If the switches are different, you'll have to do some cut-n-splice on the wires to adapt the appropriate assy to the wiring harness under the car.

Adapting the electric speedo to a 123 will take some serious head scratching since none of the wiring needed for that device even exists on the 123 chassis. I don't know if the 126's speedo head will fit into the 123 cluster either. If it were me, I'd hold out for a good 123 tranny, then, the only thing to worry about is the park/neutral safety wiring which is NBD if you make sure and get the connector that fits the replacement tranny.

I know without a doubt that there are some differences in the park/neutral safety switches between the various 722 trannies because we came across it last year when swapping the tranny on Passion Flower (ThingOne's 85 300TD). Supposedly, he was sold an '85 300D tranny but the switches were different between the two. That damned car has been driven completely across the US and is in Skagway right now with the neutral switch duct-taped so that you can start the engine and the whole mess is tie-wrapped to one of the fuel lines underneath the car. Jeeze.

OTOH, if you can get the SD tranny über cheap or free, maybe you could stab it in and use it while having the original one rebuilt?

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Never a dull moment at Berry Hill Farm.