Just bought this 83 300sd 5 cylinder diesel. The speedometer, tach, and cruise dont work, and the oil pressure bounces between 2 and 3 at idle. I think they may all be connected and have looked at wireing diagrams. I just dont really know where to start? Anyone here ever had this type of problem or know where to start testing?

start with checking grounds behind the dash. the cruise often is non funtional due to the amp. the tach also has its own little amp that goes bad. both the cruise and tach can usually be fixed with a simple resolder.

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Did some searching and found the cigarette butt idea for tach amp, it worked!
Now I just need help with speedo and cruise. I figure no need in worring about cruise till speedo is working, probably related problem. Not much info on the electronic speedos, read most eveything in the diy stuff ( Whunter ) did in the diy. Anybody else got a starting point in tracking down the problem?

Also what does the amps on these cars actually do?

"Usually" when the speedo fails, the problem is in the gauge. If you can get another gauge for the vehicle, then you can easily determine your problem.

The gauge gets a signal from one wire........it's the white wire in the six pin connector behind the speedo......but I don't know what the expected voltage and frequency should be.