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Discussion Starter · #1 ·
Hello all, newbie here.

I’m having a problem with the climate control on my 540. While driving with the temperature setting set for cold air (between 60 and mid 70’s), and the snowflake button activated, sometimes the air goes warm; not as warm as if the heat were on, but as though the A/C were not on, and ambient air were blowing in through the vents, warm Texas air. This phenomenon occurs in a variety of conditions: while idling in park, while driving around town… the only time it doesn’t seem to happen is when I am at highway speed. In fact, when I get off the highway and begin driving through town, the air goes warm within a minute, nearly every time.

I have tried various things to force the air cold again: manipulate all of the system controls (one at a time), turn the climate control system off and then on again, cycle the compressor (snowflake button)… Sometimes these attempts seem to force the air cold again, but not with any consistency; sometimes the air returns to cold on its own, without any input from me. The one input that has worked with some regularity is raising the RPMs, whether by holding down the accelerator while parked, or shifting into a lower gear (automatic). This seems to be a clue…

A few weeks ago I removed the Aux fan, which seems to be operating properly from what I have read, i.e. it comes on when a few moments after the compressor comes on. I removed the debris from behind the fan and around the radiator. I measured the fan voltage with the compressor on (when the fan should be on) and with the compressor off (fan should be off). There are three wires going to the fan: one is ground, one is power, and one seems to be control:

With the compressor off: power wire measured 13.8 vdc, control 3.3 vdc
With the compressor on: power wire measured 13.8 vdc, control oscillated between 2.4 and 2.6 vdc (PWM control?)

Sorry for the epic length, but I am facing a hot summer and need help. Anyone have any ideas?

Thanks for reading.

Tommy
 

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Is your outside temperature sensor working? Do you get the outside temp reading on your dash or does it drop to -40 steady or occasionally? If so, that is probably your problem.
BTW, welcome to the forum!!
 

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could it just be low on refrigerant gas? higher RPM will pump the remainder through faster so this could be the reason... other thing to check it the temp of the cold pipes leading to the cabin - they should be icy cold, if they are at idle and the air is not cold, could be bleeding warm air in from the heater due to sticky valves or incorrect temp sensors as mentioned above.
 

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Discussion Starter · #4 ·
Thanks for the replies. Outside temp sensor is working properly. I'll check the return pipes when i go out today.
 

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Discussion Starter · #6 ·
When I pulled the front bumper off to remove the Aux fan, I disconnected the outside temp sensor, and the reading on the dash was -40F. I would first verify that the sensor is connected. The sensor is mounted on the underside of the car inbetween the right front tire and right fog light.
 

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My outside air temp reading goes directly to -40. How do I fix this?
Thanks,
Spike
Probably it's broken or you've completely lost it , but it's easily replaceable and cheap. Oh, it's next to passenger side front wheel in the bumper, look for a small black pin.

 

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OP, have you manipulated the little knob on the vents that has the blue and red dots? That is also a temp control.
 

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Discussion Starter · #9 ·
Yeah, I've tried playing with the knob on the vents. What makes this problem difficult to troubleshoot is that it appears and then disappears on it's own, so I never know for sure if the action I just took really made a difference.

Armstm0 mentioned the refrigerant level might be low... I will check this next. In the meantime, days are getting hotter.

Thanks to all you have replied. I'll post the fix when I figure it out.
 

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Discussion Starter · #10 ·
Troubleshooting Update

Thought the Board could use one more A/C related post.

Restatement of symptoms:
-A/C blows warm, while idling or at low speeds, sometimes
-A/C blows icy cold at highway speeds, always
-Engine temp gauge centered or slightly below centered, always

With climate control set to MAX:
-When blowing cold at idle, A/C low-side pressure good - around 50 psi
-When blowing warm at idle, A/C pressure gauge needle pegs above 90 PSI

Aux Fan issue, right?

Right. So I built a test harness to allow me to monitor the PWM control signal sent to the Aux Fan with a multimeter (my meter measures duty cycle) while it is operating.

-When A/C is blowing cold at idle, PWM duty cycle is 25%,
-When A/C is blowing warm at idle, PWM duty cycle is 15%

I can clearly hear the fan speed increase as the duty cycle is greater. Fan seems to be doing its job (now I won't need to take out a second on the house to replace the fan).

I have read in other posts that the PWM control signal for the fan is generated in the DME and is based on inputs from the Coolant Temperature Sensor, the A/C Pressure Switch, and engine speed.

Questions:

1. Does anyone know if these are the only inputs on which the Aux fan PWM duty cycle (speed) is based?
2. Where is the A/C Pressure Switch located and is replacing it a feasible DIY?
3. Does anyone have a diagram of the entire A/C system, including control?

Thanks in advance :cry: <--- Sweating, not crying
 

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Discussion Starter · #11 ·
Just came from the mechanic...

I explained the symptoms I've posted here and explained the troubleshooting I've done, and my mechanic (indy) said the Aux Fan "final stage" was going bad and the fan has to be replaced.

Does this sound right? I would hate to spend $$$$$$$$ on a new fan and it not fix the problem.

Any advice? Anyone?
 

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The "final stage" is a resistor under the dash. I'm not sure why a bad final stage resistor would necessitate fan replacement -- the final stage resistor can be replaced independent of the fan itself.
 

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Discussion Starter · #13 ·
He said the final stage within the fan motor was going bad. From what I have read in other posts, the final state resistor has to do with the blower fan. On my car, the blower fan is working properly.

I trust the mechanic's ethical integrity, but I would like a second opinion...
 

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Thanks

I found that the sensor was completely missing. Now that I replaced it, the AC will run on max. It blows really cold on the drivers side but, The passengers side doesn't get nearly as cold. It's almost as if more outside air is being mixed with the passenger air to keep it from blowing as cold. Am I smoking crack or is there a shutter or switch in a plenum somewhere that is stuck open to allow more ambient air into the passengers side? Thanks for all of the help with the Ambient air sensor. I know that the indy that I had been taking my TT to wouldn't have ripped me off like the stealership would but, you guys saved me whatever it would have cost at the indy. Thanks
 

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Spike,

I've read that a cold drivers side and warm passenger side is a symptom of a low refrigerant charge. But I can't confirm that with personal experience.
 

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I confirmed this with my indy shop. I added a few ounces of freon and, now you could hang meat on either side of the car. I'm amazed that this car is smart enough to keep the driver cool when the freon is low. Thanks for your help
 

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Mine is always at -40...does this mean my temp sensor is not workin or disconnected? My AC seems to run fine - plenty of cold air.

papa :)
 

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Mine was fairly cold when the ambient air sensor was at -40 I just couldn't set it on MAX. I had noticed that the passenger side wasn't as cold as the Dirvers side. I originally thought this was caused by the sensor being broken or missing. After replacing the sensor the inside temp wasn't any colder on the passenger side but now I could turn the system to MAX. After reading the threads and learning I was a little low on freon, I put in a few ounce. Now, as I said above both sides are the same and, you could hang meat in there. If you don't live in a hot place, you might not notice the difference like I did. My wife happened to be driving one day and, it didn't feel like my side was as cold as hers.
 

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He said the final stage within the fan motor was going bad. From what I have read in other posts, the final state resistor has to do with the blower fan. On my car, the blower fan is working properly.

I trust the mechanic's ethical integrity, but I would like a second opinion...
Status?
 
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