Have you tried to tap on gauge cluster to see if the oil pressure gauge starts working? The gauge is electric as well as the sender. I had an 87 300D with an intermittent oil pressure gauge that would start working again after tapping on the cluster. There was a loose connection in the gauge itself because all the grounds behind the dash and connections going to the cluster were good.

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DJ


84 300D Turbodiesel 190K with 4 speed manual sold in 03/2012