Rear AC Switch Wiring?

[BlackTomC]

Hey guys, just pulled the rear AC components out of a 1980 Silverado, plan on putting rear ac in my GMC suburban. Question, the switch up on the dash that controls the blower motor speed in the rear, had 4 wires coming off of it, it looks like 1 is power, 1 is ground, then I have a blue and a green wire that appear to plug into the top of the air box. Anyone have any knowledge of the wiring for that switch? I'd like to understand a little more about the wiring before I go installing all this stuff.

[bd]

1980 models should have a three-speed rear blower.  Brown should be power into the switch.  Orange should be High blower connected directly to the rear blower motor and redundantly connected to the resistor in the housing.  The remaining two colors are Medium and Low speed circuits running between the switch and the resistor.  The blower motor grounds to sheetmetal in the vicinity of the rear housing.

[BlackTomC]

bd, you are awesome, thank you for filling in those blanks. You are 100% correct, the switch has 4 positions (Off, Low, Medium, High), with 4 wires (thick red, blue, yellow, and black). There is only 1 wire running to the rear AC box, so I'm assuming the switch acts as a voltage regulator of some sort to control the fan speed.

Could you do me a favor and explain the medium/low speed circuits briefly? I'd like to install this in my 78 Burb, I'm assuming a small amount of wiring + the addition of a resistor will be necessary.

Thanks again

[bd]

See the diagram.

The blower resistor is mounted in the rear blower housing and consists of a fiber card with two wire coils connected in series (end-to-end).  There are three terminals on the resistor card:  one terminal between the two coils that is shared by both coils (the middle terminal), and one terminal at each of the opposite extreme ends of the two coils.  One of the endmost resistor terminals (High Speed) connects directly to the blower motor and to the High Speed terminal of the blower switch.  The opposing end resistor terminal connects to the Low Speed terminal of the blower switch.  The shared or middle resistor terminal connects to the Medium Speed terminal of the switch.  Each blower speed (Lo-Med-Hi) corresponds to a different wire color connecting the switch to the resistor and motor.

Switched accessory B+ connects to one of the four terminals of the blower switch.  On Low Speed setting, the blower switch connects B+ to an endmost resistor terminal such that current flows through both coils of the resistor to the blower motor, which significantly decreases the voltage available to the motor, which in turn results in low airflow.  On Medium Speed setting, the switch connects B+ to the 'middle terminal' of the resistor such that current flows through only one resistor coil to the blower motor, which increases the voltage applied to the blower motor and increases blower speed to medium.  On High Speed setting, the blower switch bypasses the resistor entirely and connects B+ directly to the blower motor, resulting in maximum airflow.

[bd]

Just reread your original post.  If the rear A/C unit came out of a crew cab (CC), the resistor would have been mounted in the left side of the forward evaporator housing next to the engine and would have appeared redundant to the resistor mounted on the top of that housing.  I'm guessing you will need to procure a Burb rear blower resistor and determine where in the rear housing it will fit so that the blower forces air across the resistor coils to cool them.  Alternatively, you could mount a CC resistor in the forward evap housing if you can ensure adequate airflow to keep the resistor from burning.  Either way, be very careful when cutting holes in a housing so that you don't perforate an evaporator.  To complicate matters (better to know ahead of time), some blower resistors have integrated thermal fuses in series with the coils.  Such resistors REQUIRE airflow to prevent the non-serviceable thermal fuses from melting and permanently opening the blower motor circuit.

[BlackTomC]

Hey bd,

So as it sits now, I've got all the components for the system, included the dual squirrel cage blower motor setup with housing. Tested the motor out by connecting it directly to the battery and it works awesome.

Question for you now.. I'm trying to make sense of this switch. The switch has 4 wires coming off of it (brown, orange, blue yellow)

Brown - Goes to "ACC" fuse box slot? And why?
Blue/Yellow/Orange are basically spliced into the existing blower motor resistor wires on top of the air box (in engine bay).

Do I have that right? Tomorrow I am going to run power..... Thing is man, what runs main power from the switch to the blower motor? And how do I set that up? I tore the old main power wire out of the old burb (dark red color) because I figured I'd need it, but couldn't figure out where it went (I traced it back to the front, but it ran behind the sheet metal/body panel and lost it from there.

[BlackTomC]

I have to mention, the main power wire I ended up cutting that I found running along the ceiling of the burb was a red/brown color, not brown or red.

And I've looked dang near everywhere for where it comes out and can't seem to find it. I understand the wiring diagram, I just don't understand how to integrate these wires with the existing wires from the front AC controls.

[bd]

Until you state otherwise, the following discussion assumes you are installing the rear A/C unit into your 1976 K25 Burb.  You can verify the majority of information by referring to the 1976 Wiring Manual as you read along.  I posted the rear A/C (C-69) diagram earlier - click on the diagram to expand it.  For wiring details regarding the fuse block and power tap, refer to the linked manual.

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Power into the blower switch comes from the "unfused 20-amp tap" located directly below the "HTR AC" fuse in the fuse box, which is circuit 300 (10-gauge orange) coming from the ignition switch.  The wire connecting the fuse box to the blower switch (also 10-gauge) is protected by a 20-amp, in-line fuse located adjacent to the fuse box.

Ckt 300 is hot only in the "Run" position of the ignition switch; it is dead in the "Start" and "Accessory" positions.  This was done because the rear blower is a high-current circuit, drawing 20+ amps on high blower.  It is senseless for the starter and blower to compete for battery power while cranking the engine, and useless to power the rear A/C fan when the engine and A/C compressor aren't running. 

Drawing on distant memory, I recollect a chronic problem with the Suburbans resulting from the long wire run between the dash mounted blower switch and the rear mounted blower.  If you are stringing new wire, it might be prudent to bump the rear wire run size by two gauges (i.e., 8-gauge GXL wire for the run to the rear blower).

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I suspect that the brown/red wire to which you referred above is actually the 10-gauge orange (or purple) feed to the rear blower (color depends on year).  But, you have me at a disadvantage, since I cannot see the wire or how/where it is routed through the cab.

For additional details, first you need to answer whether the rear blower housing is cut for a blower resistor?  To economize, it maybe best to mount the resistor in the front evaporator housing, so that only one wire runs rearward.

[BlackTomC]

Dang dude, I apologize for making this confusing. I will post pictures of the main power wire tomorrow (pulled the rear AC kit out of a 1980 Suburban), as well as pictures for everything else. The entire setup is there, just a little lost on how to wire it back up.

The rear ac setup came out of a 1980 Silverado C10 Suburban and is going into a 1976 GMC K25 Sierra Suburban.

[bd]

I'm not confused, just missing some pertinent data.  Your project is relatively straightforward.  With the schematic for reference along with the information already provided, wiring should be fairly easy.  The more difficult task will be routing the pressure hoses.  What, about the wiring or my description, leaves you "lost?"

FYI - I haven't laid hands on a Suburban rear A/C for ~30 years.  Don't expect miracles of memory to bedazzle.  There are gaps!  lol

[BlackTomC]

Honestly after reading over it a few times, it's making more sense now. The easiest part for me was routing the pressure hoses, I cut a hole in the rear floor, and mounted them up in just under 2 hours yesterday. It's the wiring diagrams, I look at them, and my brain just shuts off for some reason.

[BlackTomC]

Ok I understand what's going on now.


Now I need to locate the rear AC blower motor resister. Have any idea where that's located? Once I have that, I can hook the system up and get it running.

[bd]

Yeah - A wiring diagram is analogous to a street map.  Each line (individual wire) is like a street and the wire number & color is the street name.  Each wire begins and ends in a parking lot.  That simple schematic relating to your A/C wiring is a good graphic diagram for learning, because it illustrates the connectors and which connector cavity is occupied by which wire by the wire number.

Back to the resistor:
The resistor could be located in the engine compartment OR in the rear blower housing.  If there isn't a small rectangular cavity already cut into the rear blower housing to mount the resistor, then the resistor probably will be located in the side of the front evaporator housing adjacent to the engine.  Take a look at the side of the front evap housing to see whether there are any marks for a small cutout (look at the image).  The way the schematic is drawn suggests the resistor wiring passes through a grommet in the firewall rather than running all the way back to the rear A/C blower.  In other words, only one 10- to 8-gauge wire runs to the rear blower.  If I understood your earlier post correctly, this agrees with what you have.  So, all that remains is to determine whether to mount the resistor in the front or the rear of the vehicle.

Edit:  added image

[BlackTomC]

That picture solved basically everything.

I found the resistor, ended up tracking down all the wiring for the entire system. Pulled it all out (had to cut some wires).

I'll post some photos of everything here when I'm done refinishing those plastic trims pieces. It's gonna be sweet.

The resistor looks all busted up so I'm going to replace it with this one:

http://www.autozone.com/cooling-heating-and-climate-control/blower-motor-resistor/duralast-blower-motor-resistor/gmc/k2500-3-4-ton-sub-4wd/1978/8-cylinders-l-5-7l-4bl-ohv/349412_0_4222_5281/