Hey all,
I have a 1981 300SD w126.
I recently puchased a speedometer from a guy (the 0-125MPH) unit to go in place of my 0-85MPH one. Here is my dilemma:

I plug the other speedo in (everything hooks up) and my gauges/headlights/seat belt stuff goes CRAZY! If I hit the high beams they stay on and the seat belt warning buzzer goes off and when I start the car the high beams flick on. Also if I touch the brakes it goes into headlight/buzz mode.

Here is what I think is happening:
I think there is a grounding issue with this new gauge. I was looking at the circuit board and a trace was burned off of the unit (IE power on the old gauge might have been ground on the other one ) where the ground from the connector on the back hooks to the chassis ground at two of the screws.

I am going to take my 85MPH speedo apart and see about the wiring and if I need to move some pins around to get things to work. This is just acting like a short/grounding issue so it shouldn't be too bad.

BTW when I put my 0-85 speedo back in last night everything works perfect again...so it hasn't damaged anything in the car....yet heh.

Thanks for any help you guys can offer. I figure this would be a good thread topic for those attemping to upgrade their speedometers in the w126.

As it turns out, its a direct swap.
The unit I got had a broken trace on the circuit board so the cluster wasn't getting ground. This wreaks havoc...
The speedo works good and is off by about as much as my last one was
Now its time to get proper sized tires.

Brandon