720 pickup gage wiring

Probably the instrument voltage regulator. On the back of the cluster (as I recall).

There is get a Haynes or Chilton manual or your local library should have a nice selection of manuals in their reference section

I've moved on to a 2000 fronty so I don't have my old manual anymore If the fuse is good, check the connection to the back of the instrument panel... the whole cluster is a bit of a challenge to remove due to the short wires, but check those connections

Jeff

That's exactly it. Some cars have the regulator built into one or the other of these gauges so if the one with the regulator in it fails, both stop working. A call to the dealer to find which one is more expencive ussually leads to the one with the regulator in it. Then again some cars do have seoerate regulators.

Oh no! I have an 84 Stanza with low readings on all gages because both regulators are faulty. They use heating elements and contacts, as I recall. They're a hassle to get to and I couldn't find a way to repair them.

Those regulators were bound to cause trouble. I wonder if their function is to provide a constant voltage under all circumstances. In that case, couldn't I use an IC regulator, a series resistor, or a two-resistor voltage divider instead?

With the truck, it would be nice if the problem were a fuse or a connector as Jeff suggested. Yesterday morning I asked the owner to stop at the library for a manual. Instead, he handed me his owner's manual.

Exactly. The alternators voltage varies with engine speed, enough to throw gauges off.

The later cars do use an IC regulator and I suppose you could adapt one if you know what the voltage is suposed to be.