AC & Tach not working in '85 300TD
 

This problem started when the AC & Tach would shut off after 10-15 min. after a cold start. While searching for the source of the problem, I resoldered the PCBs in both the overload voltage relay (126 540 47 45; up under the dash to the right) and the Klima relay(001 545 74 05), cleaned with Scotchbrite and lubed with conductive grease all electrical pins and now the AC & Tach won't work even upon a cold startup. I obtained a new replacement overload voltage relay (201 540 08 45) and the problem is that the replacement is a 5-pin relay whereas the original is a 4-pin relay and the female socket for this relay is a 4-pin and won't accept the 5-pin relay so I can't even test this to see if the replacement will solve my AC & Tach problem(s). There's a fellow from the Classic Center trying to sort this out for me at the moment, but I thought I'd post this "in progress" problem.

When I had A/C issues on my SD, my Klima relay had a bad trace on the PCB.

The tach is driven off that EGR computer, in the W126 it is in the passenger kick panel area. W123 I have no idea.

I am sure you could just jump the OVP with a length of fused wire.