RPM Gauge not working
 

'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.

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.

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:
http://www.peachparts.com/shopforum/diesel-discussion/256636-dead-tech-83-300d-2.html


Good luck -
James

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.

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

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