It wouldn't hurt to just wire the aux fan to run anytime the compressor is running. If the switch on the receiver dryer is bad then he'd need to evacuate (ie recover) the refrigerant first, then replace the switch and recharge it which is a bit of work for something that really isn't necessary. He can check the switch and if it is good rewire it into the circuit, but personally, if it were me I'd probably just tap into the wiring of the compressor clutch and let the aux fan run anytime the compressor ran. It won't hurt a thing. IIRC the aux fan is powered through a relay so the extra load on the compressor circuit to close that relay should be negligible.

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Marty D.

2013 C300 4Matic
1984 BMW 733i
2013 Lincoln MKz