Member
I'm having a weird AC issue that popped up twice this week.
Every time I drive with the AC on and my rpm passes about 2700rpm, my AC shuts off.
But when it's below the 2700rpm, it turns back on. When it's past 2700rpm constantly (when I'm driving on the freeway) the AC won't turn on. I can't figure out what the problem is...
It's a 1995 M3
Current cars:
87' Zinnorot 325is
91' Alpine white 318is
92' E30 Alpine white Mtechnic
97' E36 Alpine white M3/2/5
Member
Check system pressure.
My Drip Loops Are Better
This. Being that it's rpm related it sounds like a belt, pulley, or compressor issue. Start by checking pressure before replacing parts. It may be as simple as topping off freon.Originally Posted by marvinstockman
Member
It isn't about topping it off, it's over full, you're hitting the high pressure cutoff. Find a recovery system to use, pull it down, and weigh a required 825 grams of refrigerant in it and be done. Trying to set the system by pressures is just a shortcut to mediocre system performance and future issues like this.
Caprica Junkie
My Drip Loops Are Better
Makes sense. Pressure's building when spun faster.
Member
Actually upon a thought I'll concede it could be low pressure too, it's possible for a mediocre charge to run the A/C (albeit poorly) at low RPM, but then hit the low pressure cutoff when you spool it up.
I really should have asked for more info about the service history of the system before answering since it could legitimately be a couple things.
Caprica Junkie
Member
I'll have to look into it, thanks for the help guys!
Current cars:
87' Zinnorot 325is
91' Alpine white 318is
92' E30 Alpine white Mtechnic
97' E36 Alpine white M3/2/5
Member
So I had a shop look into the AC problem. After putting freon in and putting the correct pressure, the shop claims that I'll have to replace the low pressure valve on the AC? They said it'll cost me $480 with new parts?? Am I getting bent over?
Current cars:
87' Zinnorot 325is
91' Alpine white 318is
92' E30 Alpine white Mtechnic
97' E36 Alpine white M3/2/5
Member
The low pressure valve could be two things, first is the $20 low side fill valve which is right under the hood. That would be a bad price for that 15 minute job.
The second option, and what I suspect they mean, is the thermostatic expansion valve which is a whole different can of worms.
The expansion valve is located under the passenger side of the dashboard. Accessing this is... fun. It requires pulling the glove box and disconnecting the A/C lines where they hit the firewall under the hood. The lines can then be disconnected from the valve (where there is no room for tools) and the line that goes from firewall to valve can be backed out a bit (where it doesn't want to fit). Using more tools that don't fit anywhere you can then take the expansion valve out and replace it. It's a nasty job and at shop rates that price isn't that crazy I don't think.
Expansion valves rarely go bad though, they are destroyed by something else going bad. On my car, the compressor died and spread junk through the system. I'm not sure the valve was clogged but I replaced it anyway.
How much DIY are you comfortable with? If you have them recover the gas you can do the labor yourself (making sure to use all new O-rings, and might as well replace the receiver dryer while you're at it) to save the money.
The first thing though is getting the expansion valve out, if it's gummed up, you're going to have to flush goo out of the rest of the system which gets more involved.
Caprica Junkie
Member
Oh hey how about this... most basic RPM related problem to have with A/C... your clutch is going bad and disengaging at speed maybe.
The best diagnostic for any of this though is to get a gauge set on there and watch the pressures as you pull the RPM up past the problem point and see what happens.
Caprica Junkie
Member
bumping an AC trouble thread...
My AC is not working but the compressor seems to come on (my rpms drop at idle). Does idle drop of a few hundred rpm at idle with AC off prove that the compressor works? Why I ask is because my AC is not cold at all (no real difference between that air and ambient air). I added just a bit of refrigerant 134a (@79 degrees and 45 lbs on the low side open when adding 134a, but high side (open at high pressure valve, closed at gauge to read pressure - seems stuck at 25 lbs). I know not to charge from the high side. After putting in a little 134a so that low side reads 45 lbs with AC and car on, the high side pressure stayed at 25 lbs.
I disconnected the gauges and am now scratching my head. I checked all fuses and they seemed fine. I did not, however pull the 19 relay (19 right?) What does a fried relay look like? Blackened? Or can't tell till it's pulled? Thanks for any advice!
Member
Are you sure the high side gauge is fully connected? If your high side is not going up with the compressor active then you have a busted compressor. Although I would think in that case your low side would equal your high side, so it should've gone up to 45 if you charged up your low side to 45.
Member
high side was connected but the valve wasn't completely open. I'll try it again tomorrow. Thanks!
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