Help Solving Weird AC issues

[Rx7toAudi]

OK so i have been battling AC issues for a couple years and have finally decided to get the right tools and fix whatever has been going on. A little history

The car had an AC leak (according to a shop i took it to when it started blowing hot there was no freon left). I dunno if i buy it but they vacuumed and replaced the valve stems in the quick connect locations and recharged it according to spec. Blew cold for a couple of days. Then it started doing what it has been doing for two years. It gets cold at idle but the air blows warm as soon as you start driving. Opposite of most people's experience. If i start the car and let it idle with the AC on it will blow cold for as long as it is running and the RPM's aren't raised. I have some readings and stuff, tell me what you guys make of it.

1. Car off low side 150 and high side 150 PSI

2. Car running and at idle and AC blowing cold
Low Side ~42 PSI, High Side 225 PSI

3. Rev engine for a few seconds low side climbs slowly to about 55-60 PSI, then shoots up to 150, high side goes inversely lower and pressures go back to
Low Side 150 High Side 125-150 PSI

4. Pressures stay the same as hot air conditions when AC turned off during hot air blowing (150,150)

5. If i cycle power to AC with on off button at idle, pressure on low side drops back to 42 and high side rises to 225 and it will blow cold air again

6. If I cycle power while it is revved up low side pressure will drop to 125 for a couple seconds, then rise back up to 150 and ac wont blow cold air or drop pressures back to regular levels.

7. If i let it go to idle but don't cycle ac switch pressure on low side will still stay at high pressures and AC will blow warm (i used to not have to cycle the switch for it to blow cold at idle it would do that by itself but now i have to turn the power off and back on again)

I have scanned with Vag-Com with no errors being reported in the HVAC system. The compressor is on at all times that the AC power is on, even when the pressures shoot up. Clutch disengages on the compressor normally when AC power button is turned off.

I am thinking this has to be an electrical switch or something since i can cycle power and it starts to correct itself and blow cold if at idle type RPM's. However it does seem oddly mechanical too with how the pressures shoot up directly proportional to rpms. Thoughts? I wanna be sure i'm on the right track of what to fix before i open up the system and lose all this wonderfully expensive freon. Thanks guys

[getslideways]

My HVAC knowledge is a little old, but until I brush up on it, here we go (AKA take all of this with a grain of salt haha):

The pressure spike to 150 would be occurring when the clutch disengages correct?

I am wondering if your pressure switch (which i believe is on the top side of the passenger side of the condenser) is seeing pressures it doesnt like and overriding the system.

The high pressures on both sides i believe is normal when the compressor is off, as both sides equalize, so they should read pretty much the same while the clutch is not engaged. You should normally be seeing something around 40psi on the low side when the compressor is running and 150-200 on the high side while the compressor is running. Based on your description it looks like the pressures start creeping up to 60psi on the low side while the clutch is engaged and perhaps the pressure switch disengages the clutch as a safety precaution.

I am wondering if the system is overcharged and needs to be bled out a bit

[Rx7toAudi]

Hmmm good points. It seems to me that the clutch stays engaged even when the low side goes all the way to 150. I'll go back and double check that tonight and report. It would make sense that cycling the power would make it go back to normal pressures and run cool if that reengages the clutch. I had a professional shop charge it according to specs but maybe they did overcharge it. Would that be a symptom of pressures climbing relevant to rpm? The pressure will never spike until engine rpms reach about 2k an above

[MacDaddy]

I think its overcharged, find a shop that will vacuum and charge by weight, not pressure.

[getslideways]

I would suggest trying to lower the pressure first so the lowsided is closer to 35 vs the current 40+ at idle and see what happens. If that doesn't do it it may be something with the compressor and pressures pushing past seals or the overcharge is pushing liquid where its not supposed to (as normally faster rpm will make the low side read lower not higher so that's what's got me confused)

Either way let us know what you see, and a big plus for using gauges when trying to diagnose, that is a HUGE help. As noted by macdaddy, I wonder if the shop charged by pressure or by weight, as Temps can alter the ability to do it by pressure accurately.

[Rx7toAudi]

They charged me for amount of pounds of freon put in and told me how many pounds went in. That being said maybe they just quoted that for cost purposes and then went by pressures when actually doing it. I didn't have time to check the compressor actions in relations to the gauge pressures tonight. I'll get on that tomorrow and let You guys know. It does respond normally by dropping psi for a second on the low side when applying throttle but then starts climbing and shoots to 150 quickly after creeping up to about 60 psi gradually

[Rx7toAudi]

OK, i finally got some time this morning to do some more digging. Here is what i have. some very different numbers and behaviors, but then it went back to what i posted above eventually.

1. compressor off, high side 74 low side 84

2. compressor on low side 78 high side `75

3. revving engine to about 3k low side 60 high side 100

4. blip the throttle for a couple seconds and back off and let it drop back to idle psi on low side rises to 60, stays there a few seconds, then drops slowly back down to 35.

5. holding at rev it shoots past 60 all the way to 100 and high side equalizes at 100, but compressor is still on


Nothing to revelatory about those readings. Different PSI's but similar behavior . It was more stubborn to get it to drop down and start cooling again but eventually i could get it. I would sometimes have to cycle the compressor on and off several times and have varying rpm's for the pressures to go to normal and cool the air again.

Biggest thing out of all of this is compressor stays on across all conditions unless i cycle it off. Even when low side pressure is very high. So the pressures arent rising due to the compressor cycling on and off

I wonder i i have a clogged orifice tube, or maybe a bad expansion valve? Possible explanation as to why pressure climbs relevant to RPM? Like not enough freon can be moved or pushed through the system fast enough? That seems weird though that when the system finally decides to work that it can shoot those pressures down quickly. A clog would be pretty consistent that it would keep pressures built. Maybe a sticking expansion valve? I feel like i read somewhere that b5's don't have expansion valves but that was quite a long time ago so maybe i'm wrong.

Sorry for the long post but any thoughts? I'm usually pretty good about mechanical stuff but i am stumped on this

[Rx7toAudi]

I found this chart, thought i'd post in case it is helpful, but it doesn't necessarily address the action of the AC and it's connection to the RPM's etc. What do you guys think?

[2000s4_m]

OK, i finally got some time this morning to do some more digging. Here is what i have. some very different numbers and behaviors, but then it went back to what i posted above eventually.

1. compressor off, high side 74 low side 84

2. compressor on low side 78 high side `75

3. revving engine to about 3k low side 60 high side 100

4. blip the throttle for a couple seconds and back off and let it drop back to idle psi on low side rises to 60, stays there a few seconds, then drops slowly back down to 35.

5. holding at rev it shoots past 60 all the way to 100 and high side equalizes at 100, but compressor is still on


Nothing to revelatory about those readings. Different PSI's but similar behavior . It was more stubborn to get it to drop down and start cooling again but eventually i could get it. I would sometimes have to cycle the compressor on and off several times and have varying rpm's for the pressures to go to normal and cool the air again.

Biggest thing out of all of this is compressor stays on across all conditions unless i cycle it off. Even when low side pressure is very high. So the pressures arent rising due to the compressor cycling on and off

I wonder i i have a clogged orifice tube, or maybe a bad expansion valve? Possible explanation as to why pressure climbs relevant to RPM? Like not enough freon can be moved or pushed through the system fast enough? That seems weird though that when the system finally decides to work that it can shoot those pressures down quickly. A clog would be pretty consistent that it would keep pressures built. Maybe a sticking expansion valve? I feel like i read somewhere that b5's don't have expansion valves but that was quite a long time ago so maybe i'm wrong.

Sorry for the long post but any thoughts? I'm usually pretty good about mechanical stuff but i am stumped on this

If an expansion valve goes wrong it usually shows in the low side, example: high side rising and compressor cutting due to excess high side pressure and lower side drops and some times the gauge needle starts bouncing a little. This because compressor works and expansion valve wont let gas to cycle back to low pressure side

The way your system is behaving i would guess is a little valve that goes on the back (inside) of your compressor or the compressor itself. That because there is enough gas on lower side for the compressor to take and its not doing it


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