Electrical Tachometer Help

Vic1978 · Apr 21, 2018

I have a 1978 Cobra, 302, auto. I bought the car in rough condition and have been working to get it back on the road. I rebuilt the 302 and I’m having trouble with a few things, one being the tachometer that doesn’t work.

The cluster printed circuit board has the part number that starts with D4ZF. However the tachometer adapter has the part number D7ZF. The tachometer itself has the part number D5ZF.

I bought another complete cluster that has a printed board of D5ZF with a tachometer with part number D7ZF and the adapter / D7ZF.

It still doesn’t work. Does it matter that the printed board is a D5ZF or D4ZF?

Any ideas as to what is wrong? All the wiring looks stock.

The fuse box is so worn? I can’t read what some are supposed to be for. How many fuses deal with the tachometer?

Thank you.

Vic1978 · Apr 21, 2018

jozsefsz · Apr 21, 2018

Try skipping the adapter - you'll show double the revs but should eliminate an issue with the adapter.

There are 3 wires of consequence for the tach, + ignition-switched voltage, - ground, and the - signal-wire from the coil.
+/- should be easy enough to verify with a test-light or multimeter. Not sure if even the positive is fused - it definitely doesn't have its own fuse, possibly accessory or dash if anything. I'd need to study a wiring diagram to figure that out..

It's unfortunately not uncommon for the wiring to the tach to be chopped up in favor of a big old steering-column mounted tach. After 40 years anything could have happened. If you have + and - at the harness you're good there. The - signal-wire from the coil is definitely not fused. For that one, try running a jumper wire from the - side of the coil to your tach and see if it comes to life. That's the most likely wire to have been chopped.

Vic1978 · Apr 21, 2018

jozsefsz said:
Try skipping the adapter - you'll show double the revs but should eliminate an issue with the adapter.

There are 3 wires of consequence for the tach, + ignition-switched voltage, - ground, and the - signal-wire from the coil.
+/- should be easy enough to verify with a test-light or multimeter. Not sure if even the positive is fused - it definitely doesn't have its own fuse, possibly accessory or dash if anything. I'd need to study a wiring diagram to figure that out..

It's unfortunately not uncommon for the wiring to the tach to be chopped up in favor of a big old steering-column mounted tach. After 40 years anything could have happened. If you have + and - at the harness you're good there. The - signal-wire from the coil is definitely not fused. For that one, try running a jumper wire from the - side of the coil to your tach and see if it comes to life. That's the most likely wire to have been chopped.

Thanks for the information Jozse. If all else fails, I’ll try that.

From what I read, I know that you have to have the same DXFZ part number for both the tach and the adapter for it to work right. What I don’t know is if you have to have the same part number for the printed circuit board for it to all work.

Thank you.

COBRA 7 · May 11, 2018

Vic1978. Nice looking dash units. Better shape than mine. Hence, completely rewiring my dash.
The tach does not run through the printed circuit board. It has it's own 4 - pin connector. (PIN C on tach Gray / Black tracer is ground.) (PIN 6 on tach Red / Lt Green tracer to alternator is 12v power.) ( PIN B on tach Dark Green / Yellow tracer is tach signal from coil. ) THESE ARE THE COLORS FROM THE PDF FILE OF THE 78MII WIRING DIAGRAM.

THIS IS HOW MY CAR IS WIRED TO MY TACH. Direct to my tach. No adapter between connector and tach. PIN C AND G TAGED TOGETHER TO BLACK / WHITE TRACER WIRE. PIN 6 RED / GREEN TRACER WIRE. PIN B GREEN / YELLOW TRACER.

Vic1978 · May 28, 2018

COBRA 7 said:
Vic1978. Nice looking dash units. Better shape than mine. Hence, completely rewiring my dash.
The tach does not run through the printed circuit board. It has it's own 4 - pin connector. (PIN C on tach Gray / Black tracer is ground.) (PIN 6 on tach Red / Lt Green tracer to alternator is 12v power.) ( PIN B on tach Dark Green / Yellow tracer is tach signal from coil. ) THESE ARE THE COLORS FROM THE PDF FILE OF THE 78MII WIRING DIAGRAM.

THIS IS HOW MY CAR IS WIRED TO MY TACH. Direct to my tach. No adapter between connector and tach. PIN C AND G TAGED TOGETHER TO BLACK / WHITE TRACER WIRE. PIN 6 RED / GREEN TRACER WIRE. PIN B GREEN / YELLOW TRACER.

I tried to wire it direct - connecting the wires like you described and nothing. However, I connected the Gray black tracer, green with yellow tracer. That made the tach jump and work but as I revved, it lowered the rpms.

Do the wires on the connector look to be the right colors in the right places on the connector?

Electrical Tachometer Help

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