Rallye Dash Problems

[green440duster]

Hi my friend has a 71 Duster with a rallye dash, all of his gauges work except for his oil pressure gauge, the cluster seems fine otherwise and he said that a guy tried to send a signal (not sure if it was voltage or grounding) and the gauge responded. I told him that it might be the sending unit but he only has an aftermarket sending unit. I have never seen a factory rallye 340 oil pressure unit, are they different from a modern(78??) unit or should I get him to check something else.
Anyone shine some light on this problem???

Thanks

 

[340duster]

Have someone watch the gauge and touch the wire to ground (but do not hold it, just touch it) The gauge should respond. If not check all the wiring. If that is good the gauge is the problem.