I am trying to hunt down the reasons for my 1992 740 wagon running hot. I have found the electric radiator fan does not roar up or turn on when I start the motor. In fact I can only recall hearing it come on as the motor seems close to boiling over. Indeed the roar of the fan seems to be associated with the sickly sweet smell of coolant. Is this fan supposed to roar or start as the motor is turned on? I read the FAQ and was not sure about that. I went outside and watched the motor idle for a good 15 minutes and it never turned on. A few hours later I went out and started the motor, turned on the AC and the fan started right up. As soon as the AC was turned off the fan went off. Where do I start with this fan to see what is wrong with it. I think it is a recent (last 5 years or less) replacement by the original owner. I am fairly ignorant of cars so please feed it to me straight and easy. Thanks in advance.

The radiator fan is not on all the time. There is a temperature switch on the radiator that will turn it on when the radiator reaches a certain temp. Newer models (yours) also turn on when the AC compressors is turn on (I think, mine is an 89)

What is the temp guage at? How's yor main fan clutch? Does the main fan roar when you first start the engine?
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Paul NW Indiana '89 740 Turbo 140,000

See if you can apply 12V directly to the fan to make it run. If it works, check the relay, and read up in the archives about how to jumper the three thermal switches (there's a green, a purple and a gray one-- I can't remember which does what) on the bottom corner of the rad. I recall that you should be able to make the fan run at two speeds by jumpering the connector end.

Good luck!

The first thermal switch I would check is the one on the upper RH side of the radiator. If you connect the two terminals with the engine running, the fan should come on at high speed. If it doesn't, you need to check the relay, located down by the battery. There are two terminals that control the low and high fan speeds. These will be the lightweight wires going to the relay. You should be able to seperately ground out these terminals and get low and high speeds to run. If the fan runs properly, you have either a bad thermal switch or a break in the wiring. I found one of my wires broken between the thermal switch and the relay on my car.

Your fan seems to run when you turn on the AC, so I don't think you need to mess with the other 3 fan control switches on the lower front of the air conditioning condensor.

PS Don't ground out any of the heavy wires connecting to the relay!