I have a dash mounted combo gauge with a voltmeter as one of the components. It tends to read .5-.8 lower than when I take a reading at the battery. With the lights and heater blower on I get 12.0 on the gauge, 12.5-12.8 on the battery. Is this common?

Jim

i think it is fairly normal to have a lower reading at the gauge; after all the gauge is not hooked to the battery.
I take my voltage from the coil.

The volt meter really drops when you hit the brakes....mine is hooked up straight to the battery through an off-on switch so I can check the voltage when I want.

Mine does the same thing. Daytime it reads just under 13v. Night time with heater and lights it runs just under 12. It jumps all over the place if I use my blinker.

The console guage will never be as accurate as your volt meter. The only way to know for sure is to disconnect the guage and use your volt meter across the wires normally connected to the guage.

You hear a lot of talk about cleaning grounds. What is being described above is symptomatic of a path that has contact resistance - whether it be on the ground side or on the hot side. Each connection, even the wires themselves, has a bit of resistance. Voltage drops over the circuit proportional to the resistance of that part of the circuit. Your goal is to have the resistance of the load (headlights, blowers ...) large compared to the resistance of the wires and contacts. When the contacts are bad you see that noticable difference between the battery voltage and voltage across the load. If your voltage drops significantly when you turn you lights on you should probably search out the source.

I guess I know what I'm doing next weekend...

I found all the above oddities as well. My solution was to install a relay;

Battery to relay 30, then relay 87 to voltmeter
Fuse panel ( switched side ) to relay 85, and 86 to ground

Mounted the relay under dash, brought the wire from battery through pass side firewall along door sill to relay ( fused !)