I've been reading through several forums and pages looking at issues with tachs in the W126 cars. I'm at a loss on mine as I don't see to have any of the components in areas others describe. I have a 1985 300 SD with CA emissions. I do have ABS and I also have the Klima relay. Not sure if those make a difference but they've popped up on other pages so I thought it worth mentioning.
- My tach does not register at all.
- I do have a functioning blower motor for my HVAC however, no AC. I don't know if the AC is charged though so I can't say if it is the tach causing it not to work. Before I go looking into the AC system I want to fix the tach given the ACs dependence on the RPM signal.
- The EGR computer behind the passenger kick panel does not have any corrosion.
- I do have the Klima relay in the car.
- I do not have the tach amp in the circle thing, if I pull the top it is just empty pins.
I've seen pages mentioning the OVP relay. I am trying to test this but I can't seem to find it in the "usual" places. I've read it's behind the passenger air vent and accessible through either the glove box or removing the passenger knee board. I don't have anything there other than some klimateregel computer.
Others have stated it's behind the battery between the firewall and the "fake" firewall. Nothing there but the coolant tank.
Others have pointed to it hiding attached to the fake firewall to the inboard side of the brake booster. Nothing there either.
I do have an OVP relay in my fuse panel. I don't know if these are the same OVPs though, if someone could confirm. If it is the correct relay then the fuse is OK.
Some additional info on the AC, the auto speed blower does seem to function as it will increase/decrease blower speed according to the temperature differential.