Member
Working on a '91 850i.
The problem: No cold air blowing.
What is happening:
1. Initially while the car was shut off, the pressure was in the red (over-filled with 134a).
2. I started the car and the pressure reading on the gauge moved down to zero. (Compressor kicks on and everything.)
3. Turned the car off and the pressure goes back up to normal levels.
4. Started the car up, and turned the A/C back on...compressor kicks on and the pressure goes to zero again.
What gives? Anybody have an idea what this might be?
Notes:
-The compressor comes off and on like it's supposed to.
-All other functions on the car works fine, heat, defrost, re-circulation, yada, yada, yada.
Last edited by Cup1d; 09-07-2017 at 08:54 PM.
Member
What pressure are you reading? High side (small lines) or low side (larger line).
Are you using one of the consumer A/C system refill cans with a gauge?
The system low side pressure (refill schrader valve) can get quite high (90-150 psi) when the compressor is not running. This is NORMAL, so depending on what you are using to read the low side pressure, the pressure will be higher than normal running pressure for the consumer refill can gauge when the compressor is not running.
A/C System function: the compressor produces a high pressure gas that is turned to a high pressure liquid in the coils in front of the radiator. The high pressure liquid in the small hot line goes to the expansion valve where the hot freon is turned to a cold low pressure gas by the expansion valve which cools the coils where the A/C blower circulated the air inside the car and then the compressor draws the cold gas out of the cooling coils through the large low pressure line where it compresses it back into a high pressure gas.
Beyond that:
If the system is charged and the compressor is running:
Low side pressure should not go below 10-12 psi. If the low side pressure approaches 0 psi that would indicate there is no Freon gas returning to the compressor from in the cooling coils. That is typically due to an expansion valve not functioning properly or the high side line to the expansion valve is plugged. I once had a bad receiver/dryer that failed and completely filled the line to the expansion valve with desiccant.
High side should be above 150 psi all the time when the compressor is running.
R-134 & R-12 are both very sensitive to temperature so the actual high side pressure (200-350 psi) will be largely dependent on the ambient temp.
When the compressor is off the Low and High pressure lines will equalize after a few minutes hence the low pressure side will increase to 90-150 psi and the high pressure lines will come down to 90-150 psi .
Hope that helps.
Member
Yes. I'm using one of the consumer A/C system refill cans with a gauge. Are you saying that it's fine for the pressure to be (essentially in the "red" on the consumer gauge) while the car is off?
That's where I was thrown.
It definitely drops to zero when I start the car and turn on the A/C like there is nothing in the system.
If this is the case and it's fine, I'll be professionally embarrassed!
Last edited by Cup1d; 09-04-2017 at 06:04 PM.
Member
You're most likely undercharged. Tough to assess without proper gauges. IIRC at 70 deg F, R134a should be about 70psi with the system off.
Brett
Member
Just walked back in. Holy cow. I *was* still undercharged. @ltbrett and @m6bigdog, thanks for your responses. I'm still on the low side, but if the refrigerant leaves within the next few weeks, I'll have to go back in with the dye to chase the leak. If I'm going to do all that, I'll invest in a kit to do this properly.
I'm thinking:
https://www.amazon.com/XtremepowerUS...+gauges+vacuum
Mad Scientist
More often than not, the leak is one of the Schrader valves..
'91 Dinan 860 Stage III (new 6L engine)
'91 Dinan 850 TT stage III (brand new engine) 21st Century Tech meets 18th Century Dinan...
'91 850i 6sp (mint) (sold)
'90 Dinan 750iL TT stage III (Guido - The Beast)
'94 850 CSi The Detroit Auto Show car (restored to factory perfect) (sold)
'96 850Ci, The George Carlin car
''73 3.0 csi, '08 535i, '03 X5 4.6is
...and a few other non BMW cars
Member
Remember that by bleeding off the pressure in your system until it read in the green you probably let out almost all your refrigerant. You'll have to check the proper charge for a converted R12 system, but I think it is about 4 pounds.
Brett
Member
Yep, the consumer charge gauges with the can are only functional when the compressor is running.Originally Posted by Cup1d
I anticipate when you bleed off the Freon to get the pressure into the Green with the compressor off you have just discharged all the liquid Freon in the system, hence the system is grossly undercharged.
It will take nearly a full charge 1550 g or 54 oz. to get the system refilled.
I don't know if any of the 8's came with a bubble sight glass and if any did the R-12 systems would have had them, which are a good indicator of a system sufficiently charged.
A good way to make sure the system is charged correctly is to use a thermometer inserted into the center discharge vent. Set the the blower and A/C temp to max with the windows down or doors open.
The thermometer temp should be close to 44 deg. F and the low side pressure should read the correct pressure for the ambient temp or with a bubble sight glass there should be very few visible (single) bubbles.
Member
Yep. Total misunderstanding on my part. I ended up grabbing a can last night and sure enough, she cooled off quite nicely. It's on the low side so I'm headed out now to get a small can to top it off. The temperature outside was cool last night so that probably helped the car out quite a bit. It's getting up in the 80's today so we'll see now it goes.
I'm just happy that it actually works!
Rule #1. (From my wife). It *must* have A/C.
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