AO Silver Member
Join Date: Mar 2003
Re: advice please on hot engine
Fan/Fans worked by the engine management system. They have a low speed and high speed. The low speed is possible because the feed goes through a resistor at the top of the radiator (if the resistor gets hot the slow speed more or less gets up to the high speed any way). Two different circuits feed the fan through relays in front of the battery.
The whole system is switched (by the ECU) depending on temperature OR aircon pressure. If the pressure builds up to high (a hot day) the aircon will switch the high speed circuit. On a normal day the aircon will just cycle the low speed fan on and off.
So, a good test of the fan system is to set the aircon on max and see what happens to the engine temperature. If the thermostat is doing it right you will get a steady needle. This is because the fan system is overcooling the water radiator (to deal with the aircon radiator) and therefore delivering cold water to the engine which is then being protected by the thermostat closing to just a crack.