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Easy way to wire the Aux fan to be permanently on High

19K views 24 replies 5 participants last post by  apetrov1x5 
#1 ·
Hi,
I was recently on a week long driving tour. I had to miss the last day and a half as my viscous coupling went all floppy and temps went through the roof when driving fast (repeatedly accelerating up twisty hill climbs etc). What I would like to do is wire a switch for the Aux fan so that I can just switch it on fully if I want to, bypassing all the A/C and temps logic. So next time if it happens again I can most likely continue.

What would be the best way to do this?

Thanks :)
 
#2 ·
Let me think about it, it likely will not be easy. The fan controller is in the fan and gets a PWM signal from the DME. You would have to emulate that signal. So what I am thinking is switching something else to make the DME send the full signal. It should be near 80 - 90% with the AC on. Potentially a switch with the correct resistor in parallel on the bottom hose sensor could do the trick.
 
#3 ·
It would not be easy and you would need to have volt meter skills and you would have to run wires and do some experiments.
I will just give you the beginning, if you want I could do more detail.
This from WDS
"The electric fan is activated by means of a power output stage directly on the fan motor. The motor control module activates this power output stage by means of a square-wave signal with duty factors (variable pulse width) between 10 % and 90 % thus controlling the various speeds of the electric fan. Pulse duty factors less than 5 % and greater than 95 % do not trigger activation but rather they are used for fault detection purposes. The power output stage features its own positive and ground supply.
The fan speed is influenced by the coolant temperature at the radiator outlet and the pressure in the air conditioning system. The fan speed is reduced as the vehicle speed increases."

It would be hard to duplicate that so that limits the options to tricking the DME to put out the 90% square wave. There are only two direct ways one to feed a tricked out output from the lower rad sensor or getting the AC module to lie. Here is the diagram.
Text Diagram Line Parallel Plan

The AC would have to be on and functioning to trick the DME that way so I see that as limiting. This leaves the lower temp sender as the best choice. First thing you would need to do is know what resistance value of the sensor makes the DME give you 90% output from the fan. You would have to do some testing. Next you would need to wire in a two position switch, one with no off in the middle. One position would have a resistor of the value you found in your test, but would totally remove the sensor and the other position would be just the sensor. It is possible you would still trigger a code when you switched between. You definitely would if you used a switch with off in the middle.

If you really want to have a go at that I can get some more info maybe. That said not sure a switch would help anyway because as your temp climbed the lower sensor should have had your fan to max anyway. The way these fans fail is by losing available speeds then just burn out and stop functioning. On a hot day in stop and go with the AC on the fan should be 90% and screaming, is yours? The viscous backs up the electric on the M5 and guys remove them completely with some after market electric fans. Totally different than the other models. It does not engage until 95 degrees(air temp) or something where as the other models are at 75 and don't forget those cars have a much higher normal operating temp. You might want to search those temps if you want exact numbers, my memory is not great on specific values anymore. The idea of the M5 engaging at a much higher temp is correct.

I would be suspicious of the rad not being very efficient, read that as being internally plugged or externally. I would also check the operation of the aux fan because they seem to lose the higher speeds before they totally fail.
 
#4 ·
Thanks a lot Sailor. Where did you get the nice circuit diagram from? Whats WDS?
I guess the easiest option would be the resistance trickery on the coolant temp sensor as you mentioned, will give it some thought. Though it might just be easier to carry a cheap pattern viscous coupling spare :) (I'll replace the item fitted to car with OE). The issue was that I was abroad on a driving holiday with friends and the Portuguese BMW dealer said 10 days to get the viscous coupling in!

The aux fan on mine is new this year and OE. I was running totally without one for over a year and it was actually fine, however I fitted it before the trip as I knew that high ambients were on the cards and basically asking a lot of the car in general; it was driven very hard, plus SC now too.

From what I could tell, the aux fan did come on with the A/C but at a relatively low speed by default. The fan speed didnt seem to ramp up until after 90-95c which is a bit high I thought.
If I didn't have the A/C on well then thats when I got the warning gong about coolant temp during a fast hill climb; temp was over 110c. Then the aux fan was on full high speed, however I think this is just an emergency measure. So after that I drove with the A/C on, and the aux fan did a relatively good job of cooling but I was just driving normally then like rpm mostly under 4k as I didn't want to stress the engine.

I think it's possible that the original radiator might have seen better days. Do they need replacing ever? I never really hear of them going on the M5.
 
#6 ·
WDS is the wiring diagrams. You can find them here WDS BMW Wiring Diagram System - 5 E39 from 09/98.

You have to have java working well, and you will also need to add an exemption for the site's main page in the java console and maybe install an sgz viewer, it can be a chore but worth it.

I think your lower hose sensor might be toast. There are no codes for that sensor, and the only way to test it is by listening to the fan or measuring the ohms across it. You also need to know how hot the coolant is. If you leave the car at idle on a hot day even with the ac off the electric will turn on in short order. If you can hear the viscous engaging before the electric starts you have a problem.
They are set to come on at 110 and the viscous at 95 but one is liquid and one in air so the electric should be screaming before the viscous engages much. Simple thing is just to replace the sensor as the coolant stays in place when you pull the sensor as long as the cap is on. It is a 20 dollar part and takes ten minutes to replace. Testing would likely take one hour. Lots of guys have had these fail.
I have not had to replace my rad yet but have flushed it a couple of times. I noticed it was very heavy the first time I pulled it to work on other things. That said there have been a steady supply of posts where people have been replacing rads and cleaning between the condenser and rad. No surprise these are not new cars and rads fill up with crap on the inside as well as the outside.
 
#7 ·
Nice, will have a go with that site thanks.

I did replace that sensor about 10k miles ago, however I did cheap out and buy a febi part. If I access the coolant temp figures using the OBC secret menu, where does that reading come from?
Where can I view the resistance graph for that sensor, any ideas? I'd like to test it even if it's out the car and I can test it on the cooker in a pot of water...

Maybe I'll flush the rad too then, it's never been done.
 
#8 · (Edited)
The OBC comes from the upper sensor in the t-stat housing. It has two senders in it, one for info like gauge and the other to the DME. Not sure which you see in the OBC. You might find the resistance in the WDS but it has been posted on the board a few times. IIRC add 68fb to your search. Do you have a heat temp gun? they are pretty cheap and handy. You shoot the bottom hose and do a resistance reading at the same time.

The problem with flushing is it has to get out, you really need to pull the whole drain plug after the chem has been in and all of the instructions followed. I do the flush but not everything dissolves but after I am finished I take the drain out and stick a hose in and leave it run. It is better to pull the rad and let a rad shop do it because they send pressure thru both ways and have much stronger chems. Mine still weighs twice what it should but started be twice as heavy as it is now. I live in a cooler climate and the M5 is really my winter and DD car.
 
#9 · (Edited)
I do have an infra red heat sensor gun actually yeah :)
Couldn't find the resistance graph on m5board.com even with 68fb as the guy. I'll have a wider google later on.

Seems like the Behr part for the radiator isnt that expensive. It seems like a good idea to just replace the radiator. I presume Behr is the OEM?
 
#10 ·
Regarding the radiator: The Behr units have served me well and are very cost effective. How old is yours? Sorry if I missed some info earlier in the thread.
 
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#12 ·
I will have a look too. My memory is not as good as it was. I have a short list of others that could have posted. Have you got WDS working because there are some good descriptions of how the system works under one of the sections but I am not finding it either. It talks about duty cycle and what % of max speed it operates at.

How old is your aux fan? They often fail by losing some speeds or not ever getting to full blast. The lower rad sensor, was it a OEM or just something ordered.

It is a great idea to replace an old rad, much better than trying to flush. I used semi pro chems and a shaker bath and only got good results not great results.

What still bothers me is the when the fan turns on. short of the resistance, temp could substitute. Shoot your gun at the sensor housing and give an impression of what % max it is running. Really all we need to make an educated guess is what temp it turned on at and what temp it started screaming "I at Max". You might be best to knock off the viscous during this test and save possible injury that will never happen until it does.
 
#15 ·
Is your AC charged? Is it charged with those cans or by a pro? On the AC side it is pressure in the system that controls fan speed. If you are not getting cold air then it is likely lack of charge or lack of mass of refridge. If you have cold while moving but not when stopped then more so charge and over pressure from cans or lack of refridge mass.
 
#16 ·
it was charged by a specialist, about 4 years ago, then about 2 years ago I had it charged by the same guy when I replaced the aux fan. Then the aux fan died on me a year ago, so the AC was not working, and now I finally changed the aux fan to a new bmw one, but I will go to check the pressure of the freon as you suggest.

BTW, I have noticed that the new aux fan was spinning very fast today (I would say it seemed like the max speed) when I started the car briefly to park it inside the garage. The car was not cold before I started it, but not hot ever, normal operating temps (I stopped in front of the garage, switched the engine off, open the garage, turn the car on and then I noticed the aux fan kicked in). Outside temp at that moment was no more than 20C, it seemed very strange to me...also so, because the this is the second aux fan I change in last 4 years.
 
#17 ·
If you had to have it charged 4 years ago then 2 years ago it is likely due now. But you must have a leak albeit small and slow. Get him to add some leak dye so when you go back in two more years the dye will be able to show where it is leaking. It is ultra violet so it can only be seen with a black light.

I would not worry about the fan on high. When there is no circulation of air or water temps can get high. Also no idea where the hotest goes, might go to the rad who knows. Make sure it is normal while the car is running. You might check that your recirc pump is operating just to be safe.
No idea where it is on a UK car but on a NA car it is in the hoses that make it difficult to to change bank 2 CPS.
 
#19 ·
If you had to have it charged 4 years ago then 2 years ago it is likely due now. But you must have a leak albeit small and slow. Get him to add some leak dye so when you go back in two more years the dye will be able to show where it is leaking. It is ultra violet so it can only be seen with a black light.

I would not worry about the fan on high. When there is no circulation of air or water temps can get high. Also no idea where the hotest goes, might go to the rad who knows. Make sure it is normal while the car is running. You might check that your recirc pump is operating just to be safe.
No idea where it is on a UK car but on a NA car it is in the hoses that make it difficult to to change bank 2 CPS.
actually, I do have an error about auxiliary water pump (short circuit inside it or something). I will replace it too soon, many thanks!
 
#22 ·
It is not for the aux heating alone. This is a can of worms. I can tell you how it works on an E34, E32, and E38. If it works this way on an E38 it should work exactly the same way on an E39. I have not tested it but I drive my car and the same effect is given as it was on my E34, but 68 disagreed although his only experience with these cars is his E39.
In any event the pump runs during the cold start cycle which puts heat to the cab and warms the interior fast also cools the heads. From driving in the winter I know I have cab heat before the T-stat opens. Then it runs after shut down to keep the heads cooler to prevent heat damage to the heads, IE warping. It also does the aux heat which was not on E34s or E32s but is present in the E38.

I would suggest you fix it.
 
#25 ·
The warping of heads is not wanted! Hmm.
I guess theres a test to activate it in DIS?
yes, there is a test in DIS to activate the auxiliary water pump if this is what you are asking.

I ran it and it stayed silent. It confirms the error I have about a short circuit in my aux water pump.

I asked 2 pumps from a friend to plug them (electrically only) to see if the error goes away. If it does, I will buy another pump.
 
#24 ·
Probably, don't use dis much. If there is a code then the power path is likely broken so a volt meter on ohms would show you that.
Warping heads was an 80s problem on alum heads, not an issue once the manufactures learned. Highly unlikely on a alum block. But that is when things like this started going on cars and that was the main reason it would also help a turbo car. This likely stayed because of the part about heating the cab faster on start up and the aux heat or REST.
 
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