I have been having problems with the instrument cluster. All of the described problems occured at the same time.
The tachometer is not reading accuratly, the clock is very slow, the oil pressure guage does not register.

Sounds like a bad ground.

I'm having EXACTLY the same problems on my newly purchased 1988 260E. The "oil sender unit" was replaced today, but no improvement. Tach seems low, clock is slow, oil pressure always reads zero. Where is that "ground wire" I should check? Please post if you find and fix the problem. (All fuses have been replaced.)

Thanks a lot,

Mike