Fan Clutch Always Engaged with AC On

[M4ncyy]

It's 1990 525i with M20 engine.
AC blows ice-cold, auxiliary fans work well, coolant temp remains good no matter how hard I drive, but the mechanical fan is almost always engaged when AC is on, it only goes off if I'm driving at high speed (45-60miles/ 80-100kmh) and then rev it hard like 4.5 to 5.5k rpm, then it goes silent but comes back on after a while, as soon as I turn off the AC, fan goes silent in under 30 secs, also when the car is cold it is silent and disengaged.
Is it natural?
BUT there are two things to consider:
1-It's hot af here like 40degrees,100f and sunny
2-I don't have a fan shroud, I am looking for one but haven't found one yet, since the engine wasn't overheating I didn't rush it. Could it be it?

[shogun]

you can test the visco clutch, copied from an E31 website
Fan Clutch test
First, lets discuss what the fan clutch does and why it is there. The fan clutch is just what the name says, it is a mechanism that will clutch the fan on and off depending on the need for more or less cooling air to flow thru the radiator. It is a thermostatically controlled device that when operating normally will vary the fan speed independently of the engine speed. When cruising down the road at freeway speeds, with outside temperature less then desert conditions, the fan should be merely be idling along, turning just fast enough to add a little air flow when needed, in this way the fan noise and drag on the engine is greatly reduced. When reducing speed, the fan clutch will sense higher temperatures thru the radiator and “clutch up” thereby increasing the fan
speed to help maintain constant engine temperature. It may in fact, cycle as the temperature of the air thru the radiator changes depending on airflow. If the fan clutch operation is normal, when first starting the car, the fan clutch should “clutch up” and an increase in noise and airflow should be obvious. After about 60-90 seconds, the fan will un-clutch and the noise and airflow will drop. The fan will continue to turn but at a much reduced speed. As the engine warms and the thermostat opens to regulate the temperature, the air thru the radiator gets hotter and the clutch will sense this, thereby increasing the speed of the fan to maintain a normal operating temperature.
First signs of trouble:
A normal temperature indication at freeway speeds and an increasing temperature as the vehicle slows is one of the first indications of trouble. Many other things may give this indication but if the temperature seems to be stable at speeds but climbs in traffic or while stopped, this is a good indication that the fan clutch isn’t working correctly. As the temperature continues to climb, the auxiliary electric fan should start but may not provide enough air to keep the engine from overheating.
Another sign of trouble is if the fan noise is high and never decreases after starting, and is there anytime the engine RPM is higher then idle, this means that the fan clutch is “frozen” and is not releasing. Although this will not result in immediately serious trouble, it will load the engine continually and gas MPG will be reduced. Load on the fan belt(s) will be higher and shorten the life of that component also.
Fan modifications:
It has been suggested that other models of BMW fans can be substituted to reduce the noise and load of the fan. This is NOT recommended! If the fan clutch is working properly, there should be no need to replace with a lesser fan. The noise and load of the fan should only be there when it is “clutched up” and the fan speed needed to keep that big V-12 cool. BMW designed it this way and it is never a good idea to alter the cooling system and in particular where alloy engines would be effected.
Testing the fan clutch:
If you have reason to suspect that the fan clutch is defective, here is the recommended procedure to verify the condition of the fan clutch.
1. Start the car (cold) with the hood open and note if the fan is turning, increase the engine RPM and note if the fan turns faster and the noise increases, if it does, first good indication, if it does not increase speed/noise, clutch is bad and needs to be replaced. (Remember, this must be tested after the car has been off for and extended period, over night etc.)
2. Leave engine running and note if the fan starts to slow down after 2-5 minutes, speed/noise should diminish and even raising the RPM, the fan should not make as much noise as when first starting, if it does slow, this is the second good indication. If
speed/noise does not decrease, clutch may be “frozen” and should be replaced.
3. Leave the engine idle and watch the temperature indicator. When normal operating temperature has been reached, some increase in fan speed/noise should be noted, in particular when the RPM is increased. If temperature is fairly stable and the fan
noise/speed increases or cycles, third good indication. If temperature indication continues to increase, with no increase in fan noise/speed, clutch is defective and should be replaced.
4. After the engine is at normal operating temperature or above, is the only time that the “rolled up newspaper” test that many people talk about should be performed! Take some newspaper and roll it up into a long narrow tube. Be carefull, keep hands and fingers away from the fan while performing this test! With the engine at full operating temperature and idling, take the rolled up paper and insert it on the back side of the fan and try to reach the hub of the fan avoiding the blades until close to the hub. Push the rolled paper at the fan increasing the friction to the hub area of the fan. If the fan can not be stopped easily this is the fourth good indication, if it can be stopped the clutch is defective and should be replaced. Again, this test can only be performed when the engine is at or above full operating temperature. Testing can be performed in any order but just make sure the conditions during testing are those that are specified for that specific test. Do not continue to operate the engine if the temperature continues to rise and certainly stop if the temperature approaches “redline”.

[M4ncyy]

Thanks for the guide:
Fan starts loud at cold starts but slowly drops and slows down? Check
Fan is not engaged in cruising speeds but engages in traffic? Check
Engine never overheats? Check
In summary, fan works in correct order at all times, but when AC comes on, it works overtime (extremely overtime) and yet, goes quiet seconds after turning off AC.
Is this normal? Like I said, I don't have a fanshroud yet, could it be the cause?

[shogun]

Maybe the fan shroud makes the difference, it is all about distribution of cold air in engine bay, even a missing Engine compartment screening #23 makes a difference. http://bmwfans.info/parts-catalog/E34-Sedan/Europe/520i-M50/L-A/feb1990/browse/vehicle_trim/heat_insulat_engine_compartm_screening/
The system is designed as (almost) enclosed system with hood seal, covers under engine etc, the fans push the air around the engine first and then the cool air should go around the transmission and then out, complete drive train front in, rear out, if something is missing, the air will find the easiest/other way.
The visco fan clutch just operates based on the bimetall , this explains it https://www.youtube.com/watch?v=UwM4OqsLek4
https://www.youtube.com/watch?v=N_GkuTgJr3w

[demetk]

With ac running, the condenser is adding to the heat load that is coming from the engine. Since the viscus fan is temperature controlled, the increased heat that is now passing over it activates it sooner. I think it's normal. Considering you live in a very hot climate it should give you peace of mind to actually hear the fan working like that.

[M4ncyy]

But it won't disengage until like 5000 rpm, one of the fan blades broke off today as I was revving past 5500RPM :(

[M4ncyy]

Guys, shouldn't the fan clutch limit the rpm? I mean, should it allow the fan to spin at rpms beyond 4000, regardless of the engine temp? My engine wasn't even that hot yet with AC it would just not want to let go of the fan.

[shogun]

It also spins beyond 4000 RPM if necessary. Depending on the temperature, it adjusts automatically. A thermal clutch fan operates using silicon fluid as a viscous coupling medium. When the clutch is cool and disengaged, most of the silicon fluid is stored in the reservoir allowing your fan clutch to slip relative to your water pump shaft thereby spinning at a lower RPM than the water pump. This saves you money because the horsepower from your engine is not wasted driving a clutch fan when it's not needed. As your engine heats up, the thermal spring on the front of the clutch expands, which opens a valve allowing the silicon fluid to drive your clutch at an increased RPM. This provides more air flow through your radiator, preventing your car from overheating.
https://www.youtube.com/watch?v=UwM4OqsLek4
https://en.wikipedia.org/wiki/Fan_clutch
When the engine is cool or even at normal operating temperature, the fan clutch partially disengages the engine's mechanically driven radiator cooling fan, generally located at the front of the water pump and driven by a belt and pulley connected to the engine's crankshaft. This saves power, since the engine does not have to fully drive the fan.

However, if engine temperature rises above the clutch's engagement temperature setting, the fan becomes fully engaged, thus drawing a higher volume of ambient air through the vehicle's radiator, which in turn serves to maintain or lower the engine coolant temperature to an acceptable level.

In case it always runs at full speed, independent of temperature, then the visco clutch is 'frozen" and always runs at full speed, causing high noise and also damage to the fan blades.