Bench testing gauges and sending units

[bobwires]

I've got a '78 with a 350 I'm troubleshooting. the fuel gauge, coolant temp, and oil pressure gauges all don't work. the gauge cluster has power and ground going to the individual gauges - at least the oil and coolant - and I'm trying to find out if the sending units are bad or the gauges.

what is the range of resistances for the senders? are these all negative resistances? in other words if I plug a negative on my meter to the pin of the oil pressure switch will it read 12 v with no pressure or 0 v, or somewhere in between?

at the temp sender switch I get 12v with a cold engine - so the switch is a near perfect ground with it cold.

At the gauges I read 6v with the meter black on the sending wire. on the oil pressure gauge with the engine off, when I fire it up the gauge pins to full pressure, which I know those gauges often do until it warms up.

any info is much appreciated. thanks in advance!

[bobwires]

will the gauge peg to 100% if I ground the sender wire?