W123 Instrument cluster grounding
W123 instrument clusters are notorious for jumpy temperature and fuel gauges and mine was no different. This problem seems to impact other Benzes of the era, but is particularly pronounced in the W123.
I don’t really like not being able to trust my gauges to be generally correct, so I wanted to see what I could do about this. The issue seems to be mostly due to grounding. The whole cluster gets ground from one plug, which then provides ground to each of the instruments, and lights.
A friend of mine with a W123 had fixed a cracked solder joint in his cluster that had made a big difference. I’m terrible at soldering, so he offered to take a look at my cluster and see if the same solder joint needed repair.
Last week I removed my instrument cluster. On removal, it had obviously been out before. Somebody had written an odometer reading on the back, which I assume was done when the odometer was previously repaired. The cluster was generally in very good shape, except somebody had tried to pull it out from the front before and slightly damaged the front cover, so I had to be careful.
I find the easiest way to remove these clusters is to remove the drivers side speaker and push it out from behind. The W123 instrument cluster is harder than the W126 version, because the oil pressure sender is mechanical. For reasons only known to them, Mercedes-Benz give you almost no slack to play with on the oil pressure line and speedo cable. Even an extra two to three centimeters would make a big difference.
A 10mm spanner is needed for the oil pressure line, but the rest of the connections can be done by hand.
My friend looked at my cluster, and didn’t see the same solder joint broken. There was some oil inside the cluster, and some of the screws were a bit loose. Since the oil pressure gauge does not seem to be leaking now, I wonder if it leaked in the past? Regardless he gave it a clean and also made sure it was screwed in properly. From what I understand, the screws are important for the grounds.
I also decided to add another ground line. From reading on the forums, it seems a lot of other W123 owners do this. I figured it couldn’t hurt. I grabbed a couple of ground lines with eye terminal from the 420SEL parts car. One terminal went on the other side of the ground bolt behind the W123 instrument cluster, held it on with a washer and nut, also from the 420SEL. I put the other ring terminal behind one of the screws on the instrument cluster. The only non Mercedes part was a connector between the two ground lines.
I put it all back together and took the car for a test drive. Getting the oil pressure line on is quite fiddly. Eventually I got the cluster back in place and tested all the connections.
Today is very hot, mid 30s, so I was curious to see how the temperature gauge performed. The fuel tank is full, so I will not be able to test that until I have used some fuel. At least on the 20 minute test drive I did, the gauge seemed to perform very well. It crept up to about 100 at one point in stop and go traffic, and then when I was on the move again, slowly came down. It never used to perform so predictably. I will keep an eye on it, but so far it seems like a big improvement.
