Adding A/C Technical Thread

[drcomanche]

This will be a thread to compile all the information about adding AC to non AC equipped trucks. I know there is another thread started almost a decade ago, but it seems to have very little info on the subject. Eventually, this should turn into a write up to settle all of the questions and technical info into one place. If you feel like I've missed something or want more info on a specific application, I can try to look into it and figure it out as well. This is for all the information, so I will edit the original post as we go.

 

For background, I have been taking apart a bunch of stuff on the truck(as per my usual winter tradition) and while I had the dash out I figured, why not add AC? It can't be that hard. In terms of mechanical difficulty, it isn't. The electrical became a huge issue, on the other hand. Anyone with enough mechanical skillset to remove their dash should't have too much issue installing this.

 

Obviously, the easiest way to add AC is to get a system from a matching year donor vehicle, that was you won't have to troubleshoot wiring. I chose a 96 because it was available to me for cheap and it is an r134 system, making it easy to charge and deal with.

 

The parts I grabbed from the donor were:

Compressor

Evaporator(the "radiator" in the HVAC box)

Condenser(the "radiator" behind the grill)

Drier

Expansion Valve

Suction Hose(Compressor to Valve)

Discharge Hose(Compressor to Condenser)

Liquid Hose(Drier to Condenser)

Hose(Drier to Valve)

Low pressure switch(attaches to Drier)

HVAC airbox(AC equipped airbox assemblies came with a larger compartment space to fit the evaporator)

Dash harness(My truck wasn't equipped for AC, so there were no wiring components in the harness)

HVAC Controller(the in dash selectors)

 

*These parts can change depending on what years you are scavenging parts from. I believe some systems had an accumulator, too. I took all of my parts from a 96 Cherokee.

 

Installation of the engine bay components is fairly easy, using pre existing bolts and holes. Non AC equipped trucks will have to remove the dash and replace the internal airbox with an AC optioned one. If you're looking, I recommend going to a pull yard and taking the whole system, that way you know you have everything and know it all fits together. The wiring is the tough part of this one, so based on my experiences, I've created a few diagrams to help make it possible. The Haynes manual was of some help here, but it was lacking in some of the wiring bits, which makes it all the more confusing. 

 

The wiring itself is a small grouping. I chose to dismantle the entire harness to separate it as completely as possible rather than cutting and trying to build it. This involves removing a few pins from the firewall plug and one from the fuse block. The firewall plugs require you to remove the retaining pins before they can be removed(plastic bars on the side of the block). Then just use a 90 degree pick to work it out of the hole. The fuse block side has 2 metal prongs on either side holding it in. I used the pick for this as well, and with a little manipulation got it out. The rest of the circuit is attached only to these 4 wires. 

The black and tan wire is the power in and splits into 2 wires(also black and tan) that feed into the same plug, but 2 different switches. This feeds the sliding bar selector on the dash unit. From there, it has 2 light green and white wires that exit, one going to the firewall plug and turning into a green wire that feeds the low pressure switch, the AC relay, and the ECU while the other goes to the thermostat plug attached to the HVAC box in the cab. The last wire(yellow tan) goes to two different locations, the first being the blower motor selector switch on the dash unit, and the second to the blower resistor on the HVAC box in the passengers footwell. The blower switch simply controls the blower fan speed with a green wire going the the firewall plug(and off to the blower motor itself). The thermostat plug itself grounds to the harness with the black wire and sends a blue orange wire to the firewall plug(I'm not sure if it's any different to have it ground to the metal dash structure as opposed to going through the dash harness). These are the only wires in the circuit and should run the entire system. According to the Haynes manual, this should be how 91 to 96 works. The earlier 87 to 90 is very similar, with differing colors for wires.

The main input plug. This feeds the whole system in the cab.

The blower switch plug. This selects the speed of the blower fan

The thermostat plug. Sends one wire out and grounds to the dash harness.

The blower resistor plug. Controls fan speed

The 4 pins.

 

In the engine bay I found something interesting. Despite my new(to me)harness from a 91 not have AC equipped, the engine harness still has the components for it. It has an attached low pressure switch plug(that I'll have to replace with the one on a newer harness, but uses the same color wires), and the AC relay, rather than being empty, is fully wired, so I believe the ECU is equipped to handle AC even if the cab harness is missing the necessary parts. I won't know for sure until I start the vehicle, which is going to be a long while, but I can't see any reason it wouldn't. The AC relay has dedicated wires that are only part of that system that route to the ECU.

 

Any questions or additional info, let me know. 

[dasbulliwagen]

When I did mine on my 87 with an 86 donor, all the dash wiring was plug and play. On the engine compartment side, about half of the wiring was there including the relay and the orange compressor wire. But there was also an unused connector for dealership installed AC. I saw that in the downloadable 88 FSM and found it on the left inner fender of my truck. I then built the part of the harness I needed from the diagram. It worked and I've never had an issue with the electrical side of the system. 

[Jeep Driver]

What precisely does the ECU control in the AC system?

[drcomanche]

1 hour ago, dasbulliwagen said:

When I did mine on my 87 with an 86 donor, all the dash wiring was plug and play. On the engine compartment side, about half of the wiring was there including the relay and the orange compressor wire. But there was also an unused connector for dealership installed AC. I saw that in the downloadable 88 FSM and found it on the left inner fender of my truck. I then built the part of the harness I needed from the diagram. It worked and I've never had an issue with the electrical side of the system. 

I've seen another plug with almost the exact same fitting type, so maybe that relates to the dealer installed side. It's a 2 pin connector with only a tiny difference in the internal shape. I'd be curious to see how the pre 91 harness looked. If you have some pics or more info on that, I can add it to the main post so we can compile everything.

1 hour ago, Jeep Driver said:

What precisely does the ECU control in the AC system?

The best I can guess is that when the ac kicks on, it triggers an idle adjustment for the extra load. I'll have to ask a buddy who's more well versed in the programmed side if it does more. Judging by the system, it doesn't seem to need too much from the ECU. The rest of the wires that go through the block seem to go to the relay or the blower motor. Only the green wire hits the ECU.

[dasbulliwagen]

I'll see if I can get a pic of the dealer installed plug for you tomorrow.

 

[eaglescout526]

26 minutes ago, dasbulliwagen said:

I'll see if I can get a pic of the dealer installed plug for you tomorrow.

 

You mean this one?

 

I just added AC to mine and did a write up that's more for the 2.5L comrades as everything is flooded with info on the 4.0L

[Jeep Driver]

32 minutes ago, drcomanche said:

I've seen another plug with almost the exact same fitting type, so maybe that relates to the dealer installed side. It's a 2 pin connector with only a tiny difference in the internal shape. I'd be curious to see how the pre 91 harness looked. If you have some pics or more info on that, I can add it to the main post so we can compile everything.

The best I can guess is that when the ac kicks on, it triggers an idle adjustment for the extra load. I'll have to ask a buddy who's more well versed in the programmed side if it does more. Judging by the system, it doesn't seem to need too much from the ECU. The rest of the wires that go through the block seem to go to the relay or the blower motor. Only the green wire hits the ECU.

 

Thanks. 

 

Just curious because I'll be eliminating the ECU in the future. 

[drcomanche]

1 minute ago, Jeep Driver said:

 

Thanks. 

 

Just curious because I'll be eliminating the ECU in the future. 

Apparently, from what I've heard, newer age compressors don't draw as much power, so it doesn't need the extra idle rpms. I'm not sure how true that is, but it's something to consider.

[Jeep Driver]

I'm going V8..........it ain't gonna matter. 

[Ωhm]

Did I miss something here. Doesn't ECU also control the AC Clutch Relay?

[drcomanche]

30 minutes ago, Ωhm said:

Did I miss something here. Doesn't ECU also control the AC Clutch Relay?

According to the Haynes manual(which I'm not sure how reliable it is) there are 3 wires going to the ECU from the HVAC harness. The power input is the Blue White wire to the 4 pin relay. The only one from the cab that interacts with the ECU is the white green wire that passes through, but before it gets there, it divides to the relay and the low pressure switch. Its inclusion in the relay is what dictates the compressor clutch engagement. The rest that connect with the ECU seem to feed out to the other components.  I reiterate that the Haynes manual has wires that don't exist on any of my 3 harnesses(91, 93, 96) so this is what I've gleaned from looking at the physical harness and the diagram. I figure the only thing the compressor needs from the harness is a single signal for when the interior selectors choose AC to engage the clutch. 

 

I say all of this mostly in speculation, since I can't start my vehicle and volt check it. I'm trying to understand the wiring to the best of my ability, and this just seems to be the most likely solution. I'm open to broadening my scope with more info so that we can have the most complete manual for installation. 

[Ωhm]

I missed the part in your signature about 94 Engine and Drivetrain swap. I'm looking at the 88 wiring diagrams. It shows the following:

ECU INPUTS:

AC Request & AC Select

ECU OUTPUTS:

AC Clutch Relay

[drcomanche]

1 minute ago, Ωhm said:

I missed the part in your signature about 94 Engine and Drivetrain swap. I'm looking at the 88 wiring diagrams. It show the following:

ECU INPUTS:

AC Request & AC Select

ECU OUTPUTS:

AC Clutch Relay

Which diagrams do you have? I'd like to include all possible info for everyone, so if its different then it would still be handy for people with pre 91 harnesses. I'm looking at the Haynes manual, and it is equally as confusing, but seems to have the relative same circuits for the relay.

[Ωhm]

PM sent.

[scott38582]

I am trying to put air conditioning in an 87 MJ with 4.0.  I have put an airbox from a 1993 XJ but the harnesses are not the same as the XJ.  I have thought about adding separate wiring (homemade relays) but would like it to run though the ECM if possible?  If anyone could share their experience and wiring schematic it would be appreciated.   

 

Also, I was wondering if the 6 wire plug before the heating harness is wired to the ecu?  There are more wires going in then going into the 6 wire connected then to the old non-ac control.  Would it be worth it to find a 1987-1988 XJ harness or can I get the 1993 wires to work?

 

Than you for help, 

 

Scott