Steve Brotherton has reported that the cold start valves duty is very short-lived. Used only at initial start-up. I believe that if your present climate is warm enough, it may not be a factor at all.
You'll need juice going thru the circuit to measure pressure(voltage). Check the sensor in parallel.
Go to the SEARCH facility and key in:
coolant temp sensor
in the search field. Click on "titles only" - 7 hits. Good info between all 7 posts.
1991 300-SEL - Model 126
M103 - SOHC
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