Question on low pressure switch

[jawn101]

Just took the car out for a quick leg stretch and decided to bring my IR thermometer along for fun, to measure the A/C temps. My A/C is working absolutely great, the whole system (other than the evaporator) is 100% new and full of fresh R134a.

Using the IR thermometer, I noticed that the temps at the center vents fluctuated between 37 degrees and as low as 24!

Now, that's great and all - but I know it means the compressor isn't cycling as frequently as it should. The reason for that is I made the ill-advised choice at one point to adjust the low pressure switch setpoint. I do hear it cycle periodically when driving, but not the 8-on-4-off (or is it the other way around?) that is expected.

Is there any good way to determine where it's *supposed* to be? Should I just buy a new one and call it good? Don't get me wrong, I love 24 degree air (it's 85ish outside right now) but I'm a touch worried about taking the high side pressure too high, as my early-ish VIN doesn't have a high pressure switch or a pop-off valve. And since the A/C is the first system I tackled when I bought the car, I didn't know enough to add one at the time.

[Farrar]

I think most systems' low pressure switches are set to cut out at about 20 PSI.

(Off topic: you could always add a high pressure switch to your system.)

By the way, how did you manage to get an IR thermometer to read the temperature of air? I thought those things had to be pointed at a solid object.

[jawn101]

I think most systems' low pressure switches are set to cut out at about 20 PSI.

(Off topic: you could always add a high pressure switch to your system.)

By the way, how did you manage to get an IR thermometer to read the temperature of air? I thought those things had to be pointed at a solid object.

Yeah, the switch has an inscription on it but I can't quite tell what it says. I just don't know how to measure it - I do have manifold gauges but am not sure how to apply them to this situation.

How could I add a high pressure hub without opening the system? I know I could add one but don't wanna screw with a good thing :)

And yeah, I am not 100% sure if the thermometer is reading the air temp as much as the temp of the vents themselves, but if the plastic is that cold it's pretty safe to assume the surrounding air is or was at least close to the same temperature, right? :)

[Farrar]

How could I add a high pressure hub without opening the system?

I don't think you can.

However, if you look under your dashboard at the bundle of wires there, and see what looks like a jumper made of brown wire, that's where you can wire in the high pressure switch, if what Dave Swingle told me is correct -- and if my memory is correct. I didn't know that at the time and simply wired mine in series with the low pressure switch.

[Bitsyncmaster]

If the air is below 32 deg. you will get ice build up on the evaporator. Just put your gauge set on and look when the compressor cuts off. I think the norm is 25 to 30 PSI then the compressor turns on about 40 to 50 PSI.

[Ron]

Jon, see WS Manual N:09:01 for related pressure check values.
The 134 may make them a little higher, so I would just match the values in the middle temp range...

[David T]

Jon, see WS Manual N:09:01 for related pressure check values.
The 134 may make them a little higher, so I would just match the values in the middle temp range...

You do need to adjust the low pressure switch if you convert to -134. Adjust the switch so the coil never gets below about 30 degrees F. If it does, for any length of time, it will freeze the coil and you will notice a diminished flow of air (although it WILL be very cold!) out the vents. Your cycle time will not be the same as for -12.
David Teitelbaum

[Ron]

Agreed. Good point, especially with his setup.
But I have never seen where it was necessary, in that direction, with non-adjustables (...efficiency). Granted, the discharge temp would be higher than at the evaporator if it was operating for a short while, but the last column in the chart would warn you about that in any case...

[jawn101]

Thanks for the tips, guys. My gauge set is out on loan right now (oops - I forgot!) so I will do these pressure tests when I get it back. Looking at the table in N:09:01 it doesn't seem as though things are all *that* out of whack. For example, in 80 to 90 degree ambient air, the outlet temp should be around 36 degrees. Some of my IR readings were right around that. I think I need to put a probe thermometer in the vent, something with a thermocouple and monitor that as well to get an idea of the average temp rather than the instantaneous.

I'm going to re-do my drive with the cover to the relay compartment removed and listen a little more carefully for the compressor cycling. With all the dynamat in the car, I don't hear the actual clutch anymore and the relays are very muffled. Maybe things are actually fine. :confused:

[Farrar]

If you have a fan in your garage, set it up in front of your car and turn it on during your tests to pass air over the condenser. This will improve the accuracy of air conditioning tests by simulating using the a/c while driving. Sorry if you already knew that -- I only read it recently.

[jawn101]

If you have a fan in your garage, set it up in front of your car and turn it on during your tests to pass air over the condenser. This will improve the accuracy of air conditioning tests by simulating using the a/c while driving. Sorry if you already knew that -- I only read it recently.

Thanks man, I did know that from Ron in another thread :) But it's worth posting in case someone finds this thread and not that other one sometime in the future.