[AMX guy]

Just got it running after the restoration and all my gauges are pegged. any ideas on why ?

[Peter Serio]

Check your gauge housing(s) to battery ground.


A 66 A body Pontiac has no black wire (dedicated to ground) in it's dash harness. Yet the gauges all rely on a solid ground in order to function.


During the 1960s on most GM lower priced cars the factory used overlapping pieces of steel (behind the dash bezel) to achieve the gauge ground. It's fine when they put the car together, brand new. No so good after 59 years.

[AMX guy]

Thanks I'll check that out. my factory tach does work fine I'm not sure if that means anything

[Peter Serio]

That tachometer needs ground or it will not work. In-between the main pod and the #4 pod there is a piece of folded sheet metal in behind the plastic bezel which transfers the ground over from the main to the #4 "pod." That #4 pod has 2 small gauges in it. One for coolant temperature & one for oil pressure.


After that; behind the speedometer is a steel brace, it is often rusted as when the car was brand new that brace was raw steel. It is the main ground strap from the gauge cluster to the car's body.

Photo of mouse's eye view behind the dash at the ground brace:

This style brace is year unique, 1966 Pontiac A body only. Same brace though, standard gauges &/or rally gauges.

[Peter Serio]

Here is the steel piece that lays flat (mostly) to the back of the dash bezel. It nests in underneath both the cigar lighter and the ignition switch. It also communicates ground from the #4 (gauge) pod over to the main.


On a low option standard car with just a speedometer and a fuel gauge. If the car has a clock in the dash this bracket is the ground for the clock.


This part is the same for 1966 and 1967 Tempest, Lemans & GTO. (The factory did plate this part to help keep the gauge ground intact for the first 10 or 12 years in the life of the car.)

[AMX guy]

That's great information, thank you very much. hopefully I'll have time to check that in the next couple days.

[AMX guy]

Had some time today so I dug into this a little deeper. I ran a wire from the cluster housing to ground and it didn't seem to change anything.

My tach, battery warning light, dash lights and high beam indicator all work. now my temp gauge is doing nothing (but the engine is cold) and my oil and fuel gauge are still pegged out. not sure what's going on.

[DANTIP]

Without the engine running, if your ignition is turned on, what are the gauges doing?

[AMX guy]

The fuel and oil gauges are pegging. I thought the water temp was but it isn't now.

[Peter Serio]

Just FYI. A 50+ year old GM car gauge driven by electricity (key on power) all the way to one extreme or the other, for any time longer than, say a couple of minutes; will damage that gauge.

Those gauges (fuel, temperature & oil pressure) are 12 volt DC powered electromagnetic coils.

In my opinion they are high quality gauges and were designed by GM to last for the normal expected "life" of the car with no maintenance or adjustment required.

Which is somewhere between 10 and 15 years. And up to 100,000 miles of driving. After that they can last some years longer but they will not stay in calibration forever. If they are wired incorrectly and/or run missing their dash ground they will not work.

[AMX guy]

I know some vehicles have a voltage regulator for the gauges, is there one in this system?

[Peter Serio]

No, All of the GM car gauges from the 1960, 1970s and 1980s run off of a switched (key ignition) of 12 volts battery + to the back of the electro-magnet coil(s).

On year, 1968 and newer the key on Battery 12 volts + gauge cluster supply is protected by a 10 or 15 amp fuse in the fuse-block. 1964-1967 GTO gauges are not fused.

Only ford and Mopar car gauges used a step down relay and/or a resistor to limit the gauge voltage to 5 or 5 & 1/2 volts to each gauge.