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I invent stupid usernames
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Discussion Starter · #1 ·
The tachometer I put on my 1978 GMC truck is acting up. When the truck is idling, the tach will be pegged out, then when I rev the truck up, it will drop down to zero RPM on the tach, then at highway speed (around 60 MPH), it will say somewhere around double the RPM it should be running.

Tach is Equus model 6086 and it's not a problem with the tach as it's the second one that's done this. Switch is set to 8 cylinders, and as far as I can tell all of the wires are connected properly. It could be that the ground wire is connected to the valve cover, but I doubt it.
Engine is a small block 350 with HEI ignition.

Any thoughts or comments?
 

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Super Moderator
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Low quality tach, unknown ground wiring.

Start with the ground.
 

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The ground wire needs to be located on a clean ground and not near any of the plug wires or coil wires (if your HEI is a coil in cap HEI, then this won't be an issue.) The same consideration needs to be given to the tach signal wire.
 

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Old(s) Fart
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Troubleshooting 101:

Temporarily connect the tach to a different engine and see if it works properly or not. This immediately tells you if the problem is the tach (or associated installation) or the vehicle it's installed on.
 

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I invent stupid usernames
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Discussion Starter · #6 ·
Are you sure the wiring is correct?
Sounds like you have the signal and ground wires reversed.
Does the tach light work normally?
Yes the light works perfectly fine. The funny thing is, the tach will actually sometimes work in a certain rpm range.
 

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I invent stupid usernames
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Discussion Starter · #7 ·
The ground wire needs to be located on a clean ground and not near any of the plug wires or coil wires (if your HEI is a coil in cap HEI, then this won't be an issue.) The same consideration needs to be given to the tach signal wire.
I will check this out. I had it grounded inside the cab, and to the valve cover, with no difference.
 

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I invent stupid usernames
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Discussion Starter · #10 ·
Is this an after market tach. Do you have it in v8 mode and not 4 or 6 cylinder mode. Most can support all three and will act up if not in v8 setup.
Yep it's aftermarket, and yes it's set to 8 cylinders as I said before. This is really baffling, but it's too cold to work on it now. You're right that it can support all three, I made very sure of that
 
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