What cause A/C compressor not engage?

[tr125]

Hi Everyone,
I just bought a 2006 X5 3.0I a month ago wit 112K.
The A/C system is not blowing cold air, because the A/C compressor is not engage, AUX fan is not turning either.
Previous owner told me that the system just got recharged. Compressor could be engaged by jumping 12v on it. Everything else works fine on the climate control units except blowing cold air.
Anyone seen this symptom before? Can this problem cause by evaporator temperature sensor or may be failure of the DME?
Thanks

[upallnight]

If you have a bad aux fan, a scan of the DME will tell you that you have a bad Aux fan. A bad aux fan may prevent the ac from coming on. The DME sends a signal to the compressor to engage the clutch. If the 50 amps fuse in the fuse box behind the glove box is blown that a dead giveaway that you need to replace the aux fan. Don't try replacing the fuse because 50 amps fuse don't blow for no reasons.

Either buy the right software and scan the X or take it in and get the X scan.

I hate to say it again, but if you're not a DIYer and have a great knowledge of automotive repairs, a used BM Trouble is not the right car for you.

[g300d]

Ugh. Yeah, try to scan it for codes so you have somewhere to start.

That diagram shows a lot of places to check. Evaporator temp sensor, pressure switches, and compressor relay.

I've not personally worked with this system yet at that level but diagnostics should be able to get show you actual values for these sensors and switches. An extreme improbable value would be a strong indicator of a faulty sensor/switch.

For example, I once was checking a Mercedes 210 that would not engage the compressor clutch. It had on-board diagnostics for the AC system and it showed one of the evaporator sensor's actual value as "0" (I forget if it was a pressure sensor or temp sensor, it's been a while). Replaced the sensor and AC function was restored.

[tr125]

Thanks for your reply.
I checked the 50 amp fuse. all of them are looking good, but it looks like none of those are assigned to the Aux fan. I have #61 light module #62 secondary air pump #63 ABS,ASC,DSC #64 blower. not sure if i'm looking at the right fuse box. but that is the only one i can see behind the glove box.

Is there a way to test the aux fan by applying a 12v on it? It looks like the diy aux fan replacement is not going to be difficult. what i'm worry about is something gets wrong inside the HVAC assembly housing which i have to remove the whole dash panels in order reach it.

[g300d]

Dont assume it's the aux fan just yet.

It could not be turning because the dme is not sending a signal to turn it on along with the compressor because of a system fault.

I would get it scanned for codes and get the actual values of all the involved electrical sensors.

[upallnight]

You can disconnect the power connector to the aux fan and use a test light to see if power is getting to the aux fan. There are three wires to the aux fan. Positive negative and a signal wire. The skinny wire is the signal wire.

Leave it to the engineers at BMW to take a simple circuit and make it complex with the signal wire to run on the fan. The signal wire receive a pulse signal from the DME to turn on the fan and control the speed of the fan. So the only way to turn on the fan is if you can send a pulse signal thru the signal wire.

[J.Belknap]

Here is an experience I have seen: aux fan fails, temperature rises, pressure rises, relief vent purges, refridgerant volume is low, compressor will not turn on.

[g300d]

True J., OP states system was recently recharged though?

OP, maybe you should confirm refrigerant level as well?

[tr125]

Originally Posted by g300d

True J., OP states system was recently recharged though?

OP, maybe you should confirm refrigerant level as well?

That’s what the previous owner told me. but anyway I just came back from a shop, the mechanic charged me $90 for a inspection with recharging the system, but compressor still not engage. He told me that may be there are something wrong with the controls, but did not say anything about the Aux fan. He told me to make an appointment to drop of the truck in order to do further inspections and not able to do any repair estimate yet. So I just wait until this Sat and hopefully I should know what is wrong.

[jcp240z]

Where are you taking it? If you are near the East Bay, there is a great shop here.