Micro Filter causing a/c to malfunction?

[mossyoaks]

A/C works well until the car is shut off. Then won't come back on until car sits with engine off for at least 30 minutes.

Microfilter due for change (95 525iT w/160K) so wondering -before I do the change- if anyone thinks this may solve my problem.

Thanks.
Chip in SC

[joshua43214]

compressor won't come on? fan won't come on? indicator lights won't come on?

Los of air movement? no air movement? some air movement?

[mossyoaks]

Everything status quo...both green indicator lights (a/c and recirculation) on, fan blows all settings, compressor kicks in, air temp is frigid. A/c will blow all day long UNTIL the engine in shut off.

Then crank back up...and no indicator lights, fan blows all settings, but no cold air.

Let the car sit for 15 mins and everything is back to normal.

This has been going on for 6 months now.

Just annoying and of course now the SC weather is trying my wife's patience!

Hope this helps and thanks for any ideas~!

Someone mentioned a sensor somewhere???
But the microfilter would be the best first try?

Chip:D

[joshua43214]

the AC compressor is actualy controled by the DME on your car.

next time it acts up, verify that you can switch it to recirc, peer in the little grill on the panel and see if the fan in there is running. Make sure the rear defoger comes on. These 3 items are electricaly controled by the panel or the climate module.

If the compressor is not coming on, then the DME is seeing something that it does not like or is itself faulty(highly unlikely). Check the temp gauge, is it high when you restart the car? DME may think the car is overheated.

Most likely its pressure related, the compressor will not come on if the high side is over pressurized. This could be caused by the condensor fan not operating, verify the fan runs when the compressor is running. Also verify the fan comes on when you turn on the ac but it is not working. It is possible that the high side pressure is not bleeding off when the compressor is stoped, this is often caused by a faulty expansion valve, but could be faulty valving in the compressor, or both, or neither.

You can try bypassing the pressure sensor and see if the compressor operates. Do not leave the sensor bypassed, if the sensor is not faulty, it is doing its job and protecting the ac system. The sensors do go bad, if you have the single sensor with 2 plugs, you can swap it with out evacuating the r134a.

There is also an evaporator temp sensor that will turn on the compressor if the evaporator freezes over. This can happen with the engine off, once again often caused by a faulty expansion valve.

Needless to say, its very hard to diagnose pressure related problems with out hooking up a set of gauges. And to make things worse, most techs don't know how to imterpret the results beyond the basics. Most pressure related problems are caused by the expansion valve, since they are cheap and easy to replace, they often get tossed on to eliminate that possibilty.