1997 850 glt me4.4 ac compressor won't come on

[abscate]

If the compressor clutch won't pull in with direct 12 vdc it's either burnt out or the wiring is bad

You can also backside probe the signals while connected to see if the car is commanding AC on

[dbooth914]

Jumping in as this looks like a relevant thread.

-My AC quit last week. It was working fine until then, although I think the lights on the dash over the Recirc and AC buttons were flashing each time I started the car.

- I put 12v to the gray wire at the compressor and it comes on fine (did not bypass the onboard temp sw).

- I suspected the AC relay behind the glove compartment, so I replaced it. No joy (no AC)

- I have 12v at what I believe to be the low pressure switch (wire colors do not confirm) at the firewall (see pic).

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-12v on a blue wire into the switch, nothing on the grey wire out. Resistance test on the switch shows it's open, so I think I found the problem. So, I jumped the blu to gry with the connector off of the switch. No joy.

2, maybe 3 questions.
Is this the low pressure switch for the AC?
Is the low pressure switch normally closed when the AC is not running?
The wiring diagram seems to show that jumping the switch (AC Pressostat) should power the compressor (assuming the 12v on the one side).

Snap 2015-09-04 at 18.41.56.png (26.65 KiB) Viewed 1288 times

What am I'm missing here?
Broken gray wire from low pressure switch to compressor?

TIA

[Ozark Lee]

The "pressostat" is the low pressure switch and it is normally closed. Its state is based on system pressure, not whether or not the system is on or not. They system pressure that the low pressure switch will drop however when the system is operating but the switch is designed to shut the system down when the pressure drops below somewhere around 28 PSI (I don't know the actual set point but that is what I observe while servicing the systems and it is pretty consistent).

If you have no voltage coming out of of the switch you either have a low R-134A charge or a bad switch. The pressostat switches rarely go bad but they sell a lot of them since everyone likes to dream.

You really need to get a gauge on the system and see what you have for pressure. With the compressor off, on a toasty day, a pressure in the 100 to 125 PSI range would be more or less in the ballpark and would most certainly satisfy the pressostat switch.

The fact that the compressor doesn't engage when the switch is bypassed would indicate a wiring problem but there is a thermal limit switch on most of the compressors and it is a fairly high failure rate item. Most of us just wire around the thermal switch.

...Lee

[dbooth914]

I hot wired the compressor without bypassing the thermal switch and it came on. I had hoped for an easy fix, but the the thermal switch seems ok.

It seems like I have two problems.
Tomorrow I will buy a can of 134A and see if a charge kicks the low pressure switch to close.
I will also check for continuity from that switch to to the compressor.

Cheers.

[abscate]

Yes, it seems you have a break in the gray wire. You could run a wire from gray terminal to compressor and see if your AC comes back.

There is also a thermal safety on some AC compressors that is in the grey wire circuit in series..were you Possibly applying 12 VDC behind that ?

[dbooth914]

Ok, so
- I added some freon as the guage said I was at about 65psi @ 75F ambient.
- I checked the low pressure switch with my ohm meter and it was closed.
- Reconnected low pressure switch but no ac.
- I ran a wire from the blue side of the low press sw, and the compressor ran.
- I move the wire to the gray side of the low press sw, and the compressor ran, so
- I made the new wire a permanent fixture, replacing the grey wire from the press sw to the compressor.
- The compressor came on, but then started chattering or momentarily shutting off for a second, but was mostly on. I took the car out for a ride and the compressor quit running altogether.
- Jumping the connector at the low temp sw got the compressor to run again.

I now think my remaining issue (grey wire is good now) is that the low temp switch is failing intermittently, or the freon charge is incorrect.
I never got the freon to flow up to the suggested/indicated 35psi (compressor on) and had to settle for 27psi, but I had the exact same problem on my daughter's jeep. The cheapo pressure guage on the freon can from IDQ is suspect.

Thoughts?

[Ozark Lee]

You are still low on R-134A. Avoid hot wiring around the low pressure switch with a low charge since you don't get a proper flow of oil and you can ruin the compressor. You are looking for a pressure closer to 35 to 40 PSI with the compressor running.

...Lee

[abscate]

The pressure readings on AC aren't good guides unless you get the system to a known, equilibrated state. The engine has to run at about 1500 rpm to get the compressor to full capacity output, numbers at idle can be over a huge range.pressure with the AC off just tells you the presence of some R134A

[Ozark Lee]

The pressure readings on AC aren't good guides unless you get the system to a known, equilibrated state. The engine has to run at about 1500 rpm to get the compressor to full capacity output, numbers at idle can be over a huge range.pressure with the AC off just tells you the presence of some R134A

To be perfect, there is truth to that statement but life in the 4th decimal place isn't really necessary for a car air conditioner. The low side pressure differential between idle speed and 1500 RPM typically isn't much more than a few PSI if the compressor is good and the system has a charge in the ballpark. If you really want to be perfect you don't even look at pressure and you weigh the R134A in to an accuracy of a 10th of an ounce with a refrigerant scale or a charging cylinder (on the latter you need to pay attention to parallax error since it is a graduated cylinder that is scaled to convert liquid volume to weight). Even then you never can totally account for residual refrigerant in the lines.

In this case my bet is that he has an evaporator leak anyway and whatever he does it will be different tomorrow.

...Lee

[dbooth914]

After warming the can in the sun, I was able to get more of the freon to move. After a couple more charging sessions, I got the pressure up to 35psi with the compressor on. Done.

The low pressure switch is bad and I'll see if I can get one at the junkyard tomorrow.

I'll post back when the replacement pressostat is in place.

Thx!