Air Conditioning Wiring Question

[BrianX128]

I'll upfront say this is from two bricknose straight sixes and not IDI's, but I believe the wiring is the same so I figured I would ask here anyways.

I have a 1990 that had no AC, that I desperately want ac in. I have an 89 beater winter truck that has a perfect running AC system that never gets turned on due to when I usually drive it. Since it's not like my bullnose with no ac vents, I know this is possible to swap over. I just watched this video of someone doing what I'd like to do here almost to a tee:

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The only part of this project that concerns me is the wiring portion. He mentions that the non ac vehicle he's swapping the ac into was "wired for AC" as if all bricknose trucks had the harness plugs hanging near the hvac fan in the engine for the AC hvac controls in the cab.

If that isn't true and my truck doesn't, my other question is isn't all of the wiring differences between the two just two extra hvac slots that send power from the hvac controls to the low pressure switch on the ac (as well as defrost turning it on), and if it has 25 pounds then it sends the hot power to the plug on the ac compressor to lock the clutch and the other wire on the compressor just goes from there to a ground?

Couldn't I just leave the non hvac controls in my good truck that's getting the AC, and just wire a toggle switch to a relay and send power to the low pressure switch that way to turn the AC on instead of swap out all of the controls? I'd lose the "max ac blend door" but I'm ok with that. This is all assuming the good truck doesn't have the harness leads like the guys in the video did, if mine has that I'll just swap everything. It really doesn't look like that bad of a project, only difference for me is I will need a different belt.

 

[BR3]

There's always an option for a manual switch when it comes to wiring. Your main problem in my opinion would be your HVAC box because the heat only models did not come with the same design box as the evaporator coil equipped models. Even that part of the swap should be relatively straightforward, but I've never done it myself so I can't say for sure.

 

[tractorhead]

From a wiring standpoint the only difference is the power wire to the ac clutch, the blower wiring is all the same

 

[riphip]

You will need a clutch cycling switch at the evaporator core or accumulator to keep it from freezing over when it gets too low of pressure

 

[BR3]

The op had acknowledged that. His intent was to just run it through the pressure switch essentially.

On that note, a relay may be ideal, I'm not sure if the pressure switch is up to the current draw of the clutch

 

[laserjock]

I’m pretty sure it runs through the pressure switch stock but don’t hold me to that. I can check the manual if needed.

 

[BR3]

It might, I've not seen the schematic, but the wire gauge is definitely much smaller on the p.s. feed than the compressor feed. Probably around 12awg to the compressors 8ish awg. It would probably be fine for a long time, but Id always rather be safe than sorry

Sent from my Moto Z (2) using Tapatalk

 

[BrianX128]

Either way as long as I know I'm running from the pressure switch on the drier to the switch on the clutch I can make it work, I just wanted to make sure that's all it was cause I'm probably going to be doing this without a hvac control box from an ac truck so I just want to make sure I'll be able to turn it on "safely". Toggle switch in the dash controlling a signal wire to a relay under the hood right beside the pressure switch on the drier that sends power to it.

Then I'll just use the slider in the truck and have it on "vent" to blow the air straight at you in the cab and be able to press the toggle switch and turn the ac on.

 

[Macrobb]

If you are using a new/custom plug on the AC compressor, note that there is a component(diode? resistor?) in the harness right next to the plug for the compressor.

Not entirely sure what it is, but if it's a resistor, it's definitely needed(limit current through the coil). If it's a diode, it should be put "backwards" across the two coil wires - when the compressor turns /off/, the coil inside the clutch will generate a massive negative voltage spike, which isn't great for relay contacts, and also can screw up poorly-shielded electronics.

Also, pretty sure it's about 12awg-14awg wire going to the compressor on my trucks. It doesn't take that much current. The insulation may be thicker than other places though, considering the oil and temp resistance it needs.

 

[franklin2]

The A/C system has the blower wired differently. There is no "OFF" on the blower speed like there is on a truck that has just heat. This is so you cannot turn the blower off with the A/C system running, this would turn the evaporator core in the dash into a block of ice, and you would run the danger of liquid refrigerant making it's way to the compressor which would lock it up and smoke the belt that turns the compressor.

The low pressure switch controls the clutch directly, it's that way on most all Fords. Sometime later they changed it around and let the PCM control the clutch, with the pressure switch being a input to the PCM.

 

[franklin2]

If you are using a new/custom plug on the AC compressor, note that there is a component(diode? resistor?) in the harness right next to the plug for the compressor.

Not entirely sure what it is, but if it's a resistor, it's definitely needed(limit current through the coil). If it's a diode, it should be put "backwards" across the two coil wires - when the compressor turns /off/, the coil inside the clutch will generate a massive negative voltage spike, which isn't great for relay contacts, and also can screw up poorly-shielded electronics.

Also, pretty sure it's about 12awg-14awg wire going to the compressor on my trucks. It doesn't take that much current. The insulation may be thicker than other places though, considering the oil and temp resistance it needs.

It is a diode, and you must have it if you want the low pressure switch to survive. The diode is put in backwards, and clamps down on the kick-back from the clutch coil when the power is cut off to the coil.

 

[BrianX128]

I can take all wiring from the doner truck from the low pressure switch to the compressor so I can keep the diode, I just didnt want to switch engine wiring harnesses and such if I didn't have to. I won't need it in the doner truck anymore anyways.

 

[BR3]

I guess I'm more used to obs trucks than brick nose that the op is concerned about in Wich the diode is not immediately obvious and awg is larger. certainly not saying it isn't there, but I don't remember seeing it. The awg on all three obs trucks I have is certainly larger than 12, maybe not 8, but certainly at minimum 10. That's insulation/conductor size well considered. Pressure switch is probably 14. Still not saying it's not run through there, just visibly different wire gauge.

As for the freeze up, just make sure to kill the compressor before the fan haha or draw switch power from the fan switch if your worried about forgetting

 

[Macrobb]

The A/C system has the blower wired differently. There is no "OFF" on the blower speed like there is on a truck that has just heat. This is so you cannot turn the blower off with the A/C system running, this would turn the evaporator core in the dash into a block of ice, and you would run the danger of liquid refrigerant making it's way to the compressor which would lock it up and smoke the belt that turns the compressor.

The low pressure switch controls the clutch directly, it's that way on most all Fords. Sometime later they changed it around and let the PCM control the clutch, with the pressure switch being a input to the PCM.

It shouldn't matter if the blower turns off, though - if the evaporator core turns into a block of ice, the pressure should drop enough to turn the switch off.

Alternately, do what I did on my '88 - I added a second switch in-line, a 32F therm-o-disk temp switch. Attach it to the tube right after the expansion valve, insulate it from the high temps of the engine bay, and it'll shut the compressor down when things get too cold.


I'm going to have to check out my '87 F-350 - it's got the "push button" AC on the climate control panel, see what happens with fan off and ac depressed.

 

[BR3]

It will continue to run if the system is even just slightly overcharged. Even still, it would take a very very long time frozen to do any real damage. My dually that I've propane converted will freeze up in about 20 minutes regardless of all other factors because the coil averages about -2 'f. Been working like that for almost 3 years now. Just switch over to vent for a while and it goes back to blowing hard again. A freeze up scenario is impossible to miss, air stops blowing. In Brian's scenario all he has to do is cut his compressor off for a few minutes.Besides that high enough volumes of Liquid refergerant take a very long time to form, almost not realistic when your in a hot engine bay. If a compressor blow up due to low temps was going to happen, it should have happened to me. I've got functional engine bay cooling, super insulation over EVERY ac conponent, running propane, secondary electric fan, and the explorer heater bypass, and a picture of 7'f vent temps somowhere

Just use common sense and everything should work out ok is really my point,no need to reinrein the wheel on what can be a pretty basic system