A/C Low Pressure Switch / Compressor Switch - Need Correct OEM Spec

[dearchristopher]

Update after installing the new switch, I’m seeing 40° out of the vent!

I’ll keep monitoring temps and cutoff, but I ended up not even adjusting the new switch. The wider cutoff buffer seems to keep my system operating as it should.

I bought a proper manifold set, and everything is looking solid. Based on the fsm data I found, the current pressures at 73° ambient and 99% humidity (rainy week) showed dead on functional.

I don’t LOVE that I had to cut my factory pigtail off. But I soldered and weather sealed the new connectors, as well as left enough wire should I ever need to go back to factory plug.

 

[KennethS]

Good job! Mine does 40º at the center vent, too.

Question: At what pressure does the low side cut-off occur? You can get a lower vent temp by decreasing the low side cut-off slightly.

You're in a humid area, so be aware if you go too low, the evaporator can get so cold the water condensing on it can freeze and block air flow, and the system will have to be turned off to let the ice melt before the AC will blow cold again.

 

[dearchristopher]

Good job! Mine does 40º at the center vent, too.

Question: At what pressure does the low side cut-off occur? You can get a lower vent temp by decreasing the low side cut-off slightly.

You're in a humid area, so be aware if you go too low, the evaporator can get so cold the water condensing on it can freeze and block air flow, and the system will have to be turned off to let the ice melt before the AC will blow cold again.

According to the switch spec

• ON (cut-in): 3.1 kg/cm² ≈ 44 psi
• OFF (cut-out): 1.7 kg/cm² ≈ 24 psi

I haven’t even touched adjustment. I think the cutout being so close to factory spec is what got me functional. When I get a super hot day, Ill test and see if 44psi is too high

 

[KennethS]

Ill test and see if 44psi is too high

Maybe I'm missing something but I don't think that is a concern.

The main thing is the low pressure cut-off as this is what dictates how long the compressor stays on. The high pressure cut-in is the point that the compressor will turn back on after it turns of due to the low pressure cut-off, so it's not really going to affect anything in hotter temperatures.

 

[Rickyd]

I'm not sure how automotive systems are but the evaporator should be fluctuating between 28* and 49* with a low side pressure range like that.

 

[dearchristopher]

Maybe I'm missing something but I don't think that is a concern.

The main thing is the low pressure cut-off as this is what dictates how long the compressor stays on. The high pressure cut-in is the point that the compressor will turn back on after it turns of due to the low pressure cut-off, so it's not really going to affect anything in hotter temperatures.

Yeah good point, that makes sense now that I think about it. I’m still learning the push pull of ac systems. Here’s hoping it freezes me out all summer when the top is on

 

[Steel City 06]

This switch is not OEM style but works very well:
https://www.amazon.com/dp/B003R42YMS?tag=wranglerorg-20
Same style, just R134a tuned instead of R12.

It is adjustable setpoint so you can tune it.

I run mine with a R290/600a blend for extra cooling power versus R134a and the adjustability is very useful in tuning it.

Edit: This one is the one intended for R134A. The OP's switch is intended for R12. Both will work well with a R290/600a blend.

 

[dearchristopher]

This switch is not OEM style but works very well:
https://www.amazon.com/dp/B003R42YMS?tag=wranglerorg-20
Same style, just R134a tuned instead of R12.

It is adjustable setpoint so you can tune it.

I run mine with a R290/600a blend for extra cooling power versus R134a and the adjustability is very useful in tuning it.

Edit: This one is the one intended for R134A. The OP's switch is intended for R12. Both will work well with a R290/600a blend.

what makes you say the switch im running is geared for r12? i'm not seeing that in the product specs

 

[Steel City 06]

what makes you say the switch im running is geared for r12? i'm not seeing that in the product specs

It says it on the actual device itself. It's not in the descriptions for whatever reason but you can barely see it in the photos (look for the white text).

It probably doesn't matter too much. R12 and R134a are fairly similar, just R134a has less ozone impact and more GWP. (R12 also performs way better than R134a.)

 

[dearchristopher]

It says it on the actual device itself. It's not in the descriptions for whatever reason but you can barely see it in the photos (look for the white text).

It probably doesn't matter too much. R12 and R134a are fairly similar, just R134a has less ozone impact and more GWP. (R12 also performs way better than R134a.)

Eagle eye! I guess I’ll just watch it. It doesn’t short cycle anymore, so I’ll have to keep an eye out for freeze over. For how well it’s currently working, I’d be fine with buying the one you linked if it keeps things happy.

I drove it last night to the store and it froze me out. Can’t complain!

 

[Steel City 06]

If it works, I wouldn't bother. It's possible the difference is literally just the preset position of the screw, but I am just guessing there.


If you ever want even colder, consider using R290/600a blends like Red Tek, Envirosafe, or Duracool. They actually have significantly more cooling power and lower power consumption. The only caveat is that they are flammable (R290 = propane and R600a = isobutane) so you probably don't want to get into shootouts on the highway if you can help it.

https://www.es-refrigerants.com/products/w/id/31/t/134a-replacement/
https://www.duracool.com/shop/duracool-22a-8oz-can-with-tap-hose-and-gauge-attachment/

A single 8oz can is the perfect amount for a TJ with a vacuum pulled.

 

[dearchristopher]

If it works, I wouldn't bother. It's possible the difference is literally just the preset position of the screw, but I am just guessing there.


If you ever want even colder, consider using R290/600a blends like Red Tek, Envirosafe, or Duracool. They actually have significantly more cooling power and lower power consumption. The only caveat is that they are flammable (R290 = propane and R600a = isobutane) so you probably don't want to get into shootouts on the highway if you can help it.

https://www.es-refrigerants.com/products/w/id/31/t/134a-replacement/
https://www.duracool.com/shop/duracool-22a-8oz-can-with-tap-hose-and-gauge-attachment/

A single 8oz can is the perfect amount for a TJ with a vacuum pulled.

I was actually looking into this last night, interesting you bring it up. Are you running the same setup?

 

[Steel City 06]

I was actually looking into this last night, interesting you bring it up. Are you running the same setup?

Pretty much. Need to find tune the low pressure switch as I went too aggressive with the low setpoint, but it works well especially when it is hot and humid.

 

[dearchristopher]

Pretty much. Need to find tune the low pressure switch as I went too aggressive with the low setpoint, but it works well especially when it is hot and humid.

That’s the part that had me so interested. Coastal Texas can be brutal when it wants.

I’ll do some more reading!

 

[Steel City 06]

That’s the part that had me so interested. Coastal Texas can be brutal when it wants.

I’ll do some more reading!

Pretty much the only two ways to get colder A/C in a TJ are to switch refrigerants and to use a more powerful radiator fan. Definitely worth a shot if you want colder AC

 

[dearchristopher]

Pretty much the only two ways to get colder A/C in a TJ are to switch refrigerants and to use a more powerful radiator fan. Definitely worth a shot if you want colder AC

I went with a severe duty fan clutch but kept the 5 blade cause I read that it does move air pretty well. The new clutch is louder when engaged but doesn’t really bother me. Doesn’t help a TON when idling but I noticed colder air at low speeds

 

[Steel City 06]

I went with a severe duty fan clutch but kept the 5 blade cause I read that it does move air pretty well. The new clutch is louder when engaged but doesn’t really bother me. Doesn’t help a TON when idling but I noticed colder air at low speeds

I did measure the effects using an e-fan at various speeds and an engine at various speeds, per this post:
https://wranglertjforum.com/threads...att-electric-brushless-fan.78963/post-1576105

In short, it was good for maybe a 6° drop. This was with R134a.

I didn't have a control for this test but R290/600a blew that out of the water:
https://wranglertjforum.com/threads...att-electric-brushless-fan.78963/post-1734229

 

[Steel City 06]

So this thread inspired me to go out and tune mine properly, which I'd been meaning to do anyways.

I found that if it is humid as hell, then you have a perfect way to tune the low pressure side.

Basically, run it at ~1,500 ish RPM, with the A/C on outside intake. Watch the lines coming to and from the heater core. Start at max blower speed.

Back the pressure switch off (counterclockwise if looking directly at the screw, it should move outward) a full turn.

If at full fan, you start to get icing of the lines, you're probably undercharged. Measure and correct.

Next, run the HVAC fan at the lowest possible speed, still on intake. Your lines will probably begin to ice up. (If not, you may be overcharged, or your compressor is still cycling.)

Then, the goal is to tune the low pressure switch (by screwing it in) such that the line from the condenser begins to ice up and then melt off alternately, and the line to the compressor doesn't appreciably gain any ice at all.

This way, at full fan power, you get maximum cooling and pretty much no engine cycling below 2,000-ish RPM, but you don't get any appreciable ice build-up on the evaporator at lower airspeeds or mixed heat/cooling.

For the most part, I get about 35-45° pretty consistently at the vents this way at full fan. At low idle and full fan + fresh air, it did rise to about 55°, but that's because the intake temperature is at about 90° and very high relative humidity. (There is a constant stream of water pouring out the drain tube.) At full fan and low idle on recirculation, it quickly dropped to about 40°F once the humidity cleared out of the cabin.

 

[KennethS]

Just for information, mine blows 42 degrees at the center vent with high fan with 105 degree outside temperature. Steady driving at 35 MPH and around 1500 RPM. This is all factory original AC components.

 

[Steel City 06]

Just for information, mine blows 42 degrees at the center vent with high fan and 105 degree outside temperature. This is all factory original AC components.

The factory AC can indeed be pretty powerful even in high temperatures, especially in dry climates. It's mostly limited by humidity. Takes a lot of power to condense water...