AC Pressure Switch Diagnostics

[hour]

Hi all - I let my 4runner sit for months and months and now find the AC doesn't work. I have manual controls in my 1999 SR5 and the light for the button comes on, but there's obviously no cold air and the engine pitch doesn't change in response to pressing the button, like it always has.

I jumpered the connector I found under the hood, towards the firewall by the low pressure inlet, and noticed pushing the button does indeed change the idle of the car once again. I also very (very) briefly pushed down the valve under the low cap and was met with a whoosh.

So - wondering what my next step is. The whoosh isn't conclusive I suppose, that it's full enough to run, so it could still either be low or the switch could be bad.

I'm struggling to find any in depth details on diagnosing this further. I'd hate to buy a $60 refill can only to find that the system is full when I connect it. I'd also hate to buy and install a switch only to find the system was low. Is there something conclusive for the switch being faulty that I can deduce with a multimeter?

And as for replacing the switch (also can't find a youtube video on this for some reason), is that an easy job - will I lose all my refrigerant replacing it or is there a valve like I've heard about on some other videos that should prevent that

Thanks, from ass sweat South Carolina :banana:

 

[hour]

Well I guess the switch is a good starting point at least. Found my DMM and am not seeing any continuity between the two contacts of the switch.

So I'll go ahead and order one of those. Still curious if there's a check valve so I won't lose all of my refrigerant when I yank the old one

Eh another edit. I left the jumper in for a bit to look in sight glass by the grille and see if any cold air was coming out of the vents. Very little movement through the sight glass, not like I've seen on youtube, and the air blowing out was the slightest bit cool. May have a twofer on my hands.

FWIW it's this switch that's disconnected in the photo that when jumpered allows the compressor to kick on. Not sure which it is or if that's of any particular significance

I keep editing this as I keep googling, but I'm reading things like "You should have continuity if the pressures are correct" which makes me think there's a chance it's just low on refrigerant? I've had the car 11 years and never done anything for the AC so that'd be a win if I don't have to remove the switch.

But if continuity should be present regardless of the pressure in the system and I'm just reading some oddly worded posts scouring the internet, then that's not my sole smoking gun.

 

[Sparky 97Runner]

I would start by getting a set of gauges and see if your system is low. Charge your system with 134a with dye and find your leak with a cheap UV flashlight if it turns out to be low. Do not use 134a with stop leak. If your charge is good then move forward with troubleshooting. If you discover that the switch is bad you will have to recover the refrigerant in your system before changing that switch out.

 

[hour]

I would start by getting a set of gauges and see if your system is low. Charge your system with 134a with dye and find your leak with a cheap UV flashlight if it turns out to be low. Do not use 134a with stop leak. If your charge is good then move forward with troubleshooting. If you discover that the switch is bad you will have to recover the refrigerant in your system before changing that switch out.

Ok thanks, that's good to know about the switch not having a check valve.

Do you have any idea on whether I could expect continuity in the switch when the system is low? That confused me, since I've seen youtube videos of people testing them off the vehicle (but not sure if that's for demonstration purposes) so I'm still tangled up on whether the switch can be checked for continuity while installed, on a system with low refrigerant.

 

[AuSeeker]

Ok thanks, that's good to know about the switch not having a check valve.

Do you have any idea on whether I could expect continuity in the switch when the system is low? That confused me, since I've seen youtube videos of people testing them off the vehicle (but not sure if that's for demonstration purposes) so I'm still tangled up on whether the switch can be checked for continuity while installed, on a system with low refrigerant.

Not sure about 3rd Gens but on all AC systems I've messed with had a high pressure switch and a low pressure switch as well.

Follow the larger/low pressure AC line and try and find it, unfortunately on some vehicles the low pressure switch is buried in the dash somewhere, hopefully that's not the case on 3rd Gens.

 

[hour]

Not sure about 3rd Gens but on all AC systems I've messed with had a high pressure switch and a low pressure switch as well.

Follow the larger/low pressure AC line and try and find it, unfortunately on some vehicles the low pressure switch is buried in the dash somewhere, hopefully that's not the case on 3rd Gens.

Well, I have a picture of the switch in a post above and whichever one it is that is in the photo, when jumpered, allows the AC to engage. I'm just trying to determine if I could expect continuity on the switch in the photo, or if continuity is somehow dependent upon the pressure in the system (car off, key out)

 

[Sparky 97Runner]

Ok thanks, that's good to know about the switch not having a check valve.

Do you have any idea on whether I could expect continuity in the switch when the system is low? That confused me, since I've seen youtube videos of people testing them off the vehicle (but not sure if that's for demonstration purposes) so I'm still tangled up on whether the switch can be checked for continuity while installed, on a system with low refrigerant.

You will not have continuity on the switch if the system is low and the A/C amplifier will not send a signal to turn on the compressor if that switch is open. According to the 1997 4Runner wiring diagram manual "A/C dual pressure switch is open (no continuity) with refrigerant pressure at less than approx 28.4psi or more than approx 455psi". I'm assuming the 1997 manual AC wiring is pretty close to 1999. The fact that you jumpered the harness and tricked the compressor to come on and the A/C still didn't get cold makes me think you are probably low on refrigerant.

 

[AuSeeker]

Well, I have a picture of the switch in a post above and whichever one it is that is in the photo, when jumpered, allows the AC to engage. I'm just trying to determine if I could expect continuity on the switch in the photo, or if continuity is somehow dependent upon the pressure in the system (car off, key out)

That line in your photo is the is the smaller of the two AC lines which makes it the AC high pressure line and as mentioned by [MENTION=415906]Sparky 97Runner[/MENTION] and according to his quote from the FSM the switch is a dual pressure switch in that it does both the high pressure and low pressure.