'81 300TD odometer problem (NOT the gear)
 

The odometer on my '81 300TD wagon has started to advance only intermittently. The faster I go, the worse it gets; by 70 mph, it's reading less than half the actual mileage when compared to the freeway mileposts. Needless to say this makes my fuel economy numbers look pretty bad. ;)

A complicating factor here is that the '81 TD odometer is, I think, unique; unlike other W123s, it's electronic, with no mechanical cable connection. This means I can't just go grab a speedometer head from another W123 at the junkyard.

Importantly, the speedometer needle indicates correctly at all speeds, according to my GPS, which I think rules out the sender. The problem must be inside the speedometer head.

What I've done so far:
I pulled the instrument cluster, took out the speedometer head, and checked the plastic gears -- they were all good, and the mechanism moved freely when I worked it by hand. I also resoldered all the connections that looked at all flaky. It didn't help and the problem seems to be getting worse over time.

Any ideas on where to go from here?

not the gear eh? are you ABSOLUTELY CERTAIN?
it uses the same internals as the 81-84 300SD, so you could pull one of those and swap components if you need one. you cannot swap the entire speedometer, as it will not fit.

Yeah, definitely not the gear. This is the third VDO speedo I've had apart, so I was as surprised as you are to find out the gears weren't broken. ;)

Good to know about the SD speedo, thanks. That's the kind of info I needed to know. I suspect a bad cap or something in the internal electronics...something just far enough out of spec to cause it to miss pulses when the pulses get shorter at speed.