W210 Instrument Cluster Repair Discoveries

[Tubularfab]

Ok, I just acquired a 1997 E300d that happened to have the problem where none of the gauges worked and one of them just wiggled. After some disassembly, experiments, etc, I made some good discoveries!

If you have a cluster that has the stepper motor problem where one motor is just twitching and the others do nothing there is some hope! I've read posts where either the fuel of temp gauge was the culprit - mine was the fuel. When one of the gauges fails to initialize Mercedes figured it was best to disable all the gauges - seems like it would be better to put a warning message up and let your other gauges continue to function, but oh well.

1st discovery - if you remove the offending stepper the others will start working again. This is usually an improvement! I discovered this by accident when I tried installing a stepper with an internally damaged wire.

2nd discovery - there was not actually anything wrong with my original stepper. It was in good condition internally, and both the coils had matching resistances. Info - steppers have at least two coils inside that alternately switch polarity to move the armature in steps. Use a multimeter to measure resistance in ohms across the diagonally opposite terminals. I found mine were around 140 ohms each. So, the problem was not electrical! I further discovered that if I plugged the cluster in with the cover off I could put a little resistance on the needle as it initialized and it would suddenly work. So, time to think a minute...

Turns out there is a "clockspring" inside the steppers that holds some tension on the needle toward the zero or empty position. It seemed logical to try to add some resistance to the spring, so I gave it an extra turn of preload and re-assembled the stepper. It worked! The cluster is fully functional again!

Before I get to the pictures I need to warn you - this stuff is tiny, delicate, and requires some patience!

After you pry the covers off the cluster remove the needles on the fuel and temp gauges. Slowly rotate the needles against the zero stop as you pull and they will come off. Then you can gently lift the surround/face for the gauge pair and flex it out of the way revealing the steppers:


You have to pry the metal tabs back that hold the gearboxes to the motors, then the white plastic gearboxes lift off:


There are two "layers" to the gearbox, you need to separate the halves:


Finally here is the offending gear and spring:


Just wind it one turn tighter. When you put it back together there is a tiny groove to put the "tang" of the spring into. You can see it a few pics above.

Put it all back together and bend the tabs back to hold. Rotate the needles toward the lowest part of the gauge as you push them on, making sure the motors are at the stops and the needles line up there as well.

Important note: If you choose to unsolder the motors do not pry them off the circuit board. The pins that solder in press through the plastic motor housing and have amazingly small wire connected to the other ends for the coils. If you pry you will break the wires off inside!

Good luck!

PS - sorry for image size - posting from an ipad I don't have resizing abilities!

[KarTek]

That's amazing, great job!