A/C issue on 87 300D

My indy is having a problem diagnosing a strange problem re the A/C. There is a temperature sensor behind the battery and near the OVP relay. This is
not the one that feeds the dash display which has been dead for a long time. If this is disconnected, the center vents work. (These had not worked for many years and I had written them off as due to the vacuum pods which I had previously replaced at great expense.) A known good sensor does the same thing. The other issue is that the AC works fine until the system hits somewhere in the high 90C range and the clutch drops off. The only way to get it to work is to shut off the engine, let it cool, and start again. Indy has r/r with good control unit, OVP relay, temp sensor. He suspects that the Blower Control Unit may be the issue but does not know where it is. Earlier models had it behind the glove compartment but not this year/model. Any suggestions?
Factoids: Engine changed to factory crate engine 30Kmi ago. Total is now approx 225Kmi. Compressor, drier, and condensor changed one year ago. AC worked-not through center vents-but would go 3 hours and then shut off. Overspeed sensor changed at that time.

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