Here's a puzzle. My aircon works perfectly when car is travelling between 0 and 40 mph. Above that it runs cold, independently of the temp settings on the controls. Not good in England at the moment because it's winter here!My engineer has used the standard diagnostic device to check out the system but can find nothing wrong. He has spoken to various experts and none has experienced or knows of this problem before. Could it be the control boards. Grateful for any advice. Gordon Thomas
Possibly thermostat stuck open or removed. The faster you travel, the more efficient the radiator is at removing heat from the engine coolant. Over 40 MPH most of the engine heat transferred to the coolant is removed by the radiator and little gets to the heater matrix. Quick way to test is cover half of the grille with cardboard and test drive.
If you have the same Heater Control Valve as the slightly earlier cars, e.g. 1990. the spring weakens inside the unit.
A kit is available and the repair is straight forward.
Take a look at this web page in the SZ section http://mutley.hypermart.net/RollsSubstituteParts.html
There is something in the weak spring in the heater water valve theory. IIRC there was some info on the higher water pressure at higher engine rpm's forcing the valve closed and cutting off the hot water.