AC cycling Switch

[Pauly1119]

My truck is a 1996 with a 383 engine swap carburetor engine. I’ve owned it for a month now. Purchased knowing the AC doesn’t work but is all there. All the computers have been removed from my truck. All my instruments work. Seems to have been done right but I don’t have a lot of history on the build. System was never charged, dash controls all work, fan motor. My main question is I don’t have a cycling switch and no harness for one that I can find either. I was getting ready to start going through the AC and try to charge the system. I’m pretty sure a cycle switch is pretty important. Can I just add one? Is there an analog version of one that doesn’t need a computer? Do I even need one? Any help would be appreciated. I was thinking about just bringing it to a AC shop but looking for options first.

 

[AK49BWL]

There's not a switch on the dryer? There should be a switch there, and two at the compressor you'll want to run in series from your A/C switch to the compressor clutch relay, since the PCM is no longer in control of it. If the harnesses are gone, you're best to grab one from a donor truck at a junkyard, any 96+ GMT400 truck will work. New switches are available on Amazon and such, and unless you're 100% sure of the usable condition of the A/C system, you'd better replace the dryer and orifice tube while you're at it. If the system was ever opened, the dryer is finished. Check for metallic debris in the orifice tube, if present you'll need a condenser flush and potentially a compressor. Sanden 4440 is a great replacement. If you really want to go all out, get a parallel flow condenser and a variable orifice. I never have complaints about cooling in my 97

 

[Pauly1119]

Can I wire the cycle switch with the same wires from the high pressure wires in the back of the compressor?

 

[Pauly1119]

There's not a switch on the dryer? There should be a switch there, and two at the compressor you'll want to run in series from your A/C switch to the compressor clutch relay, since the PCM is no longer in control of it. If the harnesses are gone, you're best to grab one from a donor truck at a junkyard, any 96+ GMT400 truck will work. New switches are available on Amazon and such, and unless you're 100% sure of the usable condition of the A/C system, you'd better replace the dryer and orifice tube while you're at it. If the system was ever opened, the dryer is finished. Check for metallic debris in the orifice tube, if present you'll need a condenser flush and potentially a compressor. Sanden 4440 is a great replacement. If you really want to go all out, get a parallel flow condenser and a variable orifice. I never have complaints about cooling in my 97

The Accumulator, clutch and high pressure sensor on the compressor are all wired in already.

 

[AK49BWL]

Well the accumulator switch is the cycling (low pressure) switch. The third switch on the line set leaving the compressor is simply a second high pressure switch that isn't necessarily needed. So you should be fine as is electrically.

 

[Pauly1119]

Well the accumulator switch is the cycling (low pressure) switch. The third switch on the line set leaving the compressor is simply a second high pressure switch that isn't necessarily needed. So you should be fine as is electrically.

Thank you. Just to double check because the lines are just hanging right now. I connect to the sensors on the manifold above the compressor from the accumulator? I don’t have to connect to the other high pressure sensor on the back of the actual compressor? Sorry AC is out of my wheelhouse.

 

[AK49BWL]

Okay, well here's how I'd do it... You have your A/C switch coming from the control head. Route that directly to the A/C clutch relay (pin 85 or 86), assuming it's still in the fuse box under the hood... If not, then any old 30/40 amp automotive relay works fine. The other side of the relay coil (85 or 86) originally goes to the PCM, I would run that wire to the pressure switches, put all three switches in series, and the far end to ground. That way, when the system is working like it should, all switches are closed providing a ground for the relay coil which will be powered by you turning the A/C on in the cab. This also keeps the switches running very low current, a lot less likely to burn them out in the event of a clutch coil issue or short. Then the compressor clutch can be wired like a factory 96, hot side to relay pin 30 (hot at all times will be on pin 87 from a fused source), and the other side to ground.

But from the sounds of an earlier post you already have the compressor clutch itself wired to the compressor high pressure and accumulator switches? This is fine and you can forget the third switch even exists, as long as one high pressure switch is in there. Whatever the case, the cab button needs to be wired to a relay to activate the clutch because the control head cannot handle the compressor clutch current and will absolutely burn out.

 

[PlayingWithTBI]

Route that directly to the A/C clutch relay (pin 85 or 86), assuming it's still in the fuse box under the hood... If not, then any old 30/40 amp automotive relay works fine. The other side of the relay coil (85 or 86) originally goes to the PCM, I would run that wire to the pressure switches, put all three switches in series, and the far end to ground. That way, when the system is working like it should, all switches are closed providing a ground for the relay coil which will be powered by you turning the A/C on in the cab.

^^^This - here's what I did with my '88. I used 2 switches, the pressure cycling and high pressure switches. I wired a relay so I could get full battery voltage to the compressor clutch. I know mine is a little different but, the theory is the same.

Swapping the Clutch on an R4 A/C Compressor

From previous post. Here's what it looks like now. I zip-tied the harness to the ground cable going to the front of the block. Here's the results. It appears to be working OK, I'll drive it for a while before performing a clutch-ectomy. I covered up the splices. Then I finalized my...

www.gmt400.com

 

[GoToGuy]

You really should try using the OE service manual , the system operation is spelled out and easy to understand/ learn. Not to mention the wire digrams would be extremely helpful since yours is modded. Have thought about a idle kick when compressor cycles?

 

[Pauly1119]

Okay, well here's how I'd do it... You have your A/C switch coming from the control head. Route that directly to the A/C clutch relay (pin 85 or 86), assuming it's still in the fuse box under the hood... If not, then any old 30/40 amp automotive relay works fine. The other side of the relay coil (85 or 86) originally goes to the PCM, I would run that wire to the pressure switches, put all three switches in series, and the far end to ground. That way, when the system is working like it should, all switches are closed providing a ground for the relay coil which will be powered by you turning the A/C on in the cab. This also keeps the switches running very low current, a lot less likely to burn them out in the event of a clutch coil issue or short. Then the compressor clutch can be wired like a factory 96, hot side to relay pin 30 (hot at all times will be on pin 87 from a fused source), and the other side to ground.

But from the sounds of an earlier post you already have the compressor clutch itself wired to the compressor high pressure and accumulator switches? This is fine and you can forget the third switch even exists, as long as one high pressure switch is in there. Whatever the case, the cab button needs to be wired to a relay to activate the clutch because the control head cannot handle the compressor clutch current and will absolutely burn out.

Thank you for the help. I’m going to expose the wires and trace them.