Aux fan troubleshooting

[paramax55]

I have an 01 or 02 (manufacture date of 11/01) X5. The aux fan quit working (again). The AC acted like it needed a charge, so I started giving it a squirt, but the pressure didn't look like it was low, so I stopped. Then It quit working a little while later and I thought "I've been through this before. I guess I need to order a new fan." Sure enough, the fan was barely spinning and the high pressure line was hot enough that it burned me like a hot stove.

The fan that was in there was a junkyard unit I frankensteined in from a 4 door 3-something (I don't remember which year/model). The 3-series had a slightly smaller fan, but it was made by Bosch instead of Siemens and the junkyard was full of them - which told me people weren't pulling them to replace failed ones. It worked well for a few years until now.

The new fan arrived, I installed it, and... NOTHING. Damn. If I unplug the high pressure sensor and start the car, the fan will run full speed, so I imagine that the fan itself is OK. Other than that, nothing. I had some spare AC control units, so I tried those to no avail.

I've been looking for wiring diagrams to trace the signal wire from the fan to the DME so I can check for high resistance or even a broken wire. I've found that it's pin "4" but there are multiple connectors and I don't even know which pin is "#1" on each connector. I don't know if there is a secondary signal generator for emergency operation? Something that would let the fan work when the high pressure sensor is unplugged, but not when the AC is on? Or do all signals come from pin 4 on the DME? I'm thinking it's also possible that there is a corrosion/high-resistance spot somewhere that doesn't let the fan "see" the smaller PWM signals from the DME, but the full-on emergency signal is strong enough? I can scope the wire at the fan but, without knowing where the other end of the wire goes, It's really just a pretty picture.

I have an INPA coming, but it won't be here for a couple of days. And I'm pretty sure I'll need to trace this wire back to the DME, anyways.

Does anybody have a picture, or a good description of which DME wire goes to the fan? Barring that, does anybody have any good ideas/information?

[80stech]

If the fan runs full speed when you unplug the high pressure sensor then it's likely the wiring from the DME is fine. I think you maybe don't know enough to make good calls about what's going on with the A/C pressures.

[paramax55]

Well... I have built several AC systems. I do know that the high side should be somewhere in the neighborhood of 250 and the low side should be about 1/5 of that. And it ultimately comes down to the performance of the system - refrigerant boiling approximately 2/3 of the way through the evaporator...

And I do know that the AC system spins the aux fan for a moment when you start the car, then stops and checks for the voltage produced by the freewheeling of a properly working fan before it will let the compressor run - which let me fool the AC by starting the car while driving on the interstate so the fan was already freewheeling.

And I know that the only specification needed in order to manufacture a fan that will work in this car is that it respond to a PWM signal - be it whether it's a BLDC motor with a computer driver thay determins the proper speed via the width of the pulse or a simple DC motor that is current limited by MOSFETs or IGBTs with the gate being driven directly by the DME signal without so much as a filter.

And if it were a modified version of the second part of that - DME driven MOSFETs with a voltage divider resistor circuit - a weakened signal from corrosion (due to a pierced wire casing) could cause the fan to not get enough power to spin at short PWM signals while longer ones could push through.

But you are correct... I don't know enough to troubleshoot this car - namely which wire from the DME goes to the fan.

[Auto Parts Guy]

When you get INPA installed, there is an option to select the variable speed you want to run the fan at, like 20% or 90% or whatever from your laptop. If you are able to do that, then you dont have a signal wire issue IMO.

[paramax55]

Correct. But it will take 5 minutes to check the resistance on that wire while I am waiting for the INPA to get here - if Ican find out which wire it is.

[X5chemist]

Google: "R134 pressure chart"
It's a chart I use to recharge any A/C I do work on. I gave mine a slight boost last year. Pushed pressures to the higher temp scale. Mine will hit 42 degrees and dip a little under with highway speeds.

Mine has the same annoying issue. The aux fan does not start up with A/C activation and cold engine. Once the engine is up to temp, the fan runs as needed. With a hot engine, the aux fan kicks on at start up.

[paramax55]

Pressures will change with ambient temperature and cabin temperature, which will affect condenser temperature and evaporator temperature. That's why I stated "round about" figures. I have been cobbling AC systems together for more than 35 years... early on to save money, later to have fun and see what limits I could push. The physics behind a phase-change, compressed refrigerant system are not foreign to me.

All of that being said... the compressor will not even activate without the fan working - which negates the ability to even use a pressure chart. What I need to know is which pin on the DME sends the signal to the fan so I can check it for continuity. That and possibly all of the conditions that the programming wants to see in order to turn on the fan and the AC compressor...

[andrewwynn]

If the fan runs when ac pressure switch pulled it's not going to be a line problem. The signal is milliamps if not microamps.

That said, back probe with a meter that reads PWM% (or scope of course). I think the signal is 400hz PWM or something. You could back probe at the DME to find the same signal with the pressure sensor unplugged.

Other thoughts: AC wasn't tricked from the fan freewheeling unless you turned the car off while moving the self test only at start.

If the AC high line was hot your compressor was running. If you aren't moving and fan not spinning you won't her cold air because condenser not pulling heat from the system. Do you get cold when moving?

[paramax55]

Turning the car off and restarting while moving is exactly what I did. It was a test to see if there was a fan issue while I was away from my tools. I don't keep this car at home. It gets stored 50 miles away and I only use it when I am in the city. Once I figured out the fan wasn't working, I made arrangements to bring it home where I could work on it for real.

When I did my test, the compressor did engage and, yes, that is how the high side got so hot. I am in Florida and there really is no speed you can go that will force enough air through the condenser to cool it without the electric fan. But the compressor will not engage without a spinning fan.

I do have a scope and I understand that the signal is on the level of mA and either 5V or 12V. It's a long shot, but someone worked on this car in the past who was not exactly a NASA scientist. If they probed the signal wire and poked a hole in the casing, it could have developed a spot of corrosion. I've seen it before. On its way out, the casing can hold the wire ends together enough to make an intermittent connection. This intermittent connection can mimic low refrigerant before mimicing a bad fan. Like I stated, it's a long shot, but it's SO SIMPLE to check! All I need to know in order to check it is which pin from the DME????

It's like checking your tire pressure if the car pulls to one side - it's probably not the cause, but how long does it take to just know you are good???

[80stech]

Haha, This just gets better and better!