Pull the wire off the sender and put it to ground. Wire not connected should make the gauge(key on) go all the way up or all the way down. Wire grounded should make it go the oppisite way. If that works gauge and wire are ok and sender is bad. If that dont work you have to go to the back of the gage and do the same test, be sure you remove the wire that goes to the sender and not the one that goes to the constant voltage source. I read on another post that things get messy if you ground the wrong wire. Temp, Oil, and Fuel all have one wire that goes to the same terminal on the cluster and the other wire goes to the sending units. I think both wires on the amp gauge go somewhere else. If all gauges are strange look at the constant regulator thing and grounds. Temp and Oil share the same ground (the engine block) so if the oil is working chances are that the ground and constant source are fine. This would be true for most single wire sender gages. 2 or more wire usually dont use grounds.
If that dont help post details on the car or brand of gauges and maybe someone will have the info.
Good Luck!
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