JGrund85

My A/C was blowing ice cold on my 1400 mile trip back from Colorado in the 100 degree heat with 100 percent humidity when about 250 miles into the trip the A/C stopped working. I press the switch in and it doesn't light up. I replaced the A/C button and I still get the same thing. Someone has mentioned the A/C pressure switch and that it might be bad. I converted the system to R-134a and the Low side was reading something crazy like 120psi. The normal is around 40-60. I drained off the system and put it to about 55 psi, and the A/C light still will not come on.

Any idea's where the pressure switch is located on a 91 4Runner 3.0L.

I also have a 3.4 swap, but I am running the old A/C system...compressor and all.

Justinlhc

When the compressor is not engaged it will give you a high pressure reading. It should be quite a bit lower ONLY when the compressor is engaged.

Did you vac out the system before you charged it again?

JGrund85

Yes I vacuumed out the system before I charged it. That makes sense about it being higher. But still does not explain why it will not engage.

Anyone know where the pressure switch is located or have any other ideas why my A/C switch is not turning on and clutch not engaging?

abecedarian

pressure switches are under the dash, IIRC. also, HFC134a requires a higher head pressure than CFC12 that would explain the higher pressure in the system.

JGrund85

IIRC??? What does that stand for. Before I recharged the system it had CFC12. Which side under the dash are the pressure switches?

Justinlhc

His readings were taken when the compressor was disengaged, which will always give a higher reading regardless of what system it is.

You have to completely remove the evaporator from under the dash to get to the pressure switch. I would replace the expansion valve while you're in there. I got mine for like $15.

abecedarian

IIRC= If I Remember Correctly

HFC134a is not as efficient at thermal transfer as CFC12 is, so to compensate for that requires higher head (high side) pressure. Consequently, the higher high-side pressure means higher than normal low-side pressure since there's more refrigerant in the system.

Now, again IIRC, there are two pressure switches- one high pressure, one low pressure; and either can stop a/c operation. The first, high pressure, shuts the compressor off if the high-side pressure exceeds some amount. The other, low pressure, shuts the compressor off if there isn't a minimal amount of pressure in the system, which protects the compressor from damage if there's a catastrophic loss of refrigerant and consequently lubrication.

JGrund85

Ok, Thank you very much. Now I understand more about my A/C which is helpful in trouble shooting. However, the 2 fuses directly behind the glove box dash piece seem to be the issue. There is a 10A in the upper fuse location and a 20A in the bottom. The 10A is blown. I replaced that fuse which in turn restored switch power, thus engaging the clutch, however, when I turn on the engine, and engage the A/C within 5 seconds it blows the 10A fuse. I tried to trace the wire but this requires ripping the WHOLE dash off and taking apart a wiring harness. Also, I briefly tried to use a 20A fuse (I know it is a bad idea, it was the only spare fuse I had after blowing 2 10A spares) and it ran well and kicked on the compressor, but the air coming out of the ducts never got cold, but I could hear the compressor engage. Any ideas what this could be before I get into tracing wires thinking it might be a wire grounding out?

abecedarian

I am now at a disadvantage- I am out of town and away from my wiring diagrams so I can't say anything particularly specific.
I'd put my gauges on the compressor low and high side fitting and see what the readings are. I'd also use self-resetting breakers rated similarly to the fuses to help.

JGrund85

Well the low side is reading about 65-70 while the compressor is engaged which according to my R-134a guage is in the "ALERT" too high section. If you could let me know when you get back home to your wiring diagrams anything specific that would be great. I really don't want to tear into that harness to trace wires.

Anyone else with any other ideas??

Justinlhc

Ok, but his reading of 120 was from the low side, disengaged. His reading from the low side engaged was 55. AFAIK it shouldn't be over ~35psi on the low side when the conpressor is engaged. 55psi is waaaay too much and if it's overfilled it will cause lots of problems including lack of cooling and will cause the compressor to kick off from too much pressure.

I would discharge the system down to about 35 on the low side, engaged and see if that helps.

1. NORMALLY FUNCTIONING REFRIGERATION SYSTEM
Gauge reading:
Low pressure side:
0.18 – 0.25 MPa (1.5 – 2.5 kgf/cm

2)
High pressure side:

1.37 – 1.57 MPa (14 – 16 kgf/cm2)

JGrund85

What is the best way to discharge the system? You think that will keep the system from blowing the 10A switch fuse??

Justinlhc

Worth a shot. It could be putting too much pressure on the compressor causing the clutch to disengage/pop the fuse? Either way it's way too much pressure and it's never going to operate properly at that pressure.

I'm not going to say how you should discharge the system. 134a is supposed to be "safer" for the ozone. The "proper" way would be to bring it to a shop and have them vacate some of it.

JGrund85

That costs about a hundred bucks. Would an improper way that works be to just press the high side valve in and let the pressure out?

Justinlhc

I'm not telling you to do it, but the low side would be your best bet to discharge from. The improper way would be to put a rag over and around the low side valve and press it in for a few seconds making sure you don't breath any of that crap in.

jtm559

i have the same problem, just that when i give my car some gas up to 3000rpm the ac fuce pops. I rip everything out to find out where is the shortage but can't find anything....can anybody help please.

JGrund85

No fix yet, I kinda gave up as work set into full swing for me now. I can't figure it out. It doesn't make any sense to me. I can drain the system off, then recharge it and then it will work great until I get on the highway to my first gas stop (about 225 miles) then I turn off the rig to refuel and then the fuse blows. The low side then reads about 125 psi or so. I don't know what to do. Still stuck on this stupid thing. Any help guys??

JGrund85

bump for more help as i have time this week! Thanks

JGrund85

bump again for help!?!?

89934runr

Your issue seems to be related to heat soak... This could be several things. The tough part is the blowing of the fuse- this means you have a dynamic short, rather than a static short that is always present. You're causing the system to 'short', or use enough current to blow the fuse...but only under certain conditions. The fuse itself only feeds the AC switch in the dash (which then feeds the AC amplifier), and the AC Dual Pressure switch (that's the bugger that turns off your compressor when the pressures are too low or high- WHEN the AC system is on) This switch is inconveniently located down next to the evaporator core. If you have a 3.0 4wd auto, the 10a fuse also feeds the AC cut relay.

1. What are your water temps when you stop? On auto trans 3.0s, there is a water temp switch that kicks off the AC when the water gets 'hot'. It does this by removing the ground path for the AC cut relay- this permanently removing B+ to the amplifier, which disables your AC.
2. Are you able to test the voltage at the AC amplifier connector? (it's down on top of the evap box). This will tell you a lot about what your system is or is not doing.

Don't go cutting up your harness yet- my bet is that it's still a component.

Jeff