Complex but irritating AC problem

[Darius2]

Ready? Here goes... (99 C220 CDi). The coolant warning light never comes on unless the AC is being used. If AC is being used everything is fine on the motorway but if driving in slow traffic or stationary for any length of time the coolant warning light comes on (even if engine is relatively cold) and once that has happened then the AC starts to behave oddly - you can hear it 'racing' and intermittently cutting out. Still cools the car though. There are no problems with coolant levels or temperature (level sensor and thermostat both new).

If you got this far, any ideas very gratefully received

Darius

 

[Blobcat]

Are the radiator fan(s) working

 

[Darius2]

The engine never gets hot enough for me to find out. It rarely goes above 80C. About 18 months ago the radiator fan was stuck on permanently and that was sorted ( new control unit I think).