Quote:
Originally Posted by ah-kay
The CCU is active 'low' to turn on A/C. How you measure it is up to you. However, you can only measure resistance UNLESS you have something connected, that is the Kilma replay is plugged in. If it is plugged in then you need a mean to get to the PIN underneath the relay, like an extension card. The resistance is not going to work well as the relay on/off can be overrided by other inputs. The simplist way is to measure the voltage toggling when the CCU call for A/C. The CCU monitors the freon pressure so if it goes low ( 0 volt ) then you know the CCU is in good shape.
There is nothing to it in MBZ A/C circuit if you understand simple electronics. Kilma relay is nice as it can protect the serpentine belt, turn off A/C when over-taking etc. However, when it goes wrong, it takes a lot of effort to diagnose the issue.
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As previously mentioned, none of that is accurate for a W126. It receives a ground signal from the CCU to close the compressor clutch. You do NOT need the Klima plugged in. You simply look for continuity to ground when the CCU calls for the compressor to engage.
Gassers are definitely different and function with voltage on that socket as you have described.
The W124 may function like a gasser............but, I'd be absolutely sure before posting here with a test that has the opposite logic from other diesels.