1988 420SEL 103k miles. See at
http://home.austin.rr.com/mscotta/

A/C Compressor clutch will not engage. I checked the single blue wire that, when hot, will engage the clutch, it is not hot. I can apply a 12v jumper wire where the blue wire plugs into the compressor and the clutch WILL engage. I switched out the "N6 A/C Compressor Control Unit" with one from my '89 560SEL (exact same part number), but to no avail.

Where should I go from here? It's for sale in Texas ($7888) and will not sell w/o A/C! Thank you.