'83 300sd recently purchased,
RPM Guage not working at all some times, sometimes just jumps around, sometimes just works. Also, my dash back lights are not working, checked the bulbs, they are fine. I read somewhere something about a similar case where there was a bad ground connection. I removed the instrument cluster and checked every connnection to the cluster. Is the one with just two prongs that seems to be behind the rpm gauge the wire for the rpm gauge? I am not sure of where to check if it is a bad ground. How would I determine this?
Thanks.

Usually, the tach failure is due to a dying amp located on the inner driver side fender. A tall round upside down trash can looking thing. Unscrew it and replace it with a unit from Techguy 512.

The dash lights are usually a failing rheostat, or dimmer switch, spin it a few times, they might stay on. Or solder a jumper between the two posts, visible on the back of the dash cluster.

__________________
83 SD

84 CD

Is there a way that I can test the amp for the tach?

You may get some more life out of it by removing the amp itself from the cap and wedge a shim in so when you screw it back on, there is better contact.

__________________
83 SD

84 CD

I recently had the very same problem you describe on my '85 300D - in my case it was the OVP relay. Here is what I would try first (hope your SD is the same as mine):

Look on the passenger side at the foot well on the right side panel there is a plastic access door - remove and look in there. I had to separate the plastic side panel to get underneath. You're looking for a metal relay with a red top on it. When you see the rad top - you found it.... Try to gently pull it out of the socket (remember which way it went in there), then clean all the prongs well with 600grit sand paper (or similar) and some contact or brake cleaner. Pull out the fuse on top and clean the contacts too - and measure continuity with Ohm meter. Stuff it all back in there and start her up... I'll bet it will work. If not, you'll need a new Overload Protection (OVP) relay.

See Post #22 for the picture of the OVP relay:
Dead Tech 83 300d


Good luck -
James

__________________
1987 Mercedes 300SDL; SOLD
1985 Mercedes 300D; SOLD
2006 Honda Pilot - wife's ride; 122K;
1995 Toyota Land Cruiser - 3X locked; 182K

MB changed the rpm indicator set up in 1984.
No OVP in an 83.
84's and 85's don't have an amp, just a cover for the socket.

__________________
83 SD

84 CD

Dang it... it has the same symptoms... but apparently a slightly different setup....

__________________
1987 Mercedes 300SDL; SOLD
1985 Mercedes 300D; SOLD
2006 Honda Pilot - wife's ride; 122K;
1995 Toyota Land Cruiser - 3X locked; 182K

^ You do have an OVP, check the fuse on top.

Attached Thumbnails

 

__________________
83 SD

84 CD