230Sl Odometer Repair

[al_lieffring]

A forum member sent me a KMH speedometer out of his 230sl for repair. His discription over the phone was that the odometer no longer worked. I assumed that the pot metal wheel on the the digit wheel shaft had worked loose, as they so often do.

The package arrived and the inside of the box  was coated with a black powder that was also covering the back of the glass, all over the dial  and the odometer digit wheels.  I removed the rubber boot off of the back, the powder was also coating the inside of the boot. My first thought was this was rust (it was magnetic) that had worked its way inside the speedo from a rusty speedo cable.


A complete disassembly revealed the real problem, the input shaft had worn through the thrust plate that holds the rotating magnet and the mannet was grinding against the plate and producing the rust powder. but it also had worn the plastic cog  that droves off the imput shaft. The plastic had worn to where the odometer gears no longer were turning.

I have a speedo out of a 220sec that I hoped could be used to replace the worn pieces, they were similar but were not interchangable.
I was able to use the thrust plate, but had to find a wasy to make the existing drive gears work. Using the "well its aready broke" principle I took a knife blade and worked it between the plastic gear and the shaft working it carefully toward the end of the metal shaft, I managed to get it to move about 5mm to where a section of the plastic gear was not worn and would make contact with the worm gear on the input shaft.

 

[al_lieffring]

After the gear was relocated on the shaft i chucked it up in my jeweler's lathe and dressed down the end of the plastic that was now overhanging the bearing surface.

I used my ultrasonic cleaning tank to clean the rusty grime off of the unpainted metal parts with clock cleaning soloution, the plastic parts also went into the tank but with soapy water. The Dial and painted parts I had to clean by hand. When I reassembled the speedo, even though the drive wheel on the digits shaft was not loose I staked the shaft so it wouldnt come loose later.

This thrust plate design is only in the 230SL speedos. the 250 and 280SL's have a bronze bushing in the casting the magnet runs against with a thrust screw to hold it in place. The magnet housing shown in the photo is out of a 114 body car but is similar to what you would find in a 250/280sl.