Can I swap a CCU from a W123 into a W126? The car is a 1983 300sd. I removed the W126 unit that was sometimes acting strange and replaced it with one I had from a W123. They look similar but have different part #s. I took the back off each to do the solder repair- both looked similar and OK. I put the W123 unit in and it seems to work OK. I donot have AC working yet- needs additional stuff checked out. What is the difference between these units?

Doubt there's much difference. If the plugs have the same # of pins, should be okay.

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83 SD

84 CD

The W126 has one more circulation flap than a W123. The 123 CCU has no circuit to control said flap.