96' 850 AC not charging after holding vacuum

Post by **ZionXIX** » 07 Aug 2020, 21:49

I decided it was time to make a thread and generate some ideas. I recently replaced my compressor, drier and expansion valve. I took all the AC lines apart and replaced every seal I could find. I did my own vacuum down procedure before going to the mechanic for professional charging.

My original low side schrader valve was not holding vacuum when disconnecting manifold lines. It would make a "woosh" sound. I replaced with a new valve "4 seasons" brand from FCP. This valve had the same problem. Are these valves unable to hold closed against a -30inHg vacuum? Is the valve bad or the vacuum force too strong?

I've got 2 new valves in the mail.

My sources for r134a amount says 750g-825g for 850 models. We settled on 1.76 lbs for the total recharge amount.

Mechanic is understandably upset by the lack of highe side port. With the engine off we connected the machine and let it pull a fresh vacuum they started charging. It refused to go passed 0.24 lbs. Tried starting the engine and running the AC. Compressor would not engage. We gave up and vacuumed it back down.

The low side switch should kick the compressor when the freon reaches the minimum pressure. Ive heard of "jumping the pins" for the switches to get things moving.

Should I just it more time on the next try? Do I need to replace pressure switches? I suppose I could manually rotate the compressor while charging but that probably dangerous and impractical.

Open to ideas.

Azik09 · Post by **Azik09** » 08 Aug 2020, 01:41

ZionXIX wrote: ↑07 Aug 2020, 21:49 Are these valves unable to hold closed against a -30inHg vacuum?

Exactly. If vacuum is "stronger" than spring in the valve, it will open.
My mechanic charging system with nitrogen for leak seeking and testing compressor. When everything is ok, system is vacuuming and charging with refrigerant.

Post by **ZionXIX** » 08 Aug 2020, 07:04

I guess I never realized the valve was not designed to hold vacuum. Even still I slowly lose pressure over a couple of days but I also am using a harbor freight manifold set so maybe I'm losing some pressure there. Either way the valves are cheap so I'm replacing it just in case.

Does anyone know what pressure the low side switch needs to trip the compressor?

Post by **jimmy57** » 08 Aug 2020, 08:53

The low side cycling switch opens at 22-24 psi (compressor off) and closes at 42-44. I'm in Texas and when it is 95 or higher in my shop I never have any issue with the refrigerant charging in to the full capacity without ever running the system. I work on several vehicles with high side access and I use a scan tool on those to watch high side. Your 850 has a a/c high side pressure sensor and it will show that value in the parameter (PID) display set. The schrader valve inserts for a/c will have stronger springs to discourage them from opening from suction. I'm suspicious of a piece of the seal left behind from the original one. You might clean the opening before trying another one. Regardless, the system would take the full charge even if you didn't evacuate it (don't do this as it causes internal corrosion and the moisture left inside can also make it freeze at orifice tube). Things that can stop charging include plugged orifice tube and overcharged with oil.

Post by **ZionXIX** » 08 Aug 2020, 11:59

jimmy57 wrote: ↑08 Aug 2020, 08:53 The low side cycling switch opens at 22-24 psi (compressor off) and closes at 42-44. I'm in Texas and when it is 95 or higher in my shop I never have any issue with the refrigerant charging in to the full capacity without ever running the system. I work on several vehicles with high side access and I use a scan tool on those to watch high side. Your 850 has a a/c high side pressure sensor and it will show that value in the parameter (PID) display set. The schrader valve inserts for a/c will have stronger springs to discourage them from opening from suction. I'm suspicious of a piece of the seal left behind from the original one. You might clean the opening before trying another one. Regardless, the system would take the full charge even if you didn't evacuate it (don't do this as it causes internal corrosion and the moisture left inside can also make it freeze at orifice tube). Things that can stop charging include plugged orifice tube and overcharged with oil.

So I'm using the 4 seasons brand valve from fcp as its the only option they have. The spring is pretty flimsy.. The tube was clean but ill double check and I replaced with new orifice when I took everything apart. The machine just flat out refused to charge passed 0.24 lbs. So you're saying I should be able to fully charge the system without any involvement from the compressor or the pressure switches?

Post by **jimmy57** » 08 Aug 2020, 14:50

In hot weather I rarely have to run engine to charge. If it is a dual system like some SUVs have then I can get close to full even on those. I am talking about charging it through one hose/fitting with 30# tank on scale. It goes in faster if I charge through both hoses but I will get a lot more liquid blow out when I take off high side if I charge through that side. I find that I miss the full charge that way as the amount that stays in that hose is what was weighed. Low side will evacuate its hose once you run engine and low side pressure gets in the 20s. you have to use elevated engine speeds. It is standard a/c practice and is what Volvo has in their service info. The compressor pumping capacity is low at idle speed just like alternator charging amperage is below spec with engine idling. A stick, stake, pvc pipe, cardboard mailer tube, etc the right length between lower seat cushion and accel pedal and adjust seat will get a stable 1500 or so speed that is the speed suggested for this purpose.

Post by **ZionXIX** » 08 Aug 2020, 16:46

jimmy57 wrote: ↑08 Aug 2020, 14:50 Low side will evacuate its hose once you run engine and low side pressure gets in the 20s. you have to use elevated engine speeds. It is standard a/c practice and is what Volvo has in their service info. The compressor pumping capacity is low at idle speed just like alternator charging amperage is below spec with engine idling. A stick, stake, pvc pipe, cardboard mailer tube, etc the right length between lower seat cushion and accel pedal and adjust seat will get a stable 1500 or so speed that is the speed suggested for this purpose.

We definitely did not accelerate the engine beyond idle. I'll try that next time as well.

Post by **wheelsup** » 08 Aug 2020, 21:51

I swapped out my evaporator and did a fresh vacuum and fill myself.

So, for starters I used the HF gauge set. It's fine.
I also used the air powered vacuum pump. No, it isn't great, but it does remove most of the air in there, and more importantly will show if there are any major leaks. I don't believe a loss over a period of days is a big deal, at least big picture wise. You may have to top up every now and then.

I refilled using the small walmart r134a cans and a can tap. You hook it up to the center yellow line, bleed it, and then open up the low side valve.

The compressor should kick on a little bit, then slowly ramp up more and more as more refrigerant gets into the system. If it's not working for you, either warm up the can or just do what I did and turn it upside down - it will dump liquid refrigerant into the system but I believe it goes into the accumulator first and then to the compressor in our cars so locking up risk is minimal. Just kinda move it upside down, then right side up, then upside down for a few secs (or more) etc then back upright.

There is no high pressure that we can hook up to IIRC.

There is a weight involved (printed on the strut mount area I believe) but I ignored it. If using non-OEM stuff the size may be different. Instead, I chose to fill by low side pressure and to prevent the ac from cycling at higher RPM.

Make sure your clutch gap on the compressor is OK too.

When topping off over the years, or even from zero charge, I've never had to accel the motor. That shouldn't be what is causing your issue.