With the Mono valve unplugged it is in fail mode and fail mode is open to provide heat all the time for defrost purposes. Sounds like your mono valve IS working as designed. Though it may be sluggish from not being used in warmer times. Remember +12 = closed valve and is closed when heat is NOT on, +0 = open (or fail mode), when heat is ON. It works off the Ground instead of +12V....backwards from about anything else electrical.

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1986 Euro 500SEC "RUF" 9:1CR, "Rose"
1985 Euro 500SEC Cabriolet AMG Widebody
1982 Euro 500SEL "Blue"
2001 Texas Heeler (Aussie/Queensland X) "Sulphie",
2012 Queensland Red Heeler "Squeak"
Best dogs I've ever had.