On my 1982 300CD the speedometer needle quite often does not move until I drive for several miles. The odometer and trip odometer are working fine at all times. I called Palo Alto speedometer and they suggested I loosen the cable on the back of the speedometer head. I did this and the speedometer started to work normally for a few miles and then the needle stopped working again. The odometer and trip odometer continue to work fine. I checked again and the knurled nut on the back of the speedo head had tighten up by itself. I loosened the nut about 2 full turns. The speedo started working normal again for about 5 miles, then stopped again. Once again the odometer and trip odometer work fine all the time. Original speedometer cable that has never been removed from the car.

Any suggestions? Thanks

I just went through this on the 1984 300DT. I ended up purchasing three (3) different used instrument panels to be able to finally acquire a speedo that worked properly. First, make sure the cable is functioning properly. Pull the instrument cluster and then place the car into drive and move the car slowly. If you can see the cable end turning, all is well. If not, you need to replace the cable.

Next, try a another used speedometer, it worked for me. Either that or you can send the entire head to Palo Alto and they will rebuild the unit. Then, it is good as new!

The speedometer is really three instruments in one case: speedometer, odometer and trip odometer. It's very possible that one can fail and the others keep working. Inside the speedometer, there is a bar with a magnet on either end, which gets spun by the cable. This magnetically drags a steel cup to which the needle is attached...the faster it spins, the more force is applied to the cup. What's happening is that the steel cup is binding, probably due to dry lubricant. It's possible to disassemble the speedometer, clean it and lubricate. If you don't feel comfortable with the drill, it should be cheap enough to have a speedometer shop do it.

I experienced similar symptoms....turned out it was the speedo cable. Replaced it and all is normal.

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1985 300D 172,110mi and goin' strong

I understand and appreciate all of your advice, but still have a couple of questions. It is clear to me that the cable is operational as the odometer and trip odometer both work fine consistently. Also when I loosen the cable from the back of the speedo it works for a few miles and then stops. Is this where the dry lube is causing the problem?

Thanks

Not exactly. the cable drives the magnet bar directly, and it also drives the odometers by a geared connection. The most likely fault is in the steel cup which is only coupled to the cable magnetically. It's also possible that the center bushing is worn, causing some interference with the steel cup. You're going to have to open it up and examine the mechanism.

A binding cable can cause the speedometer to stop momentarily, but then it should snap back to life when enough force is applied to overcome friction. the usual symptom being a bouncing needle. If the cable became bound to the point that it couldn't turn at all, the cable would snap and the odometers wouldn't work.

MxFrank - do you repair speedometers? If so, what do you charge for this type of repair? Thanks

I don't, sorry. There are some photos of the internals in this write up:


How to Repair Your Broken Odometer

It looks like the magnet bar in a 123 is actually a round magnet, but the idea is the same. You can clearly see the steel cup when he takes it's apart, as well as the geared takeoff for the odometers.