1981 123 cruise problems

Hi. I picked up an 81 300TD wagon last weekend. The first problem to be attacked is the cruise control. It works only after driving for about an hour. I opened up the servo and cleaned the contacts,
everything looked ok there. I opened up the amplifier hoping I could find a bad solder joint, like often in the climate control. No visable problems there. Anyone know where I can find info for a more thorough testing of the amplifier?
Odometer doesn't work (speedo does) , it's electronic. What usually goes wrong there?
I also picked up a 79 300TD sedan for $200 from the same family, thinking it would be good for parts, but few will fit except for major driveline parts. It runs fine but has many other problems, I havent decide what do do with it yet.
Thanks
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http://www.odometergears.com/ is for your odometer repair. It is gears broken... change them all... 3 of them. Very common problem.
Cruise control is tricky to find the cracked solder... you need magnifying glass to see them. There is a place in Colorado who will test them for you and repair them if needed. Mbcoupes.com under part supplier.
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The gears are OK, turning the small spur by hand operates the odo. I cant even see the wires for it's motor without dismantling the speedo. I will look into the amplifier again and re-solder some of the larger junctions, it can't hurt. The 79 shows as a different part number, and uses a different actuator, so I doubt it will swap to an 81 Thanks
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All 123 body speedometers are mechanical.

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I don't think it's a rare prototype. Got the cruise working toady, just blindly soldered all the larger terminals.
Next, the heater. new aux pump and thermostat. blows warm on left side, cold on right. Weird
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Are your ACC pushbuttons horizontal and there are 5 of them?
If yes, replace your mono valve insert.
If you have a speedometer cable, you have a mechanical speedometer.

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