Radiator aux fan operation

[chinist]

Both high and low speeds work using a jumper at the radiator temp switch plug (after replacing ballast resistor). The Bentley says it should operate when temp level is reached or AC is on.......do they mean both conditions have to be met, or should one or the other start the fan? At normal operating temp with the AC on I still have no fan. Before I pull the switch out to do a hot water check, is there something in the AC system I need to look at ?

[whiskychaser]

It is either/or. There is a diagram at the back of Bentleys.
Does the pump clutch engage when you switch the AC on?

[chinist]

The AC seems to be working, except that it cools better as road speed increases......which is what got me to checking fan operation. So if fan should run with AC on regardless, I will hit the schematics and track down what should be switching the relay from the AC.
Thanks for splainin the either/or condition.

[Tiger]

Majority of the time the reason the aux fan doesn't come on is because of lack of refrigerant... either that or there is 'air' in the system. So what I would do is rig the Aux fan on high and check the refrigerant level... get it to the proper pressure... Let the AC run for at least 5 minuted before adding anything... also engine must be at operating temperature.

Once you got the level where it should be... set the aux fan back to normal... and see if it kicks on as it should... if not... add a little more and see if it kicks on... it is temperature switch but if not enough refrigerant 'touches' the sensor, then it will not register.

By pulling out the temperature switch for AC... you will lose all refrigerant.

[whiskychaser]

I thought the AC pressure switch controlled both the pump clutch and the fan. So if the pump is running the fan should too? Or maybe it is me that it is simple

[Tiger]

They say 'temperature' switch and 'pressure' switch to differentiate them...

The pressure switch has one function but two duties... if the pressure is too low, it prevents the compressor from running... if the pressure is way too high, it also stops the compressor from damage.

The temperature switch is for aux fan only... if the 'temperature' of the refrigerant is high, it will trigger the aux fan on... if the temperature is too low, it keeps it off. There is also another temperature switch on the engine that if the engine is really too hot with AC off, it will trigger the aux fan on high. Now, to get the low and high aux fan speed... if engine is normal temp or low and AC is on... fan speed is low. If the engine temp is high and AC is on, fan speed high.

Now, I bet you are wondering that the aux fan comes on and off all the time... the above doesn't make sense... lol... well, the AC compressor kicks on and off and the temperature of the refrigerant constantly changes... some time too hot... sometime too cold... and that's is the intermittent aux fan on and off cycles...

[whiskychaser]

The way I read Bentleys, it suggests it kicks in the high speed fan if the refrigerant pressure is OK. Now I know that is not correct as mine used to kick in the low speed fan. It also seems to imply that if you have a 525i it doesnt care as long as you have more than 18 bar. I knew the AC had a temp switch but I didnt know it controlled the fan. Thanks to Tiger I am sure we are all a lot clearer. Apologies to OP for asking questions on your thread. I am about to change the fan and thought I would ask questions beforehand. Back to AC school for me

[chinist]

So if the engine temp is normal and AC is on, I can assume the engine temp switch (same as radiator temp switch?) is bad ? Except if the refrigerant temp is low and that would keep the aux fan off via ref. temp switch as it has priority ?
Not a hijack if still on topic and we are all learning something

[Tiger]

No... it can be either one... like I said before, if your refrigerant is lower than optimal... the AC temp switch would not register properly. You can always test the sensor with ohmmeter to determine which is not working by comparing it to the spec.

The wiring of the AC temp and engine temp switch is not just a simple wiring route. On Mercedes, if I pull out the engine temperature wire, the aux fan goes high speed (It is good to know expecially if your engine is overhaeting...). So the engine temp switch works like if temp is up, the more resistance... same as pulling out the wire. Every car is slightly different but same theory.

I am not sure where the engine temp switch is on the BMW... it should not be on the radiator because if the engine is running without coolant, the radiator temp sensor would not know the engine is danger hot.

[whiskychaser]

I am not sure where the engine temp switch is on the BMW... it should not be on the radiator because if the engine is running without coolant, the radiator temp sensor would not know the engine is danger hot.

There are 3 on the M50. Two in the block for gauge and ECU. The one for the fan is on top of the rad - where it shouldnt be for the reason you stated and I heartily agree. I am not sure where the AC temp switch is-unless it is the one on the evaporator. In which case it goes round the houses to turn the fan on. But I have to go and do my homework...