What you need to do is isolate whether the problem is in the wiring, cluster, or the oil pressure itself. The easiest test to do is to disconnect the sender, and jump the sender wire to ground with a jumper wire. This should drive the oil pressure gauge to its normal pressure position. Wiggle the wire while someone watches the gauge. You can also drive the truck with the jumper in place, and see if the gauge still jumps around.
If the gauge still jumps around when the jumper wire is in place, then the problem is in the wiring or cluster. The next step is to carefully inspect circuit 31 for breaks causing an open circuit condition. Check every connector that the circuit passes through for problems. It could be a bad connection. You can run a new wire from the sender to the cluster in place of circuit 31, and see if the gauge starts working right.
Finally, if there is any question about the true oil pressure of the engine, it is recommended that a real pressure gauge be connected directly where the sender is screwed into the engine.
If the true pressure is normal, and overlaying the wire to the gauge does not fix the problem, double check the terminal on the cluster connector where circuit 31 connects. Verify the circuit is good by testing it with an ohmmeter or test light connected to circuit 31 at the cluster connector. If the circuit is good and the pressure is good, then the problem is in the cluster itself. You might be able to find a broken solder connection inside, otherwise just replace the cluster as an assembly.
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