81 GMC tach behavior/not working

[Zeeebull]

Hello fellows, lots of nice trucks on this forum. I'm interested in what can be learned here. Maybe I can share a few things I've done as well.

I've got an 81 GMC short box.
I just bought a new LMC tach, new guage cluster housing, and new plastic circuit card for the cluster.

I ran fresh wires for tach;
A ground to the stock ground location above e-brake pedal.
A wire from tachometer terminal marked 12V to the HEI terminal marked 12V first attempt, then to the connector in the fuse block mentioned in the install instructions on the second attempt. Same reaction wired either way.
Ran the third wire from the tachometer terminal marked coil to the HEI terminal marked TACH.

So when I have it hooked up and turn on the truck it sits at about 1700 rpm. Throttle up a few hundred rpm and it only wavers a bit, doesn't show the actual rpm at any time.
Unplug the 12V wire or shut off engine and needle points straight down.

My volt guage works.
Fuel gauge is new as well, new fuel tanks/sending units.
The temp guage only rises a quarter inch when at operating temperature, I use an infra red thermometer to double check and the motor really doesn't run hot at all. Pretty cool actually. Can't imagine it's reading true, but maybe.
Oil pressure gauge is basically maxed all the time, my 454 isn't new, but it was spotless inside the valve covers and the oil was pristine when I installed it last fall. I really don't think it's reading true either.

I might have Teflon tape blocking the ground contact with sensors for both temp and oil pressure. I'm not worried about those so much at the moment, just painting the whole picture.

When I bought the truck nothing worked in the dash at all.

Anybody see this problem before with the tach?

 

[Bdog1910]

Mine basically does the exact same thing. I wish someone would answer. I just ordered a new circuit board hopefully that fixes it. I can let you know but I'm new to these oem tachs.

 

[nvrenuf]

The wires you ran, they’re just straight wire?

The factory harness has a resistor(?) in-line in the (I think) wire going to the tach terminal on the distributor.

 

[ChuckN]

I’ll see if I can find my LMC instructions. I remember there being one for general power for the gauge that I ran off the fuse panel. One goes to ground- PO used a bolt off the steering and that worked fine before, so I used that again. Then the wire to the HEI distributor that is a plug- in. Mine reads fine and has no issue.

 

[ChuckN]

Hello fellows, lots of nice trucks on this forum. I'm interested in what can be learned here. Maybe I can share a few things I've done as well.

I've got an 81 GMC short box.
I just bought a new LMC tach, new guage cluster housing, and new plastic circuit card for the cluster.

I ran fresh wires for tach;
A ground to the stock ground location above e-brake pedal.
A wire from tachometer terminal marked 12V to the HEI terminal marked 12V first attempt, then to the connector in the fuse block mentioned in the install instructions on the second attempt. Same reaction wired either way.
Ran the third wire from the tachometer terminal marked coil to the HEI terminal marked TACH.

So when I have it hooked up and turn on the truck it sits at about 1700 rpm. Throttle up a few hundred rpm and it only wavers a bit, doesn't show the actual rpm at any time.
Unplug the 12V wire or shut off engine and needle points straight down.

My volt guage works.
Fuel gauge is new as well, new fuel tanks/sending units.
The temp guage only rises a quarter inch when at operating temperature, I use an infra red thermometer to double check and the motor really doesn't run hot at all. Pretty cool actually. Can't imagine it's reading true, but maybe.
Oil pressure gauge is basically maxed all the time, my 454 isn't new, but it was spotless inside the valve covers and the oil was pristine when I installed it last fall. I really don't think it's reading true either.

I might have Teflon tape blocking the ground contact with sensors for both temp and oil pressure. I'm not worried about those so much at the moment, just painting the whole picture.

When I bought the truck nothing worked in the dash at all.

Anybody see this problem before with the tach?

I wonder, since you’re having (more or less) problems with all your gauges that it may be a ground issue? Perhaps try a different ground other than the emergency brake assembly and see if you have any changes.

 

[ChuckN]

One more note- I’ve had separate issues with ALL my gauges. My voltmeter went dead all of the sudden and came back without intervention. For one day.

My oil pressure reading is “notchy “
for lack of a better word. The sender is on my list to be replaced.

My temp gauge (after I figured out that the PO had some wires crossed in the engine compartment) has been…not exactly accurate. It’s a challenge to get the right sender from a local parts store with the correct ohms to get the gauge to read right- after all, it just reads resistance to ground, and if the calibration is off, guess what, your gauge reads off too.

IMHO, you have to do due diligence and really suss stuff out. I’ve learned the hard way that there isn’t any shortcut. YMMV.

 

[75gmck25]

If it’s wired like an OEM tach it has a three wire pigtail and they are completely separate from the other cluster wiring. Its just power, ground and tach signal from the HEI.

For the main cluster it’s really important to verify you have a good ground, usually a single black wire grounded to the emergency brake bracket, or to a ground block on the firewall above the driver’s feet.

OEM tachs usually had a filter to prevent HEI interference with the signal. You can still buy them new (https://www.ecklers.com/hei-ignition-tachometer-filter-66-74728-1.html) but can also build one using a few resistors and capacitors (https://www.elcaminocentral.com/threads/tach-filter-schematic.133330/). I do not have a filter and my tach seems to work fine.