you might want to check if the pin on the balancer is still there. without the pin, your tach won't work. mine was broken by the oil pan bolt. someone strip the thread on the oil pan so they put a nut on top of it. i had to make a new pin out of a cut-off bolt.

http://pg.photos.yahoo.com/ph/mazzy_1234/detail?.dir=4519&.dnm=6695.jpg&.src=ph

Thanks, I will check on that, however, the gauge did work "occassionally" when I first got the car, sometimes it'd work great for 10 mins, then go sporatic, then die. It hasn't even jittered in years now though...

Tach Problems
 

Did you ever fix it? I have the same symptoms on my '84 300SD.

Jim

Its been snowing and freezing out lately, and I've been very busy, so I haven't had a chance to check for the pin, I'll look by wednesday though. If the pin is missing what is a suitable replacement? Does a bolt stuck in the same spot work acceptably? I am doubting the pin is missing since the tach used to be "jumpy" before it went completely dead. Who knows....

I also have an '82 300SD with a dead tach. The pin is in the pulley. Is it supposed to be at an angle to the pulley, or should it stick straight out?
I also pulled the 3 pin connector from the diagnostic plug. I could not measure ANY continuity among any combination of the three connections. Should there be some measureable resistance in that pickup coil?

the pin should stick straight out and have certain distance from the sensor. i think it might be magnetic or something.

It just has to be made of steel. There is a coil in the pickup that detects the moving pin, kind of like an electric guitar.

Ok, based on that I am pretty sure my pin is missing, I'll check for sure tomorrow....but in the past when working on the water pump/belts etc...I never remember seeing any "pin" nor do I notice anything going past when its running. Its supposed to be right in the middle of the balancer?

can someone supply a pic of what the RPM relay looks like
 

I'm having the same problem with my '82 300D, The tac is just dead.
Can someone supply a pic of what the RPM relay looks like to clairify where it is. This is something that I definitly would like to fix.

'82 MB 300D, Greasel:book:

here is what it looks like. its located on the driver side under the hood on top of the fender

http://img.eautopartscatalog.com/live/P401312022VDO.JPG

rpm location
 

i've seen this but i've been told it's some kind of diagnostic compute plug in for dealer service....is this true as well

Rolling on Greasel
1982 300D

Btt
 

Anybody fix their tach problems?
I just purchased an '82 300SD with a non-op tach. The tach amp body does not look like it comes apart on mine. I cant see how to get inside of it to put spacers in without breaking it.

Nope, as of yet no success, I am going to try crimping the ends of my amp to get the best connection possible, if that fails, I am going to put the amp in the street infront of my rear tire, and do a burnout on it, melting it to oblivian. :D Kidding. ;)

Paw, Any success yet?

I got mine working by cleaning the amplifier contacts of both the plug and the socket and then putting about a 1/4" of foam pipe insulation inside the cap behind the amplifier.

(By the way, I'm from Plainwell.)