Any a/c experts out there

[1999750il]

Hi all,

Today I decided to top off the a/c on my 1999 750il since it was nice and hot in New York (about 91F) and the a/c didnt seem as cold as it should be.
I used a digital mastercool manifold set and prior to charging the static pressure was about 102 PSI on both sides. I started the engine and with a/c running after several minutes I noticed the pressures build to 41 low and 410 high-side.
From my research, a 90 degree day and a properly filled a/c system should read about 30 to 40 on the low and 250 to 275 PSI on the high side after running for several minutes. I was worried when I saw the high side reach over 400 PSI a few minutes after turning the a/c on....can some one please comment on whether that's normal for these systems? The after revving the engine to 1500 to 2500 rpm the pressures dropped to more normal numbers like 35 and 250.
I let the car continue to idle with the windows open and fan on max and the pressures dropped further to 22 and 170. At that point I added some straight r134 from a tank and slowly added about 4 to 5 ounces of freon. I stuck a temperature gauge in the center vent. With the blower on medium speed the lowest temp on the gauge was 40 degrees.
While I understand that all systems will leak an ounce or so about every year....I did not expect to see the high pressure side reach over 400 PSI even if it was for just a short period of time.

Another odd thing was that the auxiliary electric fan did not turn on during the entire time I was working on the a/c system...about half an hour...and I had the fan replaced by a BMW dealer just over a year ago but have only seen it come on once.
I believe other e38 owners mentioned that these electric fans dont come on automatically when ever the a/c is turned on like most other car makes. I would imagine that on such a hot day and have the car stationary with no airflow through the condenser for over 30 minuted would reach a temperature high enough to turn the aux fan..but I'm not sure.

I apologize for the lengthy post and welcome any suggestions...Thanks!!

[rslifkin]

The aux fan should trigger based on A/C high side pressure. The pressure being that high and then dropping when the engine is revved definitely points to an airflow issue, so the non-functioning aux fan is likely the cause. It's possible your fan clutch is weak as well.

[1999750il]

Hi rslifkin....I had replaced the fan clutch with a new behr unit less than a year ago when I replaced the water pump and radiator. I can also hear the fan noise change as the blades change rpm with the temperature.
Not sure if there is a high pressure cut off switch or at what pressure that would open and turn the a/c off.
I still cant determine when the aux fan should kick on...I thought it would come on either by engine coolant temp too high or when a/c temp or pressure was too high.
I do not have a factory scan tool so I cant determine if there is a code for the aux fan...but its less than a year old but anything is possible I guess.
Thanks for your help

[Auto Parts Guy]

I agree with the above post. Typically the aux fan should turn on when the ac is on. You need to check the fuses and connections, then check with BMW diag software if it's still not working. There is probably a pressure switch on the high side line, if you unplug it, the fan should turn on. I'm not all that familiar with the e38 but these systems are similar for most part. 400psi is way too high.

[1999750il]

I believe these vintage e38s dont have the aux fan kick on with the a/c like most others. i believe they based on extreme engine coolant and high a/c temps . I know the fan is pulse width modulated and controlled by the ecu and I believe the compressor clutch remains engaged the entire time the a/c is requested on...unlike some other a/c systems that cycle the clutch on and off as a way of regulating pressures. While the 400 psi is definitely high...I cant explain why it happened for such a short period of time before dropping to normal numbers like 170 to 190. I did rev the engine just prior to the pressure drop but only for about 3 seconds to 2500 rpm...I dont think that would have generated enough air flow to have the pressure drop that quickly....so I am leaning towards a faulty aux fan...though I would like to learn when they are supposed to come on.

[Old525i]

The 40C temperature limit is about as low as the system will let the evaporator go. Less than that and water will freeze on the evaporator blocking airflow. There is a temperature sensor that monitors the evaporator, and if it drops much colder than that, it shuts off the compressor briefly (at least, that's the way it operates in my 1990 525i). The high pressure, as someone mentioned, could be due to lack of airflow on the condenser or a sticky expansion valve. My 2 cents.

[racer2086]

The aux fan should absolutely be running when the A/C is on. The car just controls how fast. High side pressures like that would be most likely a result of an airflow issue. The aux fan not being on is an obvious one. Leaves stuck in the radiator is another.

It's never a good idea to just add refrigerant to a system, because you don't know how much is in there, and the gauges don't tell you either.

[1999750il]

Hi racer/all

As for the airflow I know there are no leaves or debris blocking airflow to the condenser and I had rinsed both the radiator and condenser fins with coil cleaner not too long ago
As for the aux fan coming on with the a/c I heard others say that it does not activate as soon as you turn the a/c on like many other systems
I realize that simply adding refrigerant to a system is not ideal but I thought with the proper manifold gauges and taking into account the ambient temperature you could add enough to get to the target pressure ranges
I believe all systems lose some gas over time and adding an ounce or two can be done without evacuation of the system
Any thoughts?

[rslifkin]

The post-facelift aux fan doesn't necessarily kick on immediately when the A/C is turned on, but once it's been on with the car stopped for a few seconds to a minute (depending on outside temperature) it should come on. Certainly before the high side pressure reaches 400 psi.

Depending on where in NY you are, I've got a spare 99+ aux fan on hand that could be used for testing.

[1999750il]

Thanks for the offer
Hard to believe that the dealer installed fan from a year ago is bad but I guess anything is possible
I Was hoping to be able to test whether or not the computer has been sending the pwm signal to the fan
I have a icarsoft scanner but not certain if it’s able to read ac error codes
I would imagine that if the fan is commanded on and doesn’t work it would set a fault
I need to check the fuse for the six fan and make sure it’s intact as well
I may throw the gauges on it again and verify that 400 psi reading
I know the static Pressure of 103 went to 400 and the low side to 40
It didn’t remain at 400 long at all before dropping to a more normal 190-230

[acspecialists]

I believe the system holds a couple of pounds so a few ounces shouldn't be the end of the world. The pressures sound fine, don't get too hung up on start up pressures. If it is not a hot day turn the heat up to 85 and roll the windows up with the recirc on for a heat load. The system is working the hardest when the car is sitting. You should hear the engine fan when you start the car and then it should get quiet after a few minutes. Let the car sit and see if the auxiliary fan comes on? i would not claim to know what the software is doing, I would just be guessing.

It sounds like the A/C is working fine, just drive and enjoy the car.

[1999750il]

Hi ac specialist
Thanks for the reply
I believe it holds 1.5 pounds of 134
I would like to find out how to determine whether or not the fan is being commanded on bythe ecu
Not sure what you meant about placing heat on 85 with the recirc and then start the car?
I will also check that the main fuse for the fan is working
Thanks again for your help

[acspecialists]

When I did service on cold days I used the heater and recirc to get a heat load to evaluate the performance. If it is already hot outside you wouldn't need to do that. You are trying to evaluate performance not general operation. 400psi is high but not unheard of on a hot day before one of the fans kicks in. The high pressure cut out would be about 450psi and the relief valve on the compressor above that. If you are getting 40 out of the vent on a hot day you won't get much better that that.

Here are some tips, when you get in the car after it has been sitting open the widows and put the blower on high to get the heat out of the car. After it cools off close the windows and slow the blower down for better heat transfer. When its nice and cool you can hit the recirc and slow the blower down even more. The climate controls do a pretty good job except for the window part.

[Auto Parts Guy]

Does the e38 compressor have a regulator valve like the e53? Confused me on te e53 when i saw the clutch turning but no high/low pressure gauge movement..

If you have INPA, you can turn the fan on and select which percentage you want it to run. At least on my e39 (on/off only for e39) and e53 you can, i assume the e38 is similar.

[Grady in Texas]

There is a thread on here for replacing the fan resisters. Worked for me ('95)

[1999750il]

Hi Grady
Thanks for the advice
The fan was installed less than a year ago by a bmw dealership and I don’t believe resistors are serviceable only year 750
I already checked the connections at the fan motor and some fuses under the hood passenger side I read that there is another larger fuse located under the glove box but haven’t checked that one yet
There are a lot of differing opinions on when this aux fan should come On
While the engine temperature stays centered on the gauge and the air conditioner works well too I believe the air conditioner would work even better if the fan worked the way it was intended
Just need to determine if the computer is commanding it on or not
Thanks again for the reply

[chrisn7]

Lots of tech info about fan operation here:

https://www.newtis.info/tisv2/a/en/e...ry-fan/XMBUEDL

[rslifkin]

Lots of tech info about fan operation here:

https://www.newtis.info/tisv2/a/en/e...ry-fan/XMBUEDL

Based on the fan description at the bottom, this appears to cover the pre-facelift setup, not the post facelift setup.

[1999750il]

Hi Chris thanks for the link
It was very informative it I believe rslifkin is correct about the years
I have always heard that post facelift cars had tweaked systems from the pre facelift years and I think the aux fan is one of things that changed
I checked several fuses on the passenger side fuse box but read that there are 2 other hidden fuses underneath the glove box that are in the aux fan circuit too
I will have to check those if I can find them

[1999750il]

Today i looked below the glovebox to verify if there were any large fuses that would be tied into the aux fan and was not able to find them. I did find several relays of different sizes but have no idea what they control.
Does anyone out there know if this post lift 750 would have relays wired between the DME and the aux fan? I am still trying to determine why the aux fan does not activate. This is new BMW fan and while the a/c seems to work fine...I would like to determine if there is a fault like a fuse or relay that is preventing the fan from coming on. I have checked the 3 fuses labeled aux fan under the hood and they were fine.
Thanks again for all your advice/suggestions.

[rslifkin]

The post facelift setup should have 3 wires to the aux fan. A ground, power from the supply fuse (in the e-box under the hood, but you have to pull the big cover to get to it, it's separate from the other fuses). Third wire is the small one, that's a PWM signal from the DME that drives the controller on the fan itself which determines when and how fast it runs.

[1999750il]

you're correct...there are 3 wires going to the fan. I have taken the cover off the e box before when i replaced the starter and fuel pump relays. I wasnt aware that the fuse for the aux fan was buried in there as well. I would imagine the fuse for the aux fan is pretty large and should stick out. Thanks so much for the suggestion....I wonder if there is a constant 12 volts going to the wire from the e box and the ecm turns the fan on via the pwm signal when needed. It would be easier to test the voltage on the engine side of the fan harness than take the e box apart to check the fuse.

[rslifkin]

Yeah, it's a big 50A maxi fuse under there. And yes, it's a constant +12v to the fan from the e-box and the PWM signal determines whether the fan runs or not.

[1999750il]

Thanks so much for verifying the wiring
I will disconnect the fan from the harness and check for 12 volts at the harness in order to determine whether or not the 50 amp maxi fuse is good
I would think the key needs to be in the run position in order for the 12 volts to be at the harness?
If I find 12 volts probing the 2 larger wires at the harness with the engine running and a/c on I will know the fuses are good and at that point it’s either lack of pwm signal from the pcm or a bad fan motor.

[racer2086]

Nearly certain my wife's 2000 540i, which has the PWM fan is on when the A/C is on, just at the lowest setting. It's been super hot here lately, and I can hear the fan cranked up to max while in the car at stoplights, etc. The engine fan clutch also is engaged full bore at the time also. I would be surprised if the fan didn't turn on at all, ever, especially if the ambient temp was 90+. Is your fan clutch staying fully engaged to keep up with the cooling? That would be a reason why the A/C still "works" but is adding extra load to your cooling without the aux fan.

And if the dealer installed a brand new fan only about 1 year ago, I would take it back to them.