The ETR shuts the compressor off to keep the evaporator from freezing up with ice. ( Possible, but rare ) So if by open you mean current is flowing throught the ETR then that means it's allowing the compressor to run. By closed if you mean no current is flowing through the ETR then at closed it would have the compressor shut off. I forget what temp it is that it should shut the compressor off at but it is before freezing. Try your test at room temp and then after the ETR has been in your freezer 10-15 mins. You should see opposite results if it's working.

I'm not an expert, but I hope this helps.

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1982 240D 370,000 daily driver, great car!
1983 300SD
1995 350SD gone to rod bending heaven.
1986 300SDL should have never sold it.
1995 2Dr 4x4 Tahoe with a highly modified 6.5 turbo diesel, 6in lift.