Automotive Air Conditioning Information Forum

I did a major r134a conversion to my 1989 325i to R134a

Rebuild compressor (came with 3oz PAG46 oil in it, said to add 3.75oz additional oil which I did to the rec/dryer)
New parallel flow condensor
New expansion valve
New rec/dry and pressure switch.
Some not all new lines. All new green o-rings
No work to evaporator.
Pulled 30hg vacuum for an hour, checked no leaks an hour later.

Known issues:
*the condensor fan is presently broken, new resistor coming soon*
*the expansion valve has not been re-insulated and is presently exposed (plastic cover not reinstalled), I have the correct insulation tape coming soon*

Available specs from the compressor / BMW call for approx 32oz R134. I added (2) 12oz cans then read the pressures with A/C on, interior fan on full speed.
@90degf/60% humid I see:
Static: 115psi hi 115 psi low
Idle: 250psi hi 50psi low

When I build revs to about 2000 and hold them there I see the hi side creeping up, up up then shut it down around 350psi. Would this behavior be caused by he lack of condenser fan? I am not sure how quickly the condensor 'heat soaks'. I am also not sure exactly how sensitive the expansion valve is to airflow around it (not being covered in insulation tape per the factory and missing its plastic condensate cage).

Thank you all for any guidance. I do a lot of work on my cars but A/C is a mystery!

Yes that high pressure is caused by lack of air flow/fan.

The heat load in the condenser and its ability to reject the heat is nothing like a radiator IMO. There is not that lag, the results are almost instantaneous. For example all of a sudden i see the high side pressure creeping up when it had stayed steady, then i feel a SLIGHT change in breeze outside, as soon as the breeze stops pressures immediately come down. If you started the a/c system and had no fans on the condenser at all, it would go from static pressure to hitting 400psi in no time flat-depending on heat load I would guess 5 seconds or so!

A clean condenser, good air flow (all fans working right/RADIATOR clean and no debris between condenser/radiator), and all shrouding MUST be in order before you attempt to diagnose the system further. Also if you are talking the sensing bulb that gets clamped to the exit side of the evaporator, its imperative for correct performance to have a CLEAN/TIGHT/INSULATED connection for best system response. You want the absolute best fullest surface area contact you can get between the sensing bulb and the pipe and insulated from outside temperatures.

Make sure when vacuuming/charging you are taking all steps possible to eliminate air getting into the system. Meaning if you don't have auto shut off valves on the end of your vacuum/charge line that you are hooking up the can of refrigerant and purging out the air thats in the yellow line from when you disconnected it from the vacuum pump. hook up the can and loosen the yellow hose to manifold connection slightly to purge the air out and make sure its solid refrigerant before charging.

Thanks very much - this gives me a lot to work off. I will get the condenser fan working before I run the system again.

Regarding the "sensing bulb" I was not talking about the temperature sensor that I believe feeds into the compressor on/off circuit, I was talking about the actual expansion TXV valve, as seen in the photo attached.

Regarding the sensing bulb-yes I know you were talking about the TXV. There are a couple style txv's. The one you pictured was not the style I was speaking of, I falsely assumed you had the other style-the one you have/your picture shows is the "block style". The one I was speaking of looks like this .

Anyway, since you have the block style others can comment on how insulating it can effect its performance-however I will tell you this-its not even close to as important on the block style vs the sensing bulb style which it is essential. Because the sensing bulb inside the block style is internal to the block.

I think they insulate the block style inside cars when they are not inside the evap housing to keep them from sweating and dripping condensate now that I think about it. Because the only time i've seen them insulated is when they are inside the cabin, but NOT inside the evaporator housing. With that said, i've seen them mounted in the engine bay side of the firewall, inside the cabin, inside the cabin but outside the evaporator box, and inside the evaporator box inside the cabin.

Thanks for elaborating. I was unsure what the bulb/disc on the exterior of the block style TXV does ( top side of attached pic) so I did insulate it with cork tape. It lives inside the evap box.