: A/C fail. Vacuum?



Rob Benham
06-30-06, 12:14 AM
Hi, my (almost) perfect 95 Fleetwood Brm's A/C failed today. The compressor is not running at all, and the pressure on the low side was way round into the red.:mad:

I had topped it up a few weeks ago and it was fine. When it failed, it was again okay for a few mins, then failed again. It did this a few times.

A clue. The park brake does not always come off...er, so to speak...and I wondered if it was a lack of vacuum failing to acctuate the louvers etc. But, this may be a red-herring, so any ideas are welcome. Rob

N0DIH
07-03-06, 12:27 PM
You can't take any reading for it when the compressor isn't running. They are meaningless, as the low side gauge is reading the inlet side of the compressor, so no run, high pressure. That is normal. Same with high side, when compressor is off, low side pressure = high side pressure. It equalizes through the orifice (the hissing you hear when you shut the car off)

Vacuum system isn't required to make the compressor run. Freon high enough to trip low pressure cut out switch (it trips like 5 psi or less, very low, which you have plenty of), high pressure cut out is not tripped (like >500 psi/34 Bar) power to the compressor (check fuses) and a command from the dash control to the PCM (AC Request) and if the PCM approves, PCM then commands the AC compressor to turn on.

I often trip the low pressure cutout to see if low freon is the cause, 90% of the time it is, at the accumulator. This is the silver cylinder that is near the power steering resovoir. Unplug connector and short wires with AC turned on. If it turns on, that switch or the lack of freon is the answer. I have never seen that switch fail. Just freon being low.

The AC system is uber simple.

Compressor compresses freon into the heat exchanger in front of the radiator, we call that the condenser, which is where the gaseous freon goes under pressure and condenses into a liquid. But we can pump that compressor all day long if there is no restriction, we won't build pressure, so after the condenser, we have a restriction called the orifice. It has 2 filters, pre and post the restriction. It is around $7 to replace and it replaceable. In the liquid state, freon gets HOT giving off all of its heat. AFTER the restriction, the freon gets uber cold. This is because it is now after the restriction and it is seeing the suction or low pressure side of the compressor. So, being it is now uber cold, we head off to another heat exhanger, called the evaporator, which is where there freon goes from a liquid state to a gas state, and when that happens the freon gets super cold, and we blow air past the evaporator and cool our cars' interior off. We send the evaporated freon to the accumulator to go trough a dessicant bag to help ensure no moisture is in the system (bad for AC!) and if any non evaporated freon should get trapped here prior to getting into the compressor again. If liquids get into the inlet of the compressor, things break. This is how overfilling an AC is bad. The compressor will hyrdolock and shatter the piston.

I recently flushed my whole system, cleaned the orifice and vacuumed down, and have the absolute best AC I have ever seen. This thing blow ice cubes! I have a 94 FWB.

As for pressures, do NOT use a low side only gauge to judge system full or not. That is bad to do. Read my thread on it. Very bad.

http://www.cadillacforums.com/forums/rwd-19xx-1985-deville-fleetwood-1985/76120-ac-problems-fixed.html?highlight=evaporator+accumulator

I picked up a high/low side gauge from Meijers for $20. It has an adapter for either line and the gauge internally switches from high to low pressure. I use the freon can cheapie low side gauge and this as my high side and it really is well worth the $20. That and my vacuum pump are 2 very worth while tools in my box.

All my cars that are running have fantastic AC.

If you need proper pressures, let me know, I have a 94 FSM so I can tell you. It isn't a generic chart to use, it is very specific for each car and the temp and humidity outside. No guessing! You also have to have all doors and windows open when checking AC pressures. They vary alot.

Rob Benham
07-03-06, 11:32 PM
Thanks for your very comprehensive post and link N0DIH.

As you may have seen, after a couple of days with no response, I posted in the North* forum as well. I have spent a lot of time in that place cos of owning a 94 SLS !!

Your detailed description has led me to believe that fluid or oil reached the pump, as this morning there was a horrible mechanical bang on start-up. I guess that I'm in for some big $.

I would appreciate the graph/ tables for the temp corrections when you have time, as I will have to do most of the work myself. Do you have any recommendations for a vac pump? It seems that the vac pressure does not need to be very great...I have heard 23" of WATER! That seems very little, but I have yet to confirm this.

N0DIH
07-04-06, 11:50 AM
I had a Ford Aerostar with a broken input shaft, clutch would engage and run, but no compressing. If the compressor was lunched, the clutch would still try to engage. So hope isn't lost yet. I wouldn't rush into replacing a compressor if it doesn't kick on, it might be (likely is) something else that isn't not engaging it. Check power on the low pressure cut out switch, you should see +12v on one side of the switch when AC is requested on (and PCM actually approves the request!), and the other side is the clutch. So you should be able take the side that doesn't have +12v on it and ohm it to ground to see if the clutch windings are ok.

I pump mine down with a custom hose off the engine (a large vacuum pump! Made from an older R134a hose that I cut the can end off and just plugged a vacuum Tee into) to around 18-19 inches, whatever it will do. Then I pinch the line with some vice-grips and then hook on my hand pump and shoot for 25. I can't really get much more. But that is good enough in my book. Moisture will boil and expand at a great rate under a vacuum (like space, water boils is exposed to outerspace), so the engine will see 99% of the moisture. If you hit a spot pumping it down that it just won't go past a certain point then suddenly does go farther, that was moisture and you just got it out.

Mine was making some hideous noises at cold start, then went away, what I found it was high pressure on the high side, going as high as 34 bar (roughly 500 psi) and the high pressure cut out was kicking in. It even blew some freon out some of the lines at the compressor! Sounded like a air grinder going off and would stop when the compressor kicked off.

Well, as for a chart, it is pretty long, what is your average humidity there? Humidty has a effect from 60% relative humidity and higher.

Here is 20% humidity:

Temp......Low Side Max.........High Side Max.......Max right side outlet temp
70F.........23 psi...................190 psi................42F
80F.........30 psi...................250 psi................50F
90F.........35 psi...................300 psi................54F
100F.......37 psi...................330 psi................57F

Here is 30% humidity:

Temp......Low Side Max.........High Side Max.......Max right side outlet temp
70F.........23 psi...................200 psi................42F
80F.........30 psi...................280 psi................51F
90F.........35 psi...................310 psi................55F
100F.......37 psi...................335 psi................58F

Here is 40% humidity:

Temp......Low Side Max.........High Side Max.......Max right side outlet temp
70F.........23 psi...................200 psi................43F
80F.........31 psi...................285 psi................52F
90F.........36 psi...................320 psi................56F
100F.......43 psi...................370 psi................64F

Here is 50% humidity:

Temp......Low Side Max.........High Side Max.......Max right side outlet temp
70F.........23 psi...................200 psi................43F
80F.........33 psi...................300 psi................56F
90F.........39 psi...................340 psi................60F
100F.......47 psi...................385 psi................69F

Here is 60% humidity:

Temp......Low Side Max.........High Side Max.......Max right side outlet temp
70F.........23 psi...................200 psi................43F
80F.........36 psi...................315 psi................56F
90F.........43 psi...................365 psi................64F
100F.......55 psi...................375 psi................78F

Here is 70% humidity:

Temp......Low Side Max.........High Side Max.......Max right side outlet temp
70F.........26 psi...................240 psi................46F
80F.........38 psi...................325 psi................58F
90F.........47 psi...................380 psi................67F

Here is 80% relative humidity

Temp......Low Side Max.........High Side Max.......Max right side outlet temp
70F.........30 psi...................260 psi................49F
80F.........40 psi...................340 psi................60F
90F.........49 psi...................380 psi................72F

Here is 90% relative humidity

Temp......Low Side Max.........High Side Max.......Max right side outlet temp
70F.........32 psi...................275 psi................52F
80F.........41 psi...................345 psi................62F

Remember, to have all doors windows open and on 60F (AUTO MODE) on climate control (max, high blower) when doing all pressure measurements. ENGINE RPM @ 2000! GM recommends a high volume fan placed in front of the radiator for non V08 cars (aka, mech fan), still a good idea for any car to have it.

This is for R134a, 1994 Fleetwood, I would expect a 93-96 all to be close to the same, even for Roadmaster, Caprice as well, except wagons, they will very likely be different as there is much more interior volume, so AC freon capacity will be different (larger evaporator and condensers most likely, so more freon and more pressures). The 93 may be an R12 system, use this chart and the amount of R134a in a 94-96 for a R134a conversion. Do not overfill freon to get it colder, it won't, the system is designed for that amount and won't get better if you stuff more in. To improve cooling system performance, you need to increase the airflow across the heat exchangers and have system to proper freon levels.

Rob Benham
07-04-06, 12:14 PM
Thanks again, will post when I've had a chance to look.

Being a 5.7 ltr I tend to use this as a luxury only car...


BTW, I have got a set of central house gauges...can I get adaptors to make use of these, or is it a different ball-game?


Thanks again Rob