Thanks for the comeback. I have tried jumping positive 12 volts from the battery to each terminal of the Klima socket. Nothing at the clutch.
Does this mean the control circuit is negative, and that the brown (ground) is being switched?
The Klima relay easily comes out of its case, as though someone else has been probing the innards. When I install the Klima and manually close the two small relays on the board, again, nothing happens. It appears that relays are normally closed and that power opens them both.
It also appears that not all the positions on the pc board have been filled. My parts supplier tells me that this model 85 300TD does not use a signal from the tach to control the compressor.
When I apply 12 volts positive to the relay socket, I can hear the blower start at one pin, and a relay clicking at another, although it is not the compressor.
What conclusions can I draw from these obserations?

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Stage Wizards Lighting and sound designer
1981 SD 328,000
1985 300TD, 266,000
1984 300 TD, 267,000