Topic: 1978 Tach Reads High

[Dusk Blue Z]

My wife has a 78 TA that the tach reads about twice engine speed. It does the jump up thing when the engine is shut off which I read here was normal. It is consistently reading fast from idle on up to over red line on the shifts. Looking for any ideas of places to look for the cause. Thanks.

[ta78w72]

Either you have the wrong tach for the car or the tach is bad.  In either case, you need to replace the tach.

[Wallington]

Does it return to 0 or stick higher when off?

[tata]

I had a problem like that it was a bad ground to the firewall, hopefully it's that simple for you

[ta78w72]

The tach grounds through the cluster housing.

[Dusk Blue Z]

Thanks for the reply's. The tach jumps up to @5K with the key off. I will try and run a separate ground wire off the housing. I thought the Pink and White wires covered that.

Going to the Nationals this weekend and looking forward to meeting some new friends.
Mike

[ta78w72]

The connector has the power and signal wires.  Pink is power and the brown wire is the signal.  The Tach is set up to calculate RPM's.  In a V8, every eight pulses equal one revolution.  In a six, every six equals one revolution.  You can check the signal by connecting a light bulb to the brown wire and ground, the light should grow dimmer as the RPM's increase.  Personally, I think your tach is bad.  78 had two tach designs.  The first design had a round cap on the end and the second design had a more rectangular end cap.  The lens, and circuits are different for the first and second designs and are not interchangeable.  The second design incorporated a quartz clock.

[tata]

Yes looking forward to meeting everyone too. Please stop by other booth/inclosed trailer with lots of goodies inside.

[Dusk Blue Z]

Thanks for all the help. I ran a ground wire from the panel to a screw on the firewall and all seems to be good now. This car now has the second design circuit board with the quartz clock (which doesn't work). It came to us with the first design circuit board hacked, trimmed, and wire jumpers running everywhere???

Mike

[ta78w72]

It sounds like the ground circuit for the TACH is broken.  You could run a jumper from a ground from another spot on the cluster to the ground for the TACH.  Or, replace the circuit.  You could test the circuit with an ohm meter.  You may be able to get the clock going by oiling it.