Do I need a different pressure switch converting from R12 to R134?

I've got everything new going in today/tomorrow and I bought a new switch per the original R12 specs. I'm not sure if the pressure on/off is different for R134.

And then if I do need a different switch, I may need a thread adapter if the threads are different and mock up the electrical contacts if the connector is different.

1993 Explorer with just a single pressure switch.

 

Anyone please?

 

The pressures are not that much different so I don't see why you would need a different switch.

 

I've looked up all kinds of r12/r134 conversion links but none of them call for a different switch. And the only time the switch was mentioned it was a recommendation to by pass it for other trouble shooting.

There are "134" in some catalogs I've seen. Perhaps it's just the different fittings thast the 2 systems have.

 

Low Side Pressure Switches for R-134a have different mounting threads and won't screw on to the port. The old pressure switch is adjustable. I believe many replacements are too.
Look in between the connector terminals, there will be a screw. Adjust it counterclockwise so that the compressor cuts out at 22-24psi on the low side. That's usually 1/4-1/2 turn, but I've seen it take as much as 1 full turn.
Don't just "fiddle" blindly with the switch, make the adjustment BY THE PRESSURE READING. Moving the screw back and forth to "tweak" the pressure can cause it to leak.

 

This advice must follow some other system than what I have. First, there is no adjustment screw anywhere on the AC system for anything. The pressure switch just screws into a port on the receiver/dryer with 2 wires on the connector. Second, how can you adjust the pressure up and down to make the switch activate? Sure I have the gauges and used them...but the only way to to change the pressure is unplug the compressor and watch the pressure equalize on both sides, or instill a leak so the freon escapse (not too enviormentally friendly). And I would need to know the switch changes state so maybe a volt/ohm meter in parallel. I can understand an adjustable switch, but if you can't change the pressure in the system how can you calibrate it?

Anyway...this is what I did:

Changed out every part of the AC system for new parts. Only the switch on the dashboard is original. I thought I'd be safe leaving that alone.

Rented a vac pump from autozone and pumped it down. It held a good vacuum but when I shut off the isolation valves, the pressure did creep up....back to atmosphere in about 5 minutes. Pumped down again and isolated the sytem...jiggled all the "loose" slip together fittings and found one that did leak appreciably. So I got the next size o-rings on that on, 3 of them, and pushed on it for about 5 minutes of sweat and swear words until it finally clicked closed. This was on the low side...hose back to the compressor. Under slight pressure, it was probably OK but I wanted it very leak tight.

Vacuumed it down for 10 minutes and rechecked...no leaks. Vacuumed down for a good hour. Isolated the system and hooked up my first can of R134. Fired off the motor and opened the valve for the freon. First 12 OZ can went in and the compressor started to cycle. Second can went in and the compressor stayed on all the time. I knew from the 32 OZ of R12 required x 80% for R134, I needed a little shot more than the 2 full cans. So I tried to give a little shot out of a third but who knows how much got in...I did not weigh the can.

High pressure then was about 300 PSI and low about 45-50. It seemed a little high so I didn't add any more freon. The vents were blowing pretty chilly at this point.

I shut the sytem down and waited for 10 minutes and then fired it up again. Pressures seemed to relax a little...275 high and 40 low. Took it for a test drive and seemed OK.

This was all yesterday. Today, on the way home from work, it was about 105F out. First got in the car and it was hot for about 2 minutes but I felt the cool really quick compared to last summer with the R12 system. It used to take 4-5 miles of driving to feel the cool. Today, it was cool before I left the parking lot. On the freeway, I had to turn down the fan as I was getting too cold. That has not happened in 5 years or so around these parts.

So I'm happy. A 206K mile Explorer with brand new AC in the Arizona desert...can't beat that any day! And it is all paid for!

 

Good Job!!
You did it more than right, but you'll be chillin' for a long time. Kudos.

Take a look at your old cycling switch. Look into the electrical connector end of the switch, between the terminals. There is an an adjustment screw. Your new switch may also have the same adjustment screw.
That's not an "adjustment" mentioned in any manual or recommended procedure, but it is a viable adjustment nonetheless on an R134a conversion. It sets the calibration of the Cycling Pressure Switch.

 

If you want to do it by the 'book', you will adjust the low pressure cut off/out switch - LPCO - to a little above the boiling pressure of R134 at 32F. That should keep the evaporator from freezing up. Look at a P/T chart for 134, and you will see the pressure you want to keep the temp just above 32. Professionals may chime in that you want more than a few degrees, but that's the general idea.
tom

 

So this is really the basis for my initial question - do I need a different pressure rating for the switch going from R12 to R134? Obviously the gasses work differently. Several references I found called for 80% of the R12 amount going to R134 but not one reference on the cycling pressures.

And I did find the adjustment screw on the switch...I never thought to look in there. So OK I can go find the optimal P on an R134 chart but it's still not easy to calibrate the switch since I can't regulate the system pressure at all.

I can put a volt meter on the connector and see when the switch goes off if P ever does drop to the right amount. My understanding is that pressure drops on the low side as things get super cold and that is when the evap freezes. I have seen this here in Phoenix on building roof top units. On my truck, I don't think it can ever getthat cold. I could vent the freon to ATM to drop the P but isn't that a bit illegal? Maybe I just need to biuld a jig and us a bicycle pump to control the pressure and calibrate the switch off line.

 

The compressor will run and run and run until it reaches the lowest pressure you set in the LPCO. If it cannot develop that 'low', it will continue to run. You can find out what that setting is by using your gauge set & monitoring the low side. A little at a time, you adjust the LPCO until the compressor releases the clutch. If that pressure is above 32F, and you are happy with the vent temp, quit. If the pressure is below the 32F, you may get frost on the evaporator.
You get the LPCO to operate by putting the least demand on the system you can. Fan on lowest setting, recirc enabled, doors & windows closed, in the shade, on a mild day. With those conditions, what you are looking for is a 'satisfied' system. The A/C is satisfied with the amount of refrigerant that is boiling off in the evaporator. There's enough there already to boil. The compressor doesn't know that. It will continue to keep trying to push refrigerant towards the condenser, and pull it from the accumulator. If it is left to its own, it will drop the pressure in the evaporator(accumulator, too) below freezing(32F). This is because it has more capacity RIGHT NOW, than is needed. The LPCO saves the day by noting the pressure is below freezing, ice / frost will coat the evaporator, and it must interrupt the power to the compressor clutch. You adjust that low pressure using the screw.
What is the pressure at which R134 will boil at 32F? If you can, adjust the LPCO point to be above that pressure using the above conditions. If you don't expect to ever get that light of an A/C load, close the hood, and walk away.
tom

 

Tom - I think that is where I'm at right now, end of May in Phoenix. I won't be able to max the AC system out and push the evap towards freezing until December. So yeah, the hood is closed for now. Thanks for the advice.

 

OK how about this one....When I took the system apart this time around, there was no orifice tube in the evap core. Not like it fell out when I pulled the AC box out. They insert so tightly that you need one of those special tools that thread onto the evap core fitting to extract them. I did look all over the engine compartment though and no, it did not fall out.

There was one in there last September. I changed the compressor, drier and the tube. Then I brought it back to Napa to recharge the R12. It worked fine for a month or so then winter came and I didn't try to use AC. The first time I used it this year, about 3-4 weeks ago, it worked fine for 15-20 min's and then I heard this hiss sound under the dash and it started to blow hot. If you every notice the sound when you shut the car off when the AC was on, the hiss is that same sound - the liquid expanding through the evap core and the system equalizing pressure. Actually sounds just like liquid flowing.

So 1 of 2 things occurred. 1) The Napa guys were pissed that all I needed from them was a freon charge and they pulled the orifice tube to each me a lesson. I have had shops complain to me before that I didn't give them the full job. Could the AC even work at all (as it did) with no orifice tube?

2) The orifice tube failed, somehow imploded and pieces of it ended up in the evap core and/or drier. I was thinking of cutting those old parts up with a hack saw and looking for orifice tube shrapnal.

There were signs to feon leakage everywhere - fittings, compressor, hose assembly...there was no freon at all left 3 or so weeks after the system started to blow hot a few weeks ago.

What could have occurred to build up so much pressure to possibly blow up the orifice tube and definitely blow freon out all the connection points?

 

I am not an A/C mech, probably barely qualified to pass the EPA test, on a good day, so don't quote me in bar bets.
The orifice 'tube' can be located in two positions that I know of in FoMoCo products. In the inlet to the evaporator, and in-line, about a foot rearward of the radiator support, in the high pressure line that feeds the evaporator. There would be a metallic, likely aluminum, section, or a section where it is 'crimped down', that has the orifice trapped there.
If you replace the high side line, the replacement could have had the orifice embedded.

What makes a system blow up is high pressure and lack of lubrication. Ford made(bought) a compressor that had coated pistons. The Teflon coating apparently wore off after time, and the pistons started to form a sludge of oil and metal. This sludge happened to be transportable by the refrigerant, as the oil was miscible with the R12. The sludge would then fall out of circulation or just deposit itself in the low spots. Eventually blocking the flow of refrigerant, blowing the HP release valve off, shedding refrigerant and oil all over the underhod area.
Won't work without an orifice, as the refrigerant would just flow 'round and 'round, never getting any pressure buildup, and likely shelling the compressor as it does NOT like to eat liquids.. The juice would be compressed, fed to the condenser, cooled, and pass as quick as it wanted right into the evaporator. Then out to the accumulator, which may have been able to retain the liquid to a certain degree, as it would not be pulled out because of the bleed at the top of the internal loop. Not sure of the final, but if liquid got to the compressor, it's toast.
Were the signs of freon leakage oil spots, with dirt mixed in? Those are the only things that visibly will show freon leakage. When you put it together, did you use Nylog on the snap ring fittings? It works.
tom

 

I changed the tube last year. I pulled the old one with that special tool borrowed from Autozone and put the new orifice tube in the evap core tube. I even double checked my receipts and yes I did buy a new one. There is not a second in the liquid line though I know what you mean. I did my brother in law's Mustang last year and in order to change the orifice tube, I had to buy the entire line.

Not sure what nylog is so I'm sure I did not use it. I just coated the o-rings with the same PAG 46 oil I put in the system.

Yes the old parts have oil/dirt at all the joints. Even where the hose assemblies are pressed together.

I'm not sure, with no orifice tube in place, that the freon ever got to a liquid. Like you say Tom, it needs a restriction to build up enough P to turn to liquid. I think excessive pressure built up from the orifice tube getting clogged, some leakage occurred then. And then the tube finally blew up. Aftwerwards, in troubleshooting and running the compressor, I could feel the drier getting a little cold but eventually not at all. The next time I got around, got the time, to look at it, all the freon was gone and compressor wouldn't even go on - the switch worked, LOL.

 

One additional thing the LPCO does is to prevent the compressor from running when the system is low on refrigerant. That helps protect the compressor as the refrigerant carries the oil around the system. Hope I am not lecturing to someone who already has heard all this stuff. Your last triggered the thought, and I remember someone commenting somewhere somewhen that they had to jumper the LPCO to get the compressor to run when they first started inserting the refrigerant. Must've been too cold out? After thinking about it, must be they were in a hurry, as the pressure had to be above 32, you'd think. Or the LPCO was mal-adjusted.
tom