A/C significantly colder while driving than at idle

[tbrewster]

Needed to recharge my A/C system so I deciced to convert to r134a. We notice that the A/C blows very cold while driving the car but not so much at idle speed. The shop believes that the fan clutch is the culprit and it's not pulling pulling sufficient air across the condenser. Before I rush to replace my fan clutch I'd like to hear some thoughts on this.

I know r143a is less efficient than r12 but I did this conversion on several E28s and never encountered this problem.

Thanks for your inputs.

[Huff]

If you replace anything, it should be your aux fan. That's what moves air across your condensor.

[CorvallisBMW]

I have the same problem on my e34. I believe it has to do with the size of the condensor and compressor. When spinning at slow (idle) speed, the compressor designed for R12 doesn't have the capacity to do as much work as a larger r134a-designed compressor would. The condensor is also undersized and therefor works better at rejecting heat when it has lots of airflow, i.e. when driving at speed. The aux fan moves air, but not like driving down the freeway at 65.

[tbrewster]

Replaced the aux fan already and it's working fine. Perhaps the "not so cold a/c" was because the engine was not at operating temp. I'll have to see how it feels over the next couple days as I drive aroudn town.

[bmwstephen]

which fan did you purchase if you don't mind me asking

[CorvallisBMW]

Replaced the aux fan already and it's working fine. Perhaps the "not so cold a/c" was because the engine was not at operating temp. I'll have to see how it feels over the next couple days as I drive aroudn town.

Engine temp will not have any effect on A/C performance, they are separate/isolated systems.

[bmwstephen]

i thought the temp sensor for when the aux fan kicks in is based on the coolant temps?

[CorvallisBMW]

i thought the temp sensor for when the aux fan kicks in is based on the coolant temps?

The aux fan comes on automatically when the A/C is turned on, regardless of engine temp.

It will also come on if the car begins to overheat.

[tbrewster]

Engine temp will not have any effect on A/C performance, they are separate/isolated systems.

I was thinking that the aux fax was not yet on but forgot that it comes on when the a/c is turned on. Guess it's just a side effect of running r134a in a system designed for r12.

[jlevie]

A bad fan clutch won't help the situation, but marginal cooling on a R134a conversion when the car is idling or moving slowly is to be expected. The condensor and evaporator are undersized for R134a. One of the R12 replacements (Freeze 12, etc) will yield perfomance almost as good as R12.

[CorvallisBMW]

The more I hear about Freeze12, the more I like it. I've never tried it but most definitely will on my next car.

[bmwstephen]

Is freeze 12 backwards compatible with r134a retrofited system?

[jlevie]

Is freeze 12 backwards compatible with r134a retrofited system?

As far as I know, yes. Pull the R134a and charge with Freeze 12

[scottinAZ]

freeze12 is primarily R134a, with about 20% of R142b added. This is according to the can. At least in AZ you need to show certification to buy freeze12 (not a problem for me), so that could be an issue in your locale.
as for compatibility, legally, you cannot mix refirgerants. In practical useage, R134 mixes just fine with it, but you will lose some of the properties of Freeze12.

This link gives listings of the components of the different substitute refrigerants

http://www.epa.gov/ozone/snap/refrigerants/refblend.html

[DaveSmed]

Double check that the aux is in fact coming on when the a/c is switched on. There is a diode in the fusebox that likes to go bad and prevent that from working properly.

Hmm, open the can of worms that is hydrocarbon based refrigerants? Those work absolutely phenomenal in old R12 systems.... Though I can see a few E30s being blown into orbit by people trying to make a BBQ tank > R12 adapter.