Help with Air Conditioning Wiring?

[Hissing Cobra]

Hey everyone, the time has come. After driving my Cobra for the past 6 years and installing a brand new Air Conditioning system two years ago, I've decided to finally get off of my lazy butt and wire it up.

First, a little background on the situation. Nine years ago, I removed the original drive train, which consisted of the 2.3 Turbo/4-speed combination. My brother and I were careful to NOT cut any of the original wiring or throw any of it away. I also took all of the wiring from the engine upon removal. We've since replaced the drive train with a 302/5-speed combo and all has been fine thus far.

Two years ago, I purchased a lot of brand new A/C accessories for a 1990 Mustang LX with the 5.0/5-speed combo from Advance Auto Parts. Total cost was somewhere around $400.00 but it will be well worth it - should I be able to make it work! I installed the entire system which consisted of a brand new Evaporator Core, Accumulator (Receiver/Dryer), Condenser, and hoses. My brother had an extra pressure switch and pump that were in good working condition, so I will be using those. From the outside, it appears like I have a factory fresh installation and a working system - until you look at the wiring, which isn't hooked up to the pump yet. Here's a pic.



Fast forward to tonight and this is where I've started. I've got all of the plastic corrugated tubing and Black electrical tape removed from all of the wiring in the engine compartment and I've located what I think is the main wiring for the A/C pump (see both pic's below). It's on the passenger side and has a Bullet connector with a female end on it. If I remember from 9 years ago, the original pump had the male end on it but I'm not too sure. Also, there's some type of vacuum thingy attached to it. What is this and is it needed? Does this even go to the pump or is it something for the Turbocharger unit? I probed the Bullet connector with my test light and it did not light up. I then turned the key to the "On" position and moved the A/C control lever to the A/C position and turned the fan on. The test light still did not light.






I also have this as well (see pic below). What is this and what does it do? I know it connects to a female connector that I have on the passenger side of the radiator support but I'm not sure where it mounts, if I'll need it, or what it does.



As for the wiring itself, I have the 1979 Wiring Diagrams that I printed off of this site but they're very small and hard to read. I've tried to enlarge them and print the enlarged copies but have been unsuccessful thus far. I also have a Chilton's Manual and a Hayne's Manual and neither of them offer much of anything.

Any help is certainly appreciated!

[85GT-79FJ40]

Pete, try jumping the leads for the pressure switch together. That will bypass the switch in case the system pressure isn't high enough. With the switch on and car running with the pressure switch jumped you should see voltage at the wiring for the compressor clutch. If that's the actual wiring for it anyway. And provided you don't have a problem somewhere under the dash. But it's a start.

[JACook]

Knowing how the '79 wiring was done isn't going to be particularly helpful, other than
you're going to need to know what wires to tie into. The '79 system used an expansion
valve, the system you installed uses an orifice tube, and regulates the system by cycling
the compressor clutch on and off.

I don't recognize anything in those first couple photos, but the last one is an ambient
temperature sensor that disables the compressor when it's cold outside. The orifice tube
system doesn't need it, since the low side pressure switch will do essentially the same
thing.

The orifice tube system uses a compressor harness that looks something like this-


You don't need everything here, but the diode (the yellow lump) is kinda important.
Without one, the cycling switch won't live as long. It doesn't have to be a molded one
like that, but it does need to be fairly stout.

The wiring itself should be pretty simple. You need to take the wire that used to go to
the compressor clutch, and run it over to the cycling switch on top of the accumulator.
Then run the other side of the cycling switch to the compressor, and connect the diode
across the compressor coil. Make sure the diode is wired in "backward". It's job is to
shunt the reverse-current spike that happens when the clutch cycling switch opens.

[Hissing Cobra]

O.k. This is all good! My first problem is that the new pressure switch for the top of the accumulator (receiver/dryer) unit doesn't have any wires going to it yet, therefore I cannot jump them together.

Jeff,

Thanks for the info there. That wiring harness is interesting and looks pretty basic. On my driver's side, I have a wire with two diodes in it (see pic below). They look just like that Yellow diode in the pic that you posted. On mine, they're both in the same wire and this particular harness isn't hooked up to anything. When we installed the 302/5-speed combo, we didn't need this and we just covered it with corrugated tubing and wrapped it in tape. It hasn't been used since. I'm not sure if it's A/C wiring or if it's for the Turbocharger setup.



As for the wiring under the dash, it should all be good. Everything works in the car just like it was new, with the exception of the A/C and the cruise control, which I've removed because it was originally where the Accumulator is now. I'm going to relocate that to the underside of the driver's side fender when I'm done with this.

For starters, I guess I'll have to find the wire that used to go to the original A/C clutch (anyone know where this is and what color it is?) Once I find it, I'll run it over to the pressure switch. From there, I'll run the other side of it to the compressor. Now the compressor has two wires that will be coming out of it. What does the wire on the other side go to? Is it a ground wire?

[JACook]

The key to the original clutch wiring is in the third photo you posted. That ambient
temperature switch was mounted on the passenger side of the core support, and the
connector that it used to plug into had a light green/purple hash wire, and a black/
yellow hash wire. That sensor was the only thing that originally stood between the
control panel inside, and the compressor clutch itself. The light green/pink hash wire
is the one you will want to connect to your LPCO switch.

I believe that harness you have with the diodes was originally part of the turbo boost/
overboost indicator circuits. They were used for the "prove-out" function, to light up
the indicators when the ignition switch is in the "start" position. The connector at
the top of the photo would have connected to the turbo pressure switch on the back
side of the passenger side strut tower.

I can't say whether the diodes used in that application would be up to the task of an
A/C clutch coil snubber, but I have my doubts. It would be easy enough to incorporate
a diode into the clutch connector itself, such as an NTE576, or it shouldn't be too hard
to scrounge a factory style molded diode from a boneyard.

[Hissing Cobra]

Jeff,

I'm onto the right road here. Thank you! I reinstalled the battery, turned the key to the "On" position and moved the switch to the A/C position. I then probed the female connector that's on the passenger side of the radiator support and it lit up! Just to check, I went inside and moved the switch to the "Off" position and the light went out. Subsequent flips of the switch resulted in the light turning on and off.

So, this is my main wire, correct? If so, only one side of the two pronged plug lit up. If I take this main wire and send it to the pressure switch, what do I do with the other prong? Is the other side "hot" when I plug in that ambient temperature switch? If so, I'll have to find the other end of it and run from there. If I put a jumper wire from prong to prong and find the other end, it should light up? Correct?

It's been years since I've done wiring so please excuse my questions if they appear to be dumb!

[JACook]

The other prong used to go to the compressor clutch. If you can find the other end of
that wire, over by where the compressor used to be, you could use it. Basically you're
wiring in the LPCO switch in place of the ambient temperature sensor.

[Hissing Cobra]

Ah, o.k. Back to the garage!

[Mr Joshua]

take a look at my mess pete, if you want to feel better about your wiring

[Hissing Cobra]

I saw it Josh. No thanks! LOL!

[Hissing Cobra]

O.K. Jeff. I think I've got it. That wire that I found is the correct "Main" wire for the A/C system. It comes down the passenger side of the engine compartment (underneath the battery box) and up through the headlight bucket area to the passenger side of the radiator support, where it is the female end of a plug in connector. Only one side lights up, so I put a jumper wire from one terminal to the other and now there's "juice" going through the other wire, which routes back underneath the battery box and comes out by the Voltage regulator. I've probed the end of that wire where the 2nd female connector is and I have power there. This is where I have the Male part of the connector plug in (see my first two pic's in the original post).

So, I guess I have my power wire and I need to run this to the side of the pressure switch on top of the Accumulator, correct? Which side of the switch, does it matter? If that's correct, I need to make a new wire and connect it to the other side of the pressure switch and run it straight to the top of the compressor? If so, is this the wire that I need a diode in? How do you run the diode backward? Also, on top of the compressor, there are two wires. Obviously one of them would be connected to the wire from the pressure switch but what side? Does it matter? What is the other wire for? Is that a ground?

I know, there's a lot of questions but I want to be sure that I do this right, as this is probably the only time I'll do it in my life and I don't want to have the car catch on fire due to a poor wiring job!

For the record, I'm going to be soldering and using heat shrink tubing for any wire that I'll have to make or lengthen.

Thanks again!

[bscottie]

there should also be a high side pressure switch I believe somewhere on the condenser tubing, unless there is a relief valve on the compressor

[Hissing Cobra]

On my Orignal 1979 Condenser, there was a tank mounted to the side of it. I can't remember if there were wires attached to it or not. My new setup does not have the tank. It's actually mounted on the firewall and there's a Pressure Switch on top of it. I'm not too acclimated with doing any of this so any insight that you could provide would be great!

[795.0pacecar]

Out of curiosity why did you change over the system to an orifice tube set up? I have a late model compressor on my car and all I had to do was have hoses made, and connect the power wire from the HVAC switch to the compressor. I think total I spent $250 including freon. I did that 4 years ago and so far everything still works great, and I have 41º vent temps.

[JACook]

The old expansion valve system is a better system than the orifice tube kind, but what's
done is done, and this will work fine.

The factory orifice tube system in the Foxen did not have a high-pressure cutout switch.
The 6P148 compressor does have a relief valve, unless the outlet manifold was changed.

The tank on the old system was a receiver, that helps finish the condensing job, so that
the expansion valve gets liquid refrigerant. It also contained a desiccant bag. The tank on
the newer system is an accumulator, and also contains a desiccant bag. The accumulator
is there to give any remaining liquid refrigerant a place to boil off before going back to the
compressor. Compressors don't like liquid much. Two similar things that do very different
jobs.

It doesn't matter which side of the LPCO switch is wired to which. Just connect the hot
from the control panel to one side, and run the other side to the compressor. You can use
the factory ambient sensor wiring, and just extend the connection by the regulator, or run
a new wire. Whatever your preference.

If you look at the diode, you'll see a -|<- symbol on it. A normal diode would connect the
-| end to negative, and the <- end to positive, through a load. The diode would then only
allow current to flow in the normal direction. When you wire a diode as a snubber, you will
connect the <- end to ground, and the -| end to the wire from the LPCO switch. Connected
this way, the diode will not allow any current flow to ground when the circuit is energized.
But when the LPCO switch opens, the clutch coil will try to behave like a transformer, and
will try to send a reverse current back toward the switch, which would arc the contacts.
The snubber diode will shunt that reverse current spike to ground instead.

If you're using a generic diode, the -| end is denoted by a band. Just make sure you're
using one that can handle at least 50 amps surge current.

[Hissing Cobra]

Out of curiosity why did you change over the system to an orifice tube set up? I have a late model compressor on my car and all I had to do was have hoses made, and connect the power wire from the HVAC switch to the compressor. I think total I spent $250 including freon. I did that 4 years ago and so far everything still works great, and I have 41º vent temps.

Well, you would think that making hoses is something that should be easy to do, correct? I tried three different places that specialize in Air Conditioning systems. One of them wanted to install a complete aftermarket kit for $2,300.00. Another one told me to go to the junkyard and source 30 year old parts. The last one didn't even want to touch it. I guess that making hoses is something that people up here don't know how to do.

[Hissing Cobra]

Jeff,

Thanks for the info. I will be going to the junkyard within the next week and will source the Diode from a mid 80's A/C unit. I would imagine that all of them will be very similar, correct?

[JACook]

They were used on a lot of cars. It will be yellow, and it will be close to the compressor.
Grab the compressor and LPCO connectors while you're at it.

[Hissing Cobra]

Thanks! I'll let you know how I make out.

[CapriGT]

Thanks JACook for simplifying this. This saves me a lot of research time.

[Hissing Cobra]

Jeff,

I went to the three junkyards that are within a half an hour from where I live and the only wiring that I could find, came from a 4 cylinder Mustang that had air conditioning. The wiring for the pressure switch looked the same as the wiring on my '89 LX 5.0 but the compressor is on the passenger side of the engine compartment instead of the driver's side. The plug in harness for the compressor looked the same but the diode is different. There was a sticker on it that said DIODE and underneath the sticker there is a stamp that reads: ->IN4003 ->IN4003. Do you think that this will work? It's definitely a factory installation.

Also, this wiring harness was wired in a way that the three wires coming off of the pressure switch went to two different areas. One wire went through the dash and I'm assuming this one went to the switch. The other two went to some type of module on the passenger side shock tower. On the other side of the module, went the two wires to the compressor. I don't have that module but with the wiring instructions you've posted, it should be relatively easy to wire when we dig into it.

Ken (CapriGT) is coming over tonight to help me with this so I'll have an update later. We'll follow your instructions from your post above.

[JACook]

1N4003 is rated for 30A non-repetitive peak current, so it's probably enough, especially if
it was already being used as a clutch coil snubber. The clutch coil circuit doesn't much
care whether it's a 4cyl car.

[kj_80Cobra]

One wire went through the dash and I'm assuming this one went to the switch. The other two went to some type of module on the passenger side shock tower.

Correct it goes to the AC/Heater mode switch. The other two wires go to the Wide Open Throttle Relay.

[Hissing Cobra]

Thanks guys. We've got it temporarily wired to see if it works and we've got "juice" to the compressor plug in connector (bypassed the pressure switch because we know it won't work until there's pressure in the system). I'm thinking that even though the pump may be good, the clutch may be bad. I'll do some checking on that and report back when we get the results from that.

[Hissing Cobra]

UPDATE: Hey guys, I found out that the clutch on my pump was bad so I sourced another pump and I've now got it working! I buttoned up the entire wiring harness over the past three nights, wrapping it all with Black electrical tape and putting in the plastic corrugated tubing, with friction tape on the ends of the tubing so that it all looks like a factory installation).

I'm going to try and get it charged on Friday, as I'm only working half a day. We'll see if it can accept a charge.

Jeff, thank you very much for the help! I'll let you all know how the charge goes.

Edit: My brother and I took the clutch apart on my original pump and all of the steel was coated with surface rust (that's why it wouldn't work and the reason that I had to get another pump). We cleaned it on a wire wheel, put it back together and now it works. I'll be keeping that one as a spare for the future!