I installed the complete system from a 90 Ramcharger into my 85 Crew cab and I have some questions about how to control it. I have the 85 and 90 service manuals. If I am reading these correctly the 85 has positive power at the pressure switch and clutch cycle switch to power the compressor.

The 90 has ground at the pressure and cycling switch. But then the cycling switch sends power to the computer and the computer tells a relay to send power the the compressor. Does this sound correct?

So my question is how should I hook this up? The truck didn't originally have A/C. I am not useing the computer. I need 12V at the compressor. Currently when I push the A/C button it turns on a ground at the firewall. Should I run this to the low pressure switch like on the 90 and then let the electronic cycling switch turn on a relay that would turn on the compressor?

Sorry for the long text, AM I looking at this correctly?

I may have answered my own question....Even though not totally correct. When i turned the page of the 90 manual It had the Diesel A/C diagram. Here it has that wire that went to the computer (gas) going directly to the compressor instead. At this point I believe this is a ground and is what triggers the compressor. 12v positive at the compressor is just an ignition on wire and is fused.

So a new question... What is the 12V positive wire at the 1990 electronic clutch cycling switch for? Could someone tell me how this switch works?

I guess I will take a stab at this... I would guess that this is like some type of relay and the power is used to make or break the connection?

Has anyone here added A/C to their truck? If so how did you do it?

How did you go about swapping A/C into a non A/C truck? Did you change the whole dash or add vents to your existing dash?

I picked up a 90 Ramcharger that was loaded with everything to use as a donor. I wanted the grey dash (and interior) so I swapped everything needed. A/C heater box, all the venting, cluster face, control switches, and everything needed under the hood. It was all a bolt in to my 85. The only thing i have left is to make the wire connections at the fire wall. At this point I think I know how i want to do that....

I would think you could cut out the vent holes in your original dash if thats what you wanted to do. Having a donor makes it all so much easier.

A/C clutch is a simple on/off clutch. I don't know whether the plug on your compressor needs ground or power or both to activate. However two things. One, I don't think I would want to send full power or ground through the a/c switch. A simple 5 in relay would be better. Plus with the relay if you need to change your switch from positive to negative you can do it easily.

On your 90, the ECU will be controlling to a/c to shut off the compressor at full throttle, during cranking, etc. I would use a relay to mimic what the ECU was doing.

Thank you for the reply, Both the 85 and 90 systems have 2 wires at the compressor. I would agree with you that I didn't want to send full power through the A/C switch. But in 85 that is how its wired. The 90 system sends a ground through the A/C switch. I'm using all 90 components so that is how the switch is currently connected behind the dash. I like the idea of the relays.... You are correct about the 90 gas system, It does have a wide open throttle cutout relay by the computer. The 90 diesel does not have the computer or cutout relay. As far as the relays go this Electronic clutch cycling switch I asked about seems to be some type of relay already in the factory system. Its the last switch before the clutch on the 90 diesel. At this point I believe this is how I'm going to make my last connections on the 85.

On your wiring schematic for the 90, does the wire from the ECU go directly to the a/c compressor or to a relay first? If it goes direct, you would probably be ok hooking the compressor right up to your dash switch. The factory ECU's don't handle a lot of current for externals, which is why they used relays for most things. So if in this case the factory didn't, it stands to reason the compressor only needs a small amount of power to run it.

In the 90 Gas schematic there is three wires connected to the computer. One on the electronic clutch cycling switch and two on the Wide open throttle cutout relay, this relay is connected to the compressor. Now on the 90 Diesel schematic the compressor is connected to the wiper motor (IGN run side and a 20amp fuse) the ground is connected to the electronic clutch cycling switch, then the A/C low pressure switch, and then the dash switch, and to ground. Here now instead of the computer the third wire on the clutch cycling switch is connected to Ign run.

I don't believe the compressor needs a lot of power but i do agree with you about the use of relays. I would like to find and use some sort of heavy duty relay for the compressor and the cycling switch. I dont want to add load to the 85 wiring. Just something simple under the hood. On a side note I also have an electric fuel pump. So maybe some type of continuous duty relay and a small fuse block for the pump and A/C items.

QuickDodge, I looked at my old dash and on the back side the two vent holes that would be next to the glove box, are marked out where you would need to cut to add the factory A/C vents.

Sound like a generic parts store 5 pin relay and pigtail is all you need.