Hey guys, I am finishing up an electric fan conversion and need some advice on how to use the ac select signal from my hvac controller to switch on my pwm fan controller. My '99 didn't have an electric fan originally, so I have to use the a/c select signal to trigger the fan controller.

The fan controller has a trigger wire for A/C switching but the 2.4v signal from the hvac control module to the pcm isn't high enough to turn on the fan controller.

How do I boost the signal voltage up to the 12v that the fan controller needs to switch on my fan when I turn the a/c switch on? Relay? Or some kind of voltage amplifier? Is there anything I should be cautious of doing it this way? I don't want to smoke my pcm or Hvac module.

Will add pics soon

 

It looks like the HVAC control head supplies power to the various pressure switches that control the compressor. Isn't that a 12 volt circuit??

 

If I am reading this correctly, it's the same 5v reference wire that feeds both circuits. It just splits off before the c200 connector.

 

Then use the wire for the control circuit of the compressor relay. That one is 12 volts. (it's a ground though, so, just have the ground side of the relay control circuit go there as well.)

 

If I tap into the control side for the compressor clutch, wouldn't that cycle the fan on and off with the compressor?

I originally had it spliced in to the compressor clutch 12v+ But when the compressor cycled off, the fan also shut off. Without the fan running the whole system blows warm air and the compressor takes forever to cycle back on. I have a manual override switch for the fan and when I switch that on, the system cools off. With the fan override switched on, it's icy cold even in 105 degrees but I don't want to rely on the override to switch it on all the time.

That's why I think I have to tap into the signal wire or something that stays energized even when the compressor is cycled off.

Oddly enough, It worked well when the system had a low charge because of leaking service ports. When I recharged the system, I also installed a severe weather (black) orifice tube. The higher pressure from the orifice tube I think is causing the compressor to cycle off more frequently.
The previous owner had installed the orifice tube backwards in the line but it still worked.

 

Wonder if you don't have the system over charged......

If the signal from the HVAC control head isn't 12 volts, to cue the PCM in that you want a/c, then there isn't an easy way to do it. Even if you go off the pressure switches, the fan will only run while the compressor does.

 

I charged the system by weight, 32 ounces and then verified afterwards by weighing the emptied cans.

Yeah, that is my dilemma.. The fan needs to be running even when the compressor cycles off, and because my truck didn't have an electric fan to begin with, there's no fan relays or signals from the PCM to tap into.

Is there a way to use that 2.4v to trigger a relay that will send 12v to my fan controller? Most low voltage trigger relays I have seen are a 5v trigger.

(EDIT) OK I have been reading up on how some of the hot rodders have integrated a/c control of their electric fans and apparently I need to use a trinary switch in place of the binary high pressure switch on my A/C. This switch triggers the fan relay (or controller) when A/C head pressure exceeds 240-250 psi so the fan isn't running when you don't need it like on the freeway. I still don't understand it fully yet, but so far the theory makes sense.

 

What is static pressure when the system is off, and hasn't been running for a bit? What is high-side pressure when the system is operating, but, compressor cycles off.... how low does it go?

 

Will have to hook it up tomorrow, But I don't suspect that I got the charge wrong.

Here is a video that I found that explains exactly what is happening when my fan shuts off due to the compressor cycling.


5:30 is the explanation

When the fan cycles or isn't running at idle, the high side pressure isn't brought down by cool air flowing over the condenser. The high temp orifice tube that I installed is actually compounding this issue, but like I said earlier when I am driving and/or I have the fan manually on, It blows icy cold even in 105 degrees. It gets up to 120 here occasionally so, I think I am gonna keep that in there.

I am going to need to use a tee on the high side line that will accommodate the OEM high pressure switch on one end and then hook on a switch that closes at 250psi on the other end. That switch will then send the 12v signal from bat+ to the fan controller and bring down the temperature and pressure of the r134 flowing through the condenser. I need to keep the OEM high side switch because it is wired in series with the low pressure switch and I don't think an aftermarket switch will cooperate with it.

One other thing that it may be: I might not have had a good connection on the 12v positive lead of my compressor clutch. I had just installed new valve covers and that wire was in the way. The controller signal wire pulled free from the wire tap connector and I might not have made a good connection when I hooked it back up.. I don't know, I'm just spitballin'. But I think I'm going to test this theory first.

 

Well, I am an idiot. I checked the static pressure 112 psi hi and low in 94.3 degrees ambient.. Then I proceeded to re attach the ac signal wire to the 12v+ wire of the compressor clutch. I soldered and heat shrunk it this time.. Started up the truck and the compressor compressed, the fan ran, and the air blew cold. With the fan running solidly, the compressor never built up enough pressure to cycle off. I turned the blower motor down to 1 to see if that would make it cycle and it kept running like a champ. The moral of this story is to never trust a POS wire tap connector to make a good connection.

I only drive this truck when I need to haul something, so I never really tested it after I recharged the ac system. When I jumped in it to go pick up an engine for a different car, It took a few minutes to recognize that the ac wasn't blowing cold. I jumped out to see if the compressor and fan was running and neither was so I thought the compressor had shut off the fan.. It was actually the fan not turning on that caused the compressor to shut off from high pressure.

Next fall after it has cooled off here, I plan on switching it over to run off of a switch hooked into the high pressure line. I like the idea of the fan shutting off when not needed like when running down the freeway.

Here are some pics of my setup as promised:

Homemade fan shroud. Not a whole lot of room between fan and engine.


Proform PWM fan controller:



Re-connected signal wire soldered heat shrunk and taped up.



3 core aluminum radiator. Keeps engine nice and cool no matter how hot the weather is. Really tight fit in the 99 though.



Showing off the pain in the **** valve covers. They caused all this mess. Found them in the junkyard and restored them myself.