Instrument issues
 

Hi Guys...I've got a weird problem. My oil pressure gauge only works very intermittenly and my tachometer only seems to work with the AC on. Whats going on here? I'm not too concerned about the tach, but the oil pressure gauge is kinda important. I'd still like to get both fixed. How do I troubleshoot whether it's the sender unit, a bad ground or the instrument gauge itself. I have an instrument cluster that supposedley works and I will try installing this weekend, but there's no guarantee.

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.