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.

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1983 300SD White with Grey interior