) If step 1 still yields a dead tach, then the best way to test the rest is to hook up an aftermarket tach directly to the brown wire where it attaches to the stock tach. This will tell if your distributor signal, tach filter and wiring is OK. Assuming the aftermarket tach works, then you most likely have a bad tach board. If not then the issue may be distributor module or wiring related.

Assuming a dead tach board, your options at that point are getting a repro replacement board or the possibility of having the board repaired for about half the price also exists.

If you need more info on tach filters or board repairs, please feel free to PM or e-mail me.

Good Luck!

 

Ground comes from the HEI module when the engine is turning over.
This creates the pulsing signal to the tach, and also to the primary coil of the ignition coil.
If the engine isn't running, the the HEI doesn't ground it, and you should see 12 volts.

ACECO's troubleshooting tips should help you determine if the filter is bad.
(If you have one)

 

Thanks for the assistance Aceco and Kilroy. With your help I should be able to solve this problem.

 

Good Luck! Let us know how you make out.

 

Hello:
Is there any issues with the tach needle movement itself? The repaired board on the backside of tack is an expensive guess to why the tach board failed in the first place. I looked under my hood and saw previous person had bypassed the filter you mentioned. It is just an axial mounted capacitor isn't it, sized of course to pass the required frequence for accurate tach readings and obviously tgo block DC from coming into the AC signal path inside the tach. Gadd Zooks you would think someone out there in the corvette world would have decoded a 30 year old simple chip far enough to offer a cheap circuit board fix by now! Ive seen a few of the old boards with a fried diode and every thing else seemed OK.

Ya I got one of them fried boards in my 75 and don't wanna drop money into the new board untill I fix the problem not treat the symptom. I'd lose the stock tach in a heart beat for an accurate unit but the damn right turn indicator looks so neat in the face plate