Odd cluster grounding behaviour on '85 300d (related to seat belt sensor??)
 

On my '85 300d, I have the jumping needles/flashing seatbelt warning light syndrome that usually indicates that an additional ground is needed for the instrument cluster. This started after I had the cluster out to replace the ignition lock and switch assemblies, so I figured it was par for the course. I have just been waiting for an assist from a friend who has small hands more suited to messing around behind the cluster.

I just realized last night that in fact the behavior seems to be related to the driver's seat. The gauges jump during normal driving when I am sitting back in the seat. If I lean forward, taking some weight off the seat, the gauges go back to normal.

I have looked under the seat, where I know there is a two-pin connector, which I assume is for the seat belt receptor to sense when the belt is buckled? The connector is intact, and to my knowledge the seat has never been out of the car.

Any advice, apart from sitting on the front edge of the seat?

Take the cluster out and inspect all solder joint with a magnifier glass, touch up the ones that appear to be not sound with a soldering iron. There are various rivets to the gauges which are grounds. Wire brush them clean then add some solder paste and solder them to insure they will make good electrical connections. As I recall, there is a ground between the 3 sections which must have a screw tightened properly for the ground to pass through.

Take all the bulbs out, clean and re-seat them. Clean the circular connector teminals too.

There are also one or two grounding points behind the cluster that can be cleaned and tightened. I think pretty much everything in the dash area grounds through these two bolts.

I needed to add a separate ground wire to the instrument cluster on my 1985 300D. When you pull out the instrument cluster (remove it from the dash entirely) you will see where the majority of the brown (ground) wires are grounded to the body. Attach an additional ground wire from here to the ground attachment point (bare metal) on the back of the cluster. Your problem should be fixed by adding this extra ground wire.

There are no rivets in the gauge clusters that provide grounds. The gauges are held in place by screws that go into the plastic gauge cluster.

I really do not see how your seat position could cause the gauge needles to jump.

It was a while ago I worked on one and I was going from memory. Just looked at my parts cluster. The ground for the fuel and temp gauges are via 2 screws and nuts. Take the nut off and wire brush the connection and put the nuts back on.

The 2 lower speedo mounting screws passes ground from the circular connector to the tach.

Funola - I am not sure we are talking about the same instrument cluster. I have taken apart probably 50 W123 clusters and have never seen a grounding nut.

W123 diesel cluster. Point wrapped by the wire are all connected by PC traces and is a ground. The 2 studs and nuts are grounds for the fuel and temp gauge.

http://i31.photobucket.com/albums/c3...614_205649.jpg

I stand corrected. My apologies.