Tachometer on 300TD 1985 fails to show RPM 's. How do I check it or sensor?
 

Friend gave me a replacement. It doesn't work either. I have checked fuse. I have a voltmeter and can check ohms. What does W indicated above top terminal of the three. Understand G and +. Need direction, help. :confused:

Tach
 

You need a rpm amplifier. Sits on the left inner fender. Black round trash can like deal. Turn counter clockwise and pull. FYI 75% of the cars out there around those years need one!! No one replaces it because it is an expensive part!

How much, if I can find it. Can possibly get a replacement free. :)

Amp
 

I will check the going prices and get back to you.

You Could try resoldering the internals...
 

Hi there,
I was able to repair the tach amplifier on my '82 by carefully peeling the potting compound off the board and resoldering all the connections, also had to resolder the pins where the wires go from the board. Mercedes used terrible solder and it seems to fail on almost everything that they soldered together. The cruise control amplifier board had the same problem, also the heater control board.
Regards,
Richard Wooldridge
'82 300D/4.3L V6 - No engine problems since installing the astrovan V6 two years ago, get 28 mpg on the freeway, 20+ around town, almost twice the power now!

Its been pointed out to me in the past the '85 stopped using that......but the 84 and earlier does have the little black trashcan thats upside down looking amp.

The 1984 and 1985 does NOT have the tach amp on the fender. The little round can on the 84-85 is a diagnostic connector. The 84-85 get the tach signal via the egr circuit. My 85 tach does not work either and I have not been able to get it fixed so far. :(

The 1985 W126 300SD & W123 300DT's have a EGR computer that processes the a/c voltage from the crank sensor INTO a d/c signal. To protect that computer the car has an overvoltage relay! That relay has a replacable 10A ATC style fuse. ON the W123 chassis that relay is located by the right outside A/C vent.

START with that relay OR fuse!

Tach system problem 300TD wagon 1985
 

Many thanks to you M.B. DOC.

It was the fuse in the over voltage relay of EGR computer.

rhoconnor