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Old 06-05-2012, 11:53 AM   #1
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Default Air Conditioning: low side too high. pressure switch?

OK AC Nerds, help me out here.

My miata (2001) a/c worked a month ago. After letting the car sit for a couple weeks, it doesn't work at all. $hit blows hot air.

I did a little diagnosis.

First I verified that the compressor does not spin with the vent fan and AC button pressed. The button lites up, both cooling fans are on.

I pulled the connector on the pressure switch near the low side fill valve and jumped it. The relay clicks and the compressor loads up the motor. I forget if the center of the compressor spins. I did not leave it long enough to test for cold air blowing out of the vents.

I then tried to jam a dye charge into the system but couldn't get the nozzle to seal against the fill valve because of space constraints due to intercooler piping.

I grabbed the recharge can with hose and gauge and snapped that on there and it showed the gauge in the red "WARNING" region. This is the low side--should be 15-20 lbs at 68F ambient. I think the gauge said around 100 psi.

...

So clearly something is causing the low side pressure to increase, but the factory service manual isn't really clear on diagnosing it. You can test the switch itself but that doesn't mean it is the problem or if something else is causing the problem.

Based on my inexperience, I'd guess something in the low pressure side is preventing flow into the compressor OR the pressure switch is bad and the system just wont cycle on and pump down the low pressure side.

halp.
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Old 06-05-2012, 12:21 PM   #2
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Pressure switch is probably bad.
The low side pressure switch prevents the compressor from running if the suction pressure is too low.
A high side pressure switch prevents the compressor from running if the discharge pressure is too high.
The pressure you show is about right for the static 134 pressure in a system without the compressor running.
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Old 06-05-2012, 12:37 PM   #3
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When you measured the low pressure side, was the compressor spinning?
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Old 06-05-2012, 01:04 PM   #4
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Quote:
Originally Posted by exexx View Post
Pressure switch is probably bad.
The low side pressure switch prevents the compressor from running if the suction pressure is too low.
A high side pressure switch prevents the compressor from running if the discharge pressure is too high.
The pressure you show is about right for the static 134 pressure in a system without the compressor running.
Here is what the FSM says. Is this switch on the high side line? The diagram is confusing to me.


Quote:
Originally Posted by skidude View Post
When you measured the low pressure side, was the compressor spinning?
No, I haven't tried that. I was somewhat concerned that there would be damage bypassing the pressure switch. If I get enough responses to the contrary I'll try it.

Incidentally, I do vaguely remember seeing nothing in the sight glass.
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Air Conditioning: low side too high.  pressure switch?-pressureswitch.png  
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Old 06-05-2012, 01:24 PM   #5
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I have nothing helpful to add to this since I know jack about A/C, but:
I've a crapton of 01 AC parts if you need anything.
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Old 06-05-2012, 01:33 PM   #6
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Sounds like a bad pressure switch because the pressure you are seeing with a/c off sounds normal.

The FSM diagram appears to be for a hi side switch.
The set of numbers below the sketch describe the size of the hysteresis band.
The ones above are where the dots are located.
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Old 06-05-2012, 01:56 PM   #7
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Quote:
Originally Posted by JasonC SBB View Post
Sounds like a bad pressure switch because the pressure you are seeing with a/c off sounds normal.

The FSM diagram appears to be for a hi side switch.
The set of numbers below the sketch describe the size of the hysteresis band.
The ones above are where the dots are located.
I guess that's what confused me. the top numbers give a range and then there's a hysteresis value (or range?!) on the bottom too? Plus the words under the bottom "range" that say "OR LESS".

I read it like this:
There is continuity from about 28-32 psi UP TO 427-483 psi. If pressure drops below those lower fuzzy (tolerance) values there is no continuity, it has to come up to 31.56-35.56 before there is continuity again.

Similarly, above 427-483 somewhere, there is no continuity. continuity returns somewhere 57-113 psi below that.
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Old 06-05-2012, 01:57 PM   #8
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IF it is a bad pressure switch, that's a big pain in my ***.

and wallet.

unless someone wants to loan me some recovery equipment.
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Old 06-05-2012, 02:01 PM   #9
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paperclip. i have a bunch, ill bring some home from my desk.
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Old 06-05-2012, 02:04 PM   #10
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fwiw- the 15-20 psi is for the low side when the compressor is running, otherwise the system tends towards equalizing pressure for both sides.
Quote:
Originally Posted by y8s View Post
...This is the low side--should be 15-20 lbs at 68F ambient. I think the gauge said around 100 psi.
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Old 06-06-2012, 10:12 PM   #11
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UPDATE after slightly more checking.

The pressure switch appears to be working. When it is plugged in, the fan comes on, the relay clicks, the idle... uh... ups.

For some reason, the compressor clutch doesn't engage and start pumping.

If I turn the car off and connect the R134a can and gauge to the low side, I can turn the center of the A/C pulley by hand and watch the gauge needle move slightly.

I suppose the next thing to check is the wires at the magnetic clutch.
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Old 06-06-2012, 11:06 PM   #12
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Old 06-06-2012, 11:13 PM   #13
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Did you jump the relay to see if the clutch pulls in? If it does, jump it while the car is running and see if the air gets cold. If it does, swap the relay and see if that fixes it. If it doesn't, it's most likely low enough not to allow the compressor to kick on.

Normally, with the car running and the compressor cycling you should see readings on the low side of between 20-50psi(factors vary) alternating with each cycle. When shut off, you could see as high as 70-100psi.
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Old 06-07-2012, 04:22 AM   #14
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I've seen the A/C compressor and condenser fan relays getting EXTREMELY corroded in least 3 different cars. Maybe that is your problem.
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Old 06-07-2012, 08:14 AM   #15
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Check for power at the AC compressor. If it has power, the clutch might be getting weak. In the Jeep world, we pull a shim out of the clutch and try again. Pulling a shim or two works about 50% of the time. If it doesn't, bend over and get a new compressor/clutch combo.

I'm actually having issues where the high side is at 150 (I kept charging it to get it up to 250 though in 90 degree heat) and the low side never drops below 50psi. Duct temps sit at 50 degrees and won't get any cooler.
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Old 06-07-2012, 08:35 AM   #16
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relay is activating, you can hear it click on. I'm thinking its the clutch.
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Old 06-07-2012, 10:10 AM   #17
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Search eBay. I had to replace the clutch on my Jeep, and it only comes as an assembly (with the compressor) from most places. Ebay had the "rebuild" kit with nose bearings and clutch for about 70 bucks.
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Old 06-07-2012, 10:52 AM   #18
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the relays are fine. one turns on the fan. I swapped the connectors and the other one turns on the fan. so there is power going through both relays.

I will stick a feeler on the clutch tonight or sunday and see how big my gap is. In theory, someone who removed their A/C would have a spare clutch and be generous enough to HOOK A BROTHER UP perhaps!
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Old 06-07-2012, 11:08 AM   #19
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would the one off lars's 94 fit?
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Old 06-07-2012, 11:54 AM   #20
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The clutch is shimmed- so you might determine if the magnetic clutch is in activating when voltage is applied. It may just be worn shims vs. a bigger issue. I just picked up a last gen NB compressor and bench tested the clutch with 12v.
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