Best approach for fixing inop R12 A/C system

someotherguy · Jun 20, 2024

Getting really close to possibly DD'ing my '93 regular cab C3500 7.4, and one absolute requirement for that will be fixing the A/C.

Since it's a '93 it's still R12. As I've mentioned in my build thread, the clutch connector is broken off the compressor. I've considered trying to replace the clutch with a used one but it may be a waste of time, as the A/C has been inoperative for a who-knows number of years; don't want to assume the compressor would still be any good.

It would seem the wise choice is to convert to R134a since I won't be heading down to the local parts house to get R12. With the R134a conversion it's been mentioned I should swap the condenser, and also wire in a high pressure cutoff switch.

Gonna try the "by the numbers" approach with my questions, LOL

1. What compressor would you go with? Another regular R4, or a scroll style R4? I've heard of idle cooling issues with the scroll, and this is super hot/humid SE Texas, but it's also a regular cab. Also - before you say Sanden, I've looked into it and so far haven't seen a bracket kit that lets me mount one on a TBI 7.4?

2. Link to best way to wire in the high pressure cutoff switch? I've seen several discussions and there seem to be some variations on the theme.

3. One would *think* I could just compare 1993 to 1994 model parts since that was the change from R12 to R134a, but I am concerned about getting the correct lines and such. Any tips?

I'm sure I'm overlooking something...it feels like I am, anyway.

Richard

Nad_Yvalhosert · Jun 20, 2024

Get the A/C system running with the bare minimum of parts first: compressor, conversion fittings, and a recharge.
If it operates and theres no additional leaks found, then you can go further, tear down and replace the accumulator, orifice tube and perform a flush.
There's no sense in loading up the parts cannon if it has a wiring problem, or another blown up component.
By the way, the high pressure cut-off switch should be on the new compressor. No new wiring needed.

someotherguy · Jun 20, 2024

Nad_Yvalhosert said:
Get the A/C system running with the bare minimum of parts first: compressor, conversion fittings, and a recharge.
If it operates and theres no additional leaks found, then you can go further, tear down and replace the accumulator, orifice tube and perform a flush.
There's no sense in loading up the parts cannon if it has a wiring problem, or another blown up component.
By the way, the high pressure cut-off switch should be on the new compressor. No new wiring needed.

I definitely agree with the "evaluate the existing system" approach but the connector to the clutch is broken off flush with the rear of the clutch. I was originally hoping to scab some wires onto the broken connector back there just to test the compressor, but it looks fairly hopeless. If I have to replace the clutch.. that's $100, practically half the price of a new compressor. The existing compressor appears to be the 1993 original with over 160K miles and the level of abuse/neglect on the overall truck is pretty high. Who knows? The compressor might actually be good, but the opportunity cost of finding out is kinda high.

I don't own a vacuum pump or A/C gauge set and not sure if they're a wise investment for me at this time, though I'm not necessarily against owning these tools. With 4 vehicles to maintain, it's not out of the question.

I think you're correct on the cutoff switch. I'm not sure why I was thinking 1993 didn't have it but it does; it was just out of sight because the way the 7.4 brackets clock the compressor. Just went out there and had a peek at it. I'm a little confused looking at Rockauto for 1993 vs. 1994 as I'd expect the compressor-mounted switch to be different; doesn't R134a operate at higher pressures than R12, or am I confused?

If I replace the compressor I'll need to get a new switch, as past experience has shown me it's often impossible to remove the old one as the plastic is brittle and breaks off. Choosing the correct replacement for R134a has me a little worried.

Richard

someotherguy · Jun 20, 2024

Yeah, so further digging into the switches I see there are compressor-mounted low AND high pressure switches available. Not sure which a 1993 has IN the compressor, but I'm sure a trip through the service manual will tell me, when I'm ready to start tearing into it.

Richard

PlayingWithTBI · Jun 20, 2024

The past few R4 compressors I've bought didn't come with the HP switch in the back of them. Here's what I used...

https://www.oreillyauto.com/detail/c/murray-climate-control/lighting---electrical/switches/body-switches/a-c-pressure-switch/5e3a6dc98ab1/murray-climate-control-2-terminal-pressure-switch/mry0/35961/v/a/3009/automotive-truck-1988-chevrolet-c1500-pickup-rwd?q=a%2Fc+pressure+switch&pos=1

If your existing HP switch is a 2 wire, and you have a "Recirc" or "Max" button on your console, you probably don't need to change the wiring.

AK49BWL · Jun 20, 2024

Nad_Yvalhosert said:
By the way, the high pressure cut-off switch should be on the new compressor. No new wiring needed.

I've bought three compressors in the last 5 years for various vehicles, not one came with the high pressure switch installed, but a plug. I would not assume a compressor you buy will have one... YMMV

someotherguy · Jun 20, 2024

Yep, I'm in agreement here, all the new compressors I've ever seen have a slug there that you need to swap with your switch - if equipped.

PlayingWithTBI said:
The past few R4 compressors I've bought didn't come with the HP switch in the back of them. Here's what I used...

https://www.oreillyauto.com/detail/c/murray-climate-control/lighting---electrical/switches/body-switches/a-c-pressure-switch/5e3a6dc98ab1/murray-climate-control-2-terminal-pressure-switch/mry0/35961/v/a/3009/automotive-truck-1988-chevrolet-c1500-pickup-rwd?q=a%2Fc+pressure+switch&pos=1

If your existing HP switch is a 2 wire, and you have a "Recirc" or "Max" button on your console, you probably don't need to change the wiring.

Mine does have a 2 pin switch, and the '93 controls do have a recirculate/max button, I forget which one it's marked, but it does have it.

That blue Murray switch looks like the same blue the only listing for GM Genuine on Rockauto has for both '93 and '94 - GM # 1550812; no pressure specs listed. Four Seasons lists a similar blue color # 35961 with "on" pressure 200psi, "off" pressure 430psi. UAC # SW2830C is also blue and has the same pressure specs listed.

Richard

PlayingWithTBI · Jun 20, 2024

someotherguy said:
I'm sure I'm overlooking something...it feels like I am, anyway.

I'm sure you know, you'll need to flush the system since R12 mineral oil isn't compatible with PAG used in R134. You can use an ester oil which is compatible.

Any time you open an A/C system you should (need to) replace the drier/receiver too. Of course, when you flush the system, you do it without the new one in place. :wink:

Scooterwrench · Jun 20, 2024

I just retrofitted Smoky for 134a. Flushed the condenser and evaporator. Installed new manifold hoses,adapter fittings,accumulator/drier,Ford red orifice tube and R4 scroll compressor. I used PAG 46 oil which was also what the compressor came charged with. My truck didn't have a HP switch and I didn't wire one in. I used the 3 times ambient air temp rule for high side pressure which was 90deg that day so 270psi on the high side,about 52psi on the low side. It blows 42deg out the center vents.
I got new 4 way manifold gauges off evilbay,they set me back $46 and they work good. I use an old Ford Tecumseh compressor driven by a 1/2hp motor for my evacuation pump and let it run for about an hour. You can use any type of compressor with a V-belt by spot welding the clutch to the pulley. If you learn to do what you can with what you got it don't cost so much.

PlayingWithTBI · Jun 20, 2024

Scooterwrench said:
I used the 3 times ambient air temp rule for high side pressure which was 90deg that day so 270psi on the high side,about 52psi on the low side. It blows 42deg out the center vents.

That's fine but, when you're driving around in the heat of the day, shut it off for a few minutes to go shopping or eat lunch, come out and restart it with A/C on, it'll spike way over 430 PSI. I did that and it blew the high pressure switch out of the back of the compressor - sounded like a shotgun going off. Luckily enough I had another compressor with a smoked clutch so, I changed the clutch out of this one (since the snap ring groove was blown out too), and installed the other one's clutch. Have been running it for a few years now, after I wired in the HP switch so it worked like it's supposed to. Not just close the recirc door. The original R4 compressors came with a blowout valve so it'd let a bunch of refrigerant out to prevent blowing a hose or ruining the compressor. You just had to replace that valve and charge it back up

I'm sure EPA just loved releasing all the R12 into the atmosphere :biggrin:

Best approach for fixing inop R12 A/C system

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