Help with instrument cluster swap please

[mjohansson]

Hello again all, while I have a post of two on this forum and I do appreciate the help thank you, I have yet another question. I purchased the instrument cluster from the '79 300sd that was in the local yard for $40 and it is complete. So I was going to turn back the odometer on it from 241K to the 200k like mine has, will that be very difficult? Also I noted the '79 speedo goes up to 120 mph while my '80 only goes to 85 mph will that be a problem with a gearing then? or should it still work?
Thanks, and God bless.

[polymathman]

If the old speedometer works, just swap it out of the cluster and all problems solved.
Setting the mileage back is do-able, but by design, very difficult, and probably illegal.

[rumb]

old school way was to hook a drill up to the speedo and run it backwards for a few days.

A speedo shop will do it for you, they mark on back original miles I think. Of course you have to pay them for the service. 

Or if you are really inclined to work with little fiddly parts, take it apart yourself and reset.


When I had my speedo fixed, the set the odo forward @40K km since I knew it had not worked for over 20 years.  I had them set it to equivilant of 100K miles, which may actually be less than the car has, but it was a nice number I could use to set it to my restoration.

[orientrot]

Also I noted the '79 speedo goes up to 120 mph while my '80 only goes to 85 mph will that be a problem with a gearing then? or should it still work?
Thanks, and God bless.

I've done this same swap but haven't gotten around to setting back the odometer. I haven't driven my 300SD in a few months but I am pretty sure that I didn't have any problems with the speedo being off. The speedometer reading comes from the transmission/engine speed and swapping speedometers won't change transmission/engine speed, but changing the rear diff ratio would.

[mjohansson]

Thanks guys, I did pull the speedo out of the unit and I tell you it sure looks like a quality unit. NOT at all like my 240 Volvo that I worked on and I did reset the numbers on it to be right on for a car I bought that did not have a working speedo in. Not saying the Volvo unit was not quality, just a major difference in construction. I don't event thing I want to attempt it with the Mercedes unit.

[nathan]

its quite easily disassembled, but I've never tinkered with the counter mechanism!
http://forum.w116.org/mechanicals/instrument-cluster-disassemblycleaning/msg130923/#msg130923