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How to wire cooling fan to factory AC

mrk5

mrk5

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The Derale fan controller I'm using has the ability to turn the fans on at 60% power when the a/c is turned on.

16795.jpg

I'm wondering where I can pull a feed from the factory a/c to connect to the controller. Essentially it's just a trigger wire, so there won't be a large current draw.

I was thinking I could easily tag it off the plug for the AC compressor but then it would be cycling on and off a lot with the compressor clutch. So it seems like it would be better to come on when I slide the climate control to the "A/C" setting.
 
I use the compressor side of the low pressure switch to turn on a relay that powers the ac coil and my ac fan.
You could use a relay to power that box, or signal the box
 
ktmoutfront said:
What do you have for controls in the truck? When you switch it to AC, what wire goes hot?
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Okay, I think I fingered it out. Looked at the wiring diagram and the dark green/white wire sends power from the controls to the AC system.

AC-wiring.jpg

I didn't grab a picture from my truck but in the picture below you can barely see a terminal. Next to that one is where dark green/white wire comes in.

plug-for-fan.jpg

Luckily it's a Packard 56 terminal which I have on hand so I can make up a new terminal with my trigger wire for the fan crimped in. This should turn the fan on when the AC is on, not just when the compressor clutch is on.
 
Correct. That is why asked what controls the system. If the AC is on, fan should be running. Now you will have it all figured out by the time you add AC to the crew cab.

I know guys who run the fan only when the compressor cycles, but it does not cool well when stationary. At speed it is no big deal.
 
Thinking about this, you only have electric cooling fans? The factory strategy in older computerized electric fan only cars and trucks, was to turn on 1 of the fans full time with AC on, usually on low if 2 speeds, or the smaller.
 
When using defrost, is the fan running in cold temps going to be a problem?

No clue if modern vehicles take ambient air temps into consideration for the AC fan, just seems like a powerful fan could drive engine temps down under cold weather and low load conditions.
 
Makes sense. Wondered what the 60% was for. Hadnt considered it, but probably why a lot of the newer setups are running multi speed or PWM setups.
 
you can install whats called a trinary switch. This single switch has a high cutoff, low cut off, and fan control built into it. Usually this will replace the high pressure cut off switch in the system. I dont recall if the stock system has one though.

It will wire in line with your current fan trigger, and internally add a pressure switch for the fan, when compressor output gets above a set amount of PSI it triggers the fan. when pressure drops as the compressor turns off from cycle or shut down it kills the fan.
Ive put these in all the AC systems ive put together.

23101-VUW Diagram.jpg
 
dyeager535 said:
When using defrost, is the fan running in cold temps going to be a problem?

No clue if modern vehicles take ambient air temps into consideration for the AC fan, just seems like a powerful fan could drive engine temps down under cold weather and low load conditions.
Click to expand...
I hadn't thought about the fan running with the defrost on, too. Currently I have the AC override wired to a switch on the dash to turn the fans on manually. I might be able to wire it up so that I could use that switch to disconnect the override in the winter. For that matter, I could just set up some sort of disconnect in the wire under the hood. Just have to remember to reconnect it when I start using the AC again.

That being said, with the big block radiator I'm probably going to block off the radiator for the winter months so it won't matter too much if the fans are running.

TreeFarm said:
you can install whats called a trinary switch. This single switch has a high cutoff, low cut off, and fan control built into it. Usually this will replace the high pressure cut off switch in the system. I dont recall if the stock system has one though.

It will wire in line with your current fan trigger, and internally add a pressure switch for the fan, when compressor output gets above a set amount of PSI it triggers the fan. when pressure drops as the compressor turns off from cycle or shut down it kills the fan.
Ive put these in all the AC systems ive put together.

View attachment 356572
Click to expand...
I was reading about the trinary switch, but like you mention I'm not sure if I can put one on a factory system. All the discussions I could find about them were in regards to a Vintage Air system. I also wasn't sure if I could put it in without having to discharge/recharge the system.
 
mrk5 said:
I hadn't thought about the fan running with the defrost on, too. Currently I have the AC override wired to a switch on the dash to turn the fans on manually. I might be able to wire it up so that I could use that switch to disconnect the override in the winter. For that matter, I could just set up some sort of disconnect in the wire under the hood. Just have to remember to reconnect it when I start using the AC again.

That being said, with the big block radiator I'm probably going to block off the radiator for the winter months so it won't matter too much if the fans are running.


I was reading about the trinary switch, but like you mention I'm not sure if I can put one on a factory system. All the discussions I could find about them were in regards to a Vintage Air system. I also wasn't sure if I could put it in without having to discharge/recharge the system.
Click to expand...


pressure switches are schrader valve type, they can be removed and replaced on a charged system.

With that being said, looking at it more, Im pretty sure the square does not have any switch ports on the high pressure side of the system. It is just a simple low pressure cycle system. A trinary switch wont work on the low side.


So to make this simple and work with your stock system. you can get 12V postive any time the ac system is active from the non compressor side of the cycle switch. This switch takes power from the AC panel inside, gives it to the compressor to power the clutch. when the low side gets cold and pressure drops enough the switch opens to cycle the compressor off. The input (non compressor) side of this switch will still have power.

Use this to drive a relay to turn your fan on whenever the ac system is active.
 
I wired the Derale fan controller to turn the fans on with the AC. I didn't know until the shop that did the AC charge told me you need some air flow anytime the AC is running. Fortunately the controller has an AC override option.

16795.jpg

This is one of those times I'm glad I invested in the Delphi Packard 56 terminals so I can run wiring with factory style connectors. I just added a brown wire from the factory green & white wire that provides power to the AC system in the engine by. Crimp a new terminal onto the 2 wires and snap back in the original holder.

2020-10-13 14.16.12-1.jpg

Now the fans come on at 60% power any time the AC is turned on.
 
I hadn't gotten to all that with the '68. I just had a switch next to the AC controls that I would turn on the fans same time turning on the AC. Just let them run. Nice job
 
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