AC repair - Location of AC low pressure cut-out switch

[efan10]

Hi folks, I own a 2004 Vibe that currently has a broken air conditioner. I can confirm that the compressor clutch is not engaging under any circumstances. It is also not seized up because I can spin it with my hand. So far, I replaced the relay and checked the fuse so both of those are good. At one point I jumpered the relay because the internet suggested it, but it didn't make a difference to the clutch. I also unplugged the cable going to the clutch and verified that it is energized when I try to run the AC so there aren't any other odd electrical issues. The final relatively simple option is that I either don't have enough refrigerant in the line so the low pressure cutout switch is engaged, or the switch is bad. So I'd like to get a pressure reading and try recharging the system before going further. However, I'm up against the classic circular problem that I can't get a pressure reading or recharge unless the clutch is engaged and the AC is running, but its possible that the root cause of why the clutch isn't engaging is that the pressure is too low. The way I understand to get around this, assuming the pressure switch is part of the problem, is to bypass the switch. Alas, I can't find it. So my question is, where is the AC low pressure cut-off switch on a 204 Pontiac Vibe? Any instructions on how to jump it safely also appreciated.

Note: I realize that if there is very little/no coolant in the loop, defeating the cutoff switch and forcing the clutch to engage would be disastrous for the compressor. However, I think I can confirm that there is not insignificant pressure in the line. I can still attach a pressure gauge to the LP port when the AC is not on and it pegs out the meter. This at the very least, indicates that the system is not dry and probably wouldn't damage the compressor if turned on. If I'm mistaken here, please let me know.

[jayoldschool]

Stop overthinking this. You are probably low on 134a.

[BustedVibe]

Better to just add a little freon at a time and see if it comes on. I'm guessing running an overfilled system for a short bit is not going to wreck it like forcing an undercharged system would.

[jolt]

So you measured at least 12 volts going to the clutch and what was the ohm reading at the clutch coil?

[joatmon]

If you jumper 12v to the clutch and it still doesn't engage, then you have a clutch problem. Manually putting 12v across the clutch coil bypasses all pressure sensors, cut out switches, and ecu control. While ac clutch failures are not as common as some other vibe issues, they've been reported here enough that it is likely what's wrong in your 04, based on the testing you've done.

You may still have refrigerant level/pressure issues, but you may not. I think (from this side of the vast internet) that if you do, it's independent of an ac clutch problem in your case

[efan10]

Stop overthinking this. You are probably low on 134a.

Follow up question: So I can add coolant to the system even if the compressor isn't on? I was under the impression that a recharge was only feasible if the system is operational and the compressor is pulling the pressure low. However, I've also never worked on AC systems before so I could certainly be mistaken.

[jdmccright]

You can but not as efficiently as when the a/c is running and pulling the pressure down to ~30psi versus 90 or so when off. Take care of the clutch first, then throw in a can of dye to see if there is a leak.

[jayoldschool]

Stop overthinking this. You are probably low on 134a.

Follow up question: So I can add coolant to the system even if the compressor isn't on? I was under the impression that a recharge was only feasible if the system is operational and the compressor is pulling the pressure low. However, I've also never worked on AC systems before so I could certainly be mistaken.

Yes, the compressor will engage as soon as the low side pressure is above the cut off value. Note that you are supposed to fill with the engine on a higher RPM (1500 IRRC...)

[efan10]

Its getting hot out here so I finally got around to looking at this again and I have another follow up question. I bought the can refrigerant re-charge and hooked it up, and opened the valve for 5 seconds or so (all with the engine running of course). And nothing happened, i.e. the clutch didn't engage. How long should I have to hold the valve open for to get the pressure up without running the risk of over pressurizing the system if the problem is elsewhere?

[joatmon]

So, you're adding refrigerant, did you ever resolve the electrical problem with the clutch? If you can't force the clutch to engage, it doesn't matter how much juice is in the pipes

[efan10]

So, you're adding refrigerant, did you ever resolve the electrical problem with the clutch? If you can't force the clutch to engage, it doesn't matter how much juice is in the pipes

Well, no, but I at least showed that there is an electrical signal going to the clutch. Regardless if the pressure in the line is too low it will force the low pressure cutoff switch to engage, thus making it impossible to enable the clutch unless you add more refrigerant (or bypass the switch). At least that is my understanding.

This does make me think of another related question though. Where does the cutoff switch operate? Would it prevent an electrical signal from even going to the clutch or does it act on the clutch itself. In the former case, it would imply that the cutoff switch is not triggered in my car and there is indeed a problem with the clutch.

[joatmon]

The AC system has several sensors, including the pressure switch, which feed the car''s computer (ECU/ECM/PCM/etc) When the computer decides to engage the clutch, it send a signal to activate the AC clutch relay,. When the relay closes, it connects one side of the clutch coil to 12V from the AC fuse. The other side of the clutch coil is grounded by being bolted to the engine. The only way that the pressure switch can affect the clutch is to convince the ECU to not activate the clutch, but the pressure switch can't interrupt the signal between the ECU and the clutch Thats why a pretty definitive test of the clutch is to jumper 12V from the battery to the AC relay socket pin marked 12V to coil, that eliminates all kinds of AC system things, reduces the circuit to a wire and the clutch coil itself.

When the AC clutch engages, it will clamp the pulley to the compressor shaft. When not engaged, the end plate on the compressor can be spun by hand, when it is engaged, the end plate is clamped to the pulley and the serpentine belt makes it very hard to turn the end plate by hand. (best done when the engine isn't running, of course) Usually the clutch makes a loud "clack" when it engages.

I had the AC clutch fail in my Vibe, so I guess I figure if it happened to me, that makes it a likely thing to happen. If the clutch is bad, then that's something that would have to fixed independent of any other possible AC system problems I was interpreting you saying that you did get an electrical signal going to the clutch to mean that the clutch still did not engage, but at this point I'm not sure if your testing showed that the clutch did work or didn't work.

[efan10]

Thanks for the informative reply joatmon. Your description of the computer system indicates that since there is indeed power going to the clutch, and the clutch is not engaging, that there is not a low pressure issue.

I dropped the vehicle off at a shop this morning and explained the issue. They will check it out themselves and said that if my troubleshooting is correct, the entire clutch/compressor assembly will need to be replaced. I asked about just replacing the clutch and they said that the entire system would be replaced. i.e. The clutch and compressor come effectively as one component and you typically replace both at the same time. Total cost would be $800-$900. After googling around and looking at other posts (mainly viewtopic.php?t=37869), I'm dubious that the entire compressor needs to be replaced if only the clutch is bad and am tempted to take it to another shop. Any other opinions on this matter?

[Chiadog]

If the problem is the compressor clutch, pick up a used one from a junkyard!

[efan10]

If the problem is the compressor clutch, pick up a used one from a junkyard!

I'd like to... but I'm not comfortable enough in my repair skills and available tools to swap the clutch by myself. I talked with the guy on the phone this evening. He did indeed recommend replacing the entire compressor. When I brought up just doing the clutch again, he said he would just do the clutch if that's what I wanted, but that its a bad way to cut corners and any self respecting shop wouldn't recommend it. So basically exactly what I expected.

[brothel]

Hi efan10, I'm wondering if you ever got this issue resolved? My 2010 vibe has stopped blowing cold air, and it's been to see 3 different mechanics thus far, and I've received conflicting diagnoses from them. First guy said the compressor was shot (which sounded reasonable - the fan still blows air, fuses are okay, and the AC switch lights up). However, the second mechanic bypassed something (pressure switch?) and the compressor fired up and blew cold air. He said it required a $500 part, but since he wasn't 100% sure on his diagnosis he didn't want to go ahead with the job in case he was wrong. Third guy simply has no idea. I've been trying to avoid taking it to the dealership for fear of cost, but I don't know what/who else to try. Cheers.

[Mark]

Very informative thread, but no one actually answered the OP's original question: Where is the low pressure switch located? There is a four-pin switch on the top of the accumulator, but AutoZone shows a 3-pin switch. Can anyone tell me if this is the correct location and switch?

Also, if the switch is bad, can I remove it without first removing the refrigerant? Is there a check valve in the accumulator?

Thanks!

[joatmon]

while brothel was asking about a 2010 Vibe, which I have no pubs on, the OP, efan10 asked about a 2004 Vibe. I have an 03 Vibe service manual, should apply to his 04. I doubt this info applies to 09-10's

Very informative thread, but no one actually answered the OP's original question: Where is the low pressure switch located? There is a four-pin switch on the top of the accumulator, but AutoZone shows a 3-pin switch. Can anyone tell me if this is the correct location and switch?

Also, if the switch is bad, can I remove it without first removing the refrigerant? Is there a check valve in the accumulator?

Thanks!

Here are some shots from the 03 service manual, shows the location of the pressure switch (there's only one) and the pin out for the pressure sensor (4 pins)

g1rps.jpg (64.27 KiB) Viewed 4658 times

The procedure for removing the pressure sensor is

1. Discharge and recover the refrigerant
2. Disconnect the A/C refrigerant pressure switch electrical connector
3. Remove the A/C refrigerant pressure switch Use a second wrench to hold the switch mount to avoid bending the liquid line

so seems like there's no check valve to prevent loss of refrigerant when replacing the pressure sensor

The procedure for installing the pressure sensor is

- Use new O-rings lubricated with 525 viscosity mineral oil
- Hold the A/C refrigerant pressure switch mount with an open end wrench to avoid bending the liquid line
1. Install the A/C refrigerant pressure switch
- TIghten
Tighten the A/C refrigerant pressure switch to 10 N-m (7.0 lb.ft.)
2. Connect the A/C refrigerant pressure switch electrical connector
3. Evacuate and recharge the refrigerant system.
4. Operate the A/C system and check for leaks

Amusing (to me anyway) that in the picture they spell it "refridgerant" and "refrigerant" Also a bit annoying (to me anyway) that they randomly alternate between calling it a sensor and a switch

[Mark]

Thanks Joatmon!

That is where I found it, but I thought it was high side only. Must be just one on this car. If I understand it I should be able to jumper 1 and 2 and if the compressor kicks in then I have a bad switch. If not then I have a $$$ repair, or a cool weather only car.

[joatmon]

If I understand it I should be able to jumper 1 and 2 and if the compressor kicks in then I have a bad switch. If not then I have a $$$ repair, or a cool weather only car.

Did you try bypassing the AC clutch relay to verify the clutch is working? There's some info in viewtopic.php?p=521935