Author Topic: A/C Wiring Diagram and A/C Blower How-tos (Read 63259 times)

firebirdparts · « **on:** January 16, 2013, 08:36:26 AM »

We get many questions about A/C wiring, mostly connected to issues with the blower.

Here is the 78 wiring diagram for A/C:

Years other than 1978 differ a little bit in two ways:

1. The control head interacts with blower speed a little bit differently for different years. This is not a big deal. Since "low" fan speed does not ever need to be off, it does not need to go through the switches at all, and that is how a 78 is. Some years have a "real" set of contacts in the switch for low fan speed. For some years, MAX a/c has a set of contacts that bypass the fan speed switch, resulting in high fan speed all the time in MAX A/C.

2. The cold controls that turn the compressor off vary from year to year and a little bit with engine choice. In every case, all the components that can cut the compressor off are in series, so they have to all agree for it to be on.

firebirdparts · « **Reply #1 on:** January 17, 2013, 09:15:21 AM »

There are three major things you need to know about the blower wiring, in addition to the fact that the fan is always on, even in the "off" position.
1. There's a ground wire. The blower and the relay are mounted in plastic, and they need a ground wire. For most engines, the ground was on the compressor brackets with the compressor clutch ground. There's a plug, called the "compressor harn line conn" in this diagram. If you unplug that, the blower will stop. If you remove your compressor brackets, the blower will stop (for most engines).

2. There are two power supplies for the blower. High speed is powered off the back of the alternator for most engines, circuit #2, red, in this diagram. There is a connector in that line, or maybe two, and these get hot and corrode. Expect problems. Lower speeds are powered by the heater fuse, which sends power through wires 50A, 50B (brown) under the dash, plugs into the A/C harness, and powers all the controls there, and eventually goes through the resistors and the compressor cold controls.

3. The relay is a double-throw. It sees a big red wire with 12V coming from the alternator, and reduced power coming from the resistors. It has to choose between those two, and connect one of those two items to the blower motor. When the coil is powered by circuit 52, it chooses high. If the coil is off, it chooses the resistors. The coil is turned on by the A/C control head under the dash.

ILLTA77 · « **Reply #2 on:** January 17, 2013, 10:17:34 AM »

great write up, most of us who have and use the A/C will run into some of these wiring/connector issues failing at some time. Until you work on several, you just dont know where to begin. A shop manual is needed, they seemed to change the wiring every few years due to improvements in the A/C system parts. TXV, VIR, CCOT all different systems used over the years in Firebirds

800qjet · « **Reply #3 on:** January 17, 2013, 04:35:48 PM »

Thanks for the great writeup. As usual, you have been most helpful.

firebirdparts · « **Reply #4 on:** January 18, 2013, 11:55:20 AM »

Thanks, guys. Everybody has trouble with this sometime.

« **Reply #4 on:** January 18, 2013, 11:55:20 AM »

2020-B1-10

BlueTTA · « **Reply #5 on:** January 18, 2013, 12:36:32 PM »

Here is a drawing for 1980. (Figure 1B-3 F Series C60 A/C Wiring)

Personally, I disagree with the drawing for the (T) VIN because there is no Idle Stop Solenoid and no Wide Open Throttle switch.
So the Turbo should look much more like the (W) or 301 N/A sub drawing.

To save it, just right click on the drawing and save as a .PNG file. Then you will be able to zoom on the file to read the details.

firebirdparts · « **Reply #6 on:** January 18, 2013, 01:47:57 PM »

They must have changed their minds.

I have only seen one with WOT A/C cutout, and that was on a 1977 Formula, 301, 2-barrel.

800qjet · « **Reply #7 on:** January 18, 2013, 02:23:13 PM »

I'll keep looking for info on the WOT switch. I have a switch and wiring and hope to adapt it to the Qjet 4 bbl. Might be good for the AC compressor. I had a new '78 GP with the 301 two bbl. and it had the WOT on a Rochester Dualjet 210.

jjr · « **Reply #8 on:** January 20, 2013, 05:26:43 PM »

1980, VIN code T... with WOT switch...

Joe

800qjet · « **Reply #9 on:** January 20, 2013, 05:42:27 PM »

Thanks, I printed the wiring diagram. That will help sort out my box of wiring. I asked on the 301 forum about pix of the linkage side of the Dualjet 210.

Mint 1973 · « **Reply #10 on:** December 08, 2013, 06:02:33 AM »

Anyone have 1973 a/c wiring?

firebirdparts · « **Reply #11 on:** December 08, 2013, 07:14:18 PM »

Factory shop manual is pretty awesome. you'll love it.

Farmer2 · « **Reply #12 on:** December 29, 2014, 07:29:49 AM »

Wiring Question. I have two red hot wires coming off the same terminal of the altenator. One goes to the battery and the other has a 10" lead and a white plastic connector that goes to nothing. In alot of the photos of your cars I see this lead and it looks like it goes across the manifold to the pass side and I loose it after the ac compressor. Can someone let me know if I need to remove this or wire it in where it goes and someone short cutted it. I have a 1978 TA 400 with no AC just heat.

I looked at the schematic listed here and it looks like it would be for the high speed of the blower motor. I dont and never had ac on this car. I dont know what the previous owner did with this wire.

Thanks, Dave

firebirdparts · « **Reply #13 on:** December 30, 2014, 08:23:03 AM »

Remove it. If you think it's original, it may be a factory mistake.

Farmer2 · « **Reply #14 on:** December 30, 2014, 09:16:26 AM »

Thanks guys. I found it in an online schematic and it is for the ac relay if I had it. I will remove it. It has its own eyelet so no biggy. just wanted to make sure it wasnt from something a previous owner disconnected.

Dave

« **Reply #14 on:** December 30, 2014, 09:16:26 AM »

2020-B1-8

Casey · « **Reply #15 on:** February 10, 2015, 02:15:06 AM »

Alright, so I have a '79 A/C car that someone decided to remove the compressor and bracket. So from what I'm reading this is the reason why the blower motor won't turn on at all? I'd like to have at least the blower motor working for now to circulate air and for heat/defrost when needed. Is all I need to do is go and run a wire from the harness ground that'd normally go into the compressor to a good grounding point, like the block, in order for the blower to work again? Any help would be greatly appreciated. According to the previous owner it was because of the relay, so after buying a new one and installing it only for it to still not work is kind of irritating.

firebirdparts · « **Reply #16 on:** February 10, 2015, 07:40:52 AM »

Yes, that is the first thing you should do. That wire is black.

If that does not fix it, it'll be time for the test light after that. If you want to pre-empt us, you could check the heater fuse too.

firebirdparts · « **Reply #17 on:** February 10, 2015, 02:25:57 PM »

P.S. Chevy-powered cars may orginally be grounded to the firewall instead. Was yours originally a chevy 350 car?

Casey · « **Reply #18 on:** February 10, 2015, 09:09:47 PM »

I found the compressor harness and plugged it into the harness next to the blower box and condenser, then put the ground onto the alternator bracket. I also noticed the positive wire next to the compressor harness that plugs into the alternator wasn't connected, so I found that wire and plugged it into the harness and into the alternator. Though doing that the positive wire from the battery to the alternator will need redone, as the previous owner had it where that other wire plugs in. So I'll need to fix the battery to alternator wire before I can check and see again, as now the car doesn't have power.

I replaced all of the fuses shortly after buying it. From the VIN it was originally a 301 car, and the drivetrain is swapped.

Casey · « **Reply #19 on:** February 10, 2015, 11:31:32 PM »

Alright, finished the wiring and with the key in the ON position I can hear the relay clicking by moving the sliders on the climate control. Blower motor isn't turning on though, I wouldn't think the car not running would make a difference. The heater resistor is intact with no burn marks or anything, so I'd think it's ok too. Unless the relay isn't bridging the contacts even though it's clicking. I had put the old relay back in after the new one didn't help last time, I guess I can try swapping them tomorrow when I don't have to fumble around with a flashlight.

firebirdparts · « **Reply #20 on:** February 11, 2015, 07:44:54 AM »

You need to use a test light, see where you have power.

Some tips:
The relay should only click when you are on "HI" fan speed.
The power supply for the lower fan speeds is through the heater fuse.
The fan runs on low when the ignition is on, but the a/c slider is "off." Low fan speed is the same thing as off.

Also, something here bothers me:

Quote

I also noticed the positive wire next to the compressor harness that plugs into the alternator wasn't connected, so I found that wire and plugged it into the harness and into the alternator. Though doing that the positive wire from the battery to the alternator will need redone, as the previous owner had it where that other wire plugs in

That is not right. The red wire feeding the blower goes to the screw post on the alternator. You just stack it on top of the charge wire. Two wires on that post.

Casey · « **Reply #21 on:** February 11, 2015, 02:57:31 PM »

I have a test light, so will use that. Being the low speed doesn't need the relay, it could be the motor then I suppose. I guess worst case this weekend when I have more time I'll get to the motor and supply it with power straight off the battery to test it.

I'll pay more attention when I go and do more testing, but I think it clicked more than what it should.

The wire is on the alternator post, just a small piece of wire with a clip on it that ends in a ring connector on the post. I looked at my brother's original 301 car and this was how it was done as well with the same wire and clips between the harness and alternator.

Casey · « **Reply #22 on:** February 11, 2015, 02:58:17 PM »

Also, thanks for being so helpful!

Casey · « **Reply #23 on:** February 13, 2015, 04:19:27 PM »

Got it working somewhat, the motor will start to spin but does it very slowly. I suppose it needs to be replaced then.

firebirdparts · « **Reply #24 on:** February 13, 2015, 07:51:10 PM »

It might. If I were you, I would jumper my way over there will full battery power and see what happens. Getting that blower out of there is not fun.

Casey · « **Reply #25 on:** February 13, 2015, 08:53:25 PM »

Was going to do that to double check before buying one and going through the hassle. It started working out of the blue, was just sitting there with the key in the ON position and all of a sudden I hear a rumbling sound. At first I was like ahhh, then I realized it was the motor. The relay clicks going to high, and then clicks when moving from off to whatever zone but only when the fan is on high. I suppose that's how it's supposed to work since it's going off the fuse panel until then. If anything I've learned why so few have working heat and air now.

peter panov · « **Reply #26 on:** July 24, 2015, 01:17:09 AM »

Hello,

Please advise.
I am trying to restore my A/C electrical system on a 1978 Trans am with L78 engine.
If you look at the wiring diagram in the first post, there is a wire 987B-18 grn lt/blk that goes from the thermal switch to a compressor harness connector. After that it turns into 67-18 blu lt and goes to an idle stop solenoid (L80 only). In my case, I have an L78 where will 987B-18 grn lt/blk go after the compressor harness connector? I am missing everything after this connector and this is the only wire that I can not figure out where to place.

Thank you!

firebirdparts · « **Reply #27 on:** July 24, 2015, 06:46:07 AM »

That circuit is only used to turn on the idle speed solenoid. If you don't have one, then it goes nowhere. I am not sure I believe that is L80 only, but it's not that important. If you have the solenoid, then use it.

peter panov · « **Reply #28 on:** July 27, 2015, 05:04:55 AM »

Thank you!

elgordo77 · « **Reply #29 on:** January 05, 2017, 10:08:56 PM »

Hope someone can help me. I have a 77 TA 403 K Vin. I am fixing and having trouble on where wire goes from a sensor that is next to passenger door inside fender next to blower it is one wire in and one wire out of sensor the previous owner connected the two wires to be one and just was hanging next to air vent fender,he also put all wrong wires in car under hood. System works but I am still would like to know where it plugs into if it is important to connect it?
Thanks
John

Aus78Formula · « **Reply #30 on:** January 05, 2017, 11:05:05 PM »

John, Does the aircon actually work or just the fan? Sounds like the 'sensor' might be the pressure switch which is normally mounted on the aluminium drier and sometimes a second one mid-line in the tubing. These drop out when the AC pressure is too high or too low, essentially overcharging and freezing up, or a leaked system. You can bypass these by running the wires across them but the only reason to is to cover up another fault. Sounds fairly messed up anyway. Some also had a temperature switch on the case for similar reasons, and similar ways around. When there's more than one switch it's possible to still bypass some and operate. Later years dropped the extras.

Start a new thread, add a few pics, and we'll soon be able to tell you what's going on and what to do.

cwi inspector · « **Reply #31 on:** March 16, 2017, 07:01:34 AM »

I have a 79 Trans Am with the 403. I recently replaced the under dash wiring harness as well as the AC harness. Everything works as it should. I am putting the AC back on this car as the original AC was removed by a previous owner. I do not have the AC compressor hooked up electrically as of yet. My issue is I have the blower motor working however the ground wire that is in the harness isn't grounding so I grounded the blower motor to the body of the car. When I hook the blower motor up it drains my battery over night. I have replaced the resistor, relay, both switches on the controls and the blower motor. I'm at a loss as to what may be draining my battery. Also I can not get the motor to work in the HI position. I am installing a Classic Auto Air system. The only wire that connects to this compressor is the power wire. I was told by Classic Auto Air the plug that is plugged into the OEM compressor is no longer needed as the compressor is self grounding through the mounting brackets. There is no connection on the CAA compressor for that connector to be plugged in. Any help or ideas on where to go from here would be greatly appreciated.

firebirdparts · « **Reply #32 on:** March 16, 2017, 08:07:23 AM »

CWI inspector, welcome to TAC!

There are several paths that might cause your battery to drain. I guess that is always true. The high fan speed relay is always powered, so that might be a place to start. Since high fan speed doesn't work, I don't know for sure that yours is actually powered, but it might be. If power escapes from there with the ignition off, then the relay itself is bad. They are not supposed to do that. You can easily unplug the relay and let that be an experiment.

The heater fuse should be unpowered with the ignition off, so power coming that way is not supposed to be the problem. You could consider double-checking that the heater fuse wire is dead when it's supposed to be. There are separate contacts in the ignition switch for that circuit.

The relay has to be grounded to work, so if you are not using the original ground wire yet, that could be the reason that the high fan speed does not work.

Self grounding compressor will be great, just use the green wire to the compressor to turn it on. The black ground wire still needs to be grounded somewhere. Originally it grounded the compressor, fan, and relay. You still need something to ground the fan and relay.

cwi inspector · « **Reply #33 on:** March 16, 2017, 11:18:48 AM »

I have replaced the relay on the side of the evaporator core box twice with no luck and tried the old one as well trying to get the blower to work on hi. Is this relay the same as the hi blower relay. If they are different where is the hi blower relay located? I had the dash board out and there was what looked like a relay mounted to the back side of the dash board. I hope that's not it. I do not want to have to pull the dash again.
Update. I was looking at the diagram at the top of the page and there is no wire leading from the starter to the red wire in the AC harness. Correct me if I'm wrong. Once I connect the red wire to the starter I should have Hi speed on my blower. should this red wire be hooked to the solenoid of the starter or to the positive terminal. This doesn't sound logical but could this be the reason the battery is being drained? I apologize for asking so many questions but previous owners of this car molested the crap out of the electrical system and tore the ac out. That's why I replaced the under dash harness. I really appreciate your help. Thank You.

firebirdparts · « **Reply #34 on:** March 16, 2017, 06:37:38 PM »

that is the correct relay. As I said, that is just a place to start. The relay under the dash is rear defrost.

The red wire powers high fan speed. You will need that to get high fan speed to work. however, the battery is not draining through that wire if it's not hooked up.

That wire comes from battery power. Battery power is available on the starter where the battery cable is connected. That's where it hooks to. There is supposed to be a fusible link in there (a wire which melts if you get in trouble). You need some protection since that wire doesn't go through the fuse box and comes straight from the battery cable.

cwi inspector · « **Reply #35 on:** March 16, 2017, 07:09:39 PM »

Thanks for all the information and help. Hopefully I can get this figured out now.

cwi inspector · « **Reply #36 on:** March 18, 2017, 06:48:27 PM »

I have everything hooked up and the blower motor works as it should even on high. The electrical to the compressor is hooked up and working. I still have the electrical draw on the blower motor. I start the car and my volt gauge usually always reads 13 to 14 volts. When I turn on the blower motor it drops to 11 and takes a while to go back up once I turn the fan off. I'm at a total loss.

firebirdparts · « **Reply #37 on:** March 18, 2017, 09:06:28 PM »

The blower takes quite a bit of power, but it wouldn't really suck the voltage down to 11 volts. I am thinking that problem may be something else, but even if it is the blower, if the battery is running down, I say fix that first. The blower is not supposed to be able to get power when the ignition is off. So you've got some bypassing or something elsewhere in the system.

You really did not mention whether the blower runs when the ignition is off. If power is going through the blower, it could be running.

cwi inspector · « **Reply #38 on:** March 19, 2017, 08:31:47 AM »

No the blower does not run when the ignition is off. That's why I'm at a loss. The battery drains when the blower is plugged in. When I unplug it the battery stays charged. With the wiring harness being new, switches new, resister new, relay new and new blower motor this should not be happening. Everything as far as I can tell is hooked up the way it should be.

firebirdparts · « **Reply #39 on:** March 19, 2017, 11:00:21 AM »

Do you have a volt meter or a test light? If there is power to the blower with the ignition off, you need to see which wire has power, then work back along that wire.

« **Reply #39 on:** March 19, 2017, 11:00:21 AM »

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Author Topic: A/C Wiring Diagram and A/C Blower How-tos (Read 63259 times)