[dv657172]

I have a correct Pontiac in-dash tach in my 71 GTO however since reinstalling the engine, when the tach is connected the engine will crank but not start. It is apparently grounding the coil. It worked prior to pulling the engine so I am not sure what has happened in the interim.

Any recommendations on who could repair the tach or any ideas on whats causing this issue?

Thanks
DV

[johnta1]

Is the tach wire hooked to correct post on coil?

[steve25]

I would ohm meter test out the wires feeding the Tack looking for a short to ground.

My guess is that you will find a short on the wire going to the coil since if a hot feed wire was shorted to ground you would be popping fuses

[dv657172]

Johnta1 - yes, negative side of coil

Steve25 - thanks, will check. I have a multimeter but have only used it so far for voltage checks. What should it read on the ohm setting if wire is good vs. bad?

[steve25]

Set the meter for ohms and touch the leads to together.
That’s a short and a digital meter will read 000 , on a analog meter the needle will go all the way over to the left side as long as the meter has been zeroed with adjustment of the set knob.

[johnta1]

The engine runs when the tach wire is not connected to coil?

If so, I'd check where the tach wire comes into the inside at the firewall. Maybe got pinched when changing engine? If OK, follow to the dash see if any obvious shows up.
Then if still nothing disconnect it from the actual tach and connect the other end to the coil. (or check for continuity like said)

Probably something minor, but if nothing found may have to take tach out and check it.
(Peter Serio may be able to help out?)

[dv657172]

Thanks guys! I will do some more testing and come back with my findings

DV

[dv657172]

Well I didnt have continuity on the tach wire. Despite running it the same way it was run previously via the 3 hole grommet throught the firewall, it looks like I pinched it as it shared a hole with the ac wire harness.

Is it ok to tape this spot or should I replace the entire harness? It will definitely be routed through the firewall differently once its fixed. Hopefully thats the fix.

Thanks for the continuity lesson and diagonsis suggestions guys, much appreciated.

[johnta1]

Tape will work.

[steve25]

While tape will work for future safe sake I would cut it strip it and crimp it back together with a red or blue butt connector, whichever fits the wire gauge best.

If you do not have a assortment of crimp on connectors and a good high leverage crimper now’s the time to get them.

When you work on a car I don’t know how one survives without these basic tools.

Please make the investment just for safety sake.

[dv657172]

Back to square 1...Fixed the wire however car will still not start with tach connected. Wire harness has been confirmed good via connecting tach end to my classic or should I say antique "Sears Engine Analyser" and getting an RPM reading.

Will reach out to Peter and see what services he offers

[70GS455]

I also repair tachs. In fact I just recently added this shift light to a customer's dash tach.

[ponjohn]

40 years ago I made a new circuit board for my in-dash tach in my 68 GTO. was easy to copy.

[Jack Gifford]

Originally Posted by steve25

,,, Set the meter for ohms and touch the leads to together...
... on a analog meter the needle will go all the way over to the left side...

to the right side.

[Elarson]

Originally Posted by dv657172

I have a correct Pontiac in-dash tach in my 71 GTO however since reinstalling the engine, when the tach is connected the engine will crank but not start. It is apparently grounding the coil. It worked prior to pulling the engine so I am not sure what has happened in the interim.

Any recommendations on who could repair the tach or any ideas on whats causing this issue?

Thanks
DV

If the engine ran with tach hooked up before, it's not the tach; it's something related to the engine removal.
You verified that the tach is hooked to the correct coil terminal.
Have you verified that the 2 ignition wires to the coil aren't reversed?

Eric

[dv657172]

Elarson - Wires are all connected correctly. Car runs fine without the tach connected and the wire is good. Connect the tach and the car will crank but not fire. Somehow the tach is grounding the coil. Wire confirmed to be fine. Tach sent to Peter Serio, looking forward to hearing what the issue is.

[dv657172]

The issue was the tach. The circuit board was confirmed to be bad by Peter Serio of Precision Pontiac. He did a great job replacing the circuit board and restoring the Tach. Its now back in the car (71 GTO) and everything works fine once again. Highly recommend his services!