[Tom Wells]

Mike,

The back of the gauge is likely not easily accessible being behind the dash panel so the sending unit is where I'd begin.

The temp sender is toward the front of the intake manifold, if it's similar to mine - I have an Edelbrock Torker II - yours may be different.

Anyway, mine is just behind the thermostat housing, a little bit to the passenger side rear of the distributor.

Put a lead from the voltmeter there, ground the other one and watch it and the gauge while you increase the engine speed if the cables are long enough. Get a volunteer to watch one or the other if need be. I don't think the car needs to move, just the engine speed would need to be increased to say 2500 rpm.

If the voltmeter and the gauge are both steady from idle to 2500, then that would seem to rule out the sender and the gauge.

Caution: I haven't actually tried this - it does seem logical...

Tom

PS: The infrared temp measurement does sound like the way to go.