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How To Repair GMC CK Series Trucks HVAC Control Unit

HVAC Control Unit repair on 1996-98 line of GMC C/K series.
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How To Repair GMC CK Series Trucks HVAC Control Unit

Postby Gawain Tomlinson [OP] » May 10 2013, 12:20am

GMC CK Series Trucks HVAC Control Unit Repair
AC Delco 15-72548 Heater / AC Control

By Gawain Tomlinson
San Diego, CA

Disclaimer: Any use of the information contained in this document is at the user’s own risk. The author does not assume any liability for actual or consequential damages.

This Heater AC Control Unit was used on the 1996-98 line of Chevrolet and GMC C/K Pickups, and on 1996-99 GM Suburbans, Tahoes, and Yukons. It was no longer used after 1999, due partly to unreliability, and partly due to upgrades to newer technology.

AC Delco 15-72548 Heater AC Control

The most common problems with this vintage of AC Delco climate control systems are failure of ventilation door servos, and failures of the control unit. This document pertains to the control unit. For troubleshooting and repair of servos, see my “How To Fix Mode/Blend Door Actuator Problems” article on SilveradoSierra.com.

There are two common problems with the Heater AC Control Unit. Both problems are caused by different failure modes of the same circuit in the control unit. The circuit fails because a transistor, which switches on power to most of the control unit is undersized, which makes it a high failure rate item.

Problem #1:
a) air flow is always directed to the floor, regardless of the position of the Mode dial and,
b) the AC button does not illuminate when depressed and,
c) the Recirc button does not illuminate, and the resirc door does not operate and,
d) the AC compressor does not come on;
This is caused by an open Switched +12V power transistor, or it could possibly be caused by a defective blower speed switch.

Problem #2:
a) the AC button is depressed or the mode switch is in one of the defog positions,
b) and the blower switch is off,
c) the AC compressor runs causing the lines and evaporator to ice up;
This is caused by a shorted Switched +12V power transistor, or it could possibly be caused by a defective blower speed switch. If the compressor runs with the blower off, freeze-up will result.

Another symptom that could be evidence of a defective Switched +12V power transistor is unreliable operation of the climate controls evidenced by flickering of the AC and Recirc lights.

In most failure modes only the Temperature control dial functions, as evidenced by warm air flowing out near the floor when the dial is turned to the heating position. This is because the temperature control is powered directly from +12V whenever the ignition switch is in the run position.

Circuit Description

Page 1, shows the entire schematic of the printed circuit board (PCB). Page 2, is the blower motor and switch schematic. And Page 3, is the connector map.

I reverse engineered the AC Delco 15-72548 Heater AC Control Unit. This schematic is pretty accurate, but there are some component values / types missing, either because I could not decipher the part numbers, or I did not remove them from the circuit to measure them. These are minor problems that do not hinder understanding the circuit operation. Some component values can be guessed. When this is true I will point it out in the circuit description. The schematic drawings and connector map are shown below:

The PCB does not have component number designations, so I did not assign arbitrary designations.


Panel Light Circuit

In the lower left corner of the schematic is the panel light circuit. It is separately powered on Pin 1 from the dash light dimmer, and has no function in climate control, other than illumination.

Climate Control Power

The climate control PCB is powered when the ignition switch is in the RUN position via Circuit #141. +12V comes in on Pin 5 in the upper left corner of the schematic. Power is routed back out to Pin 7 through a 3.8 ohm resistor and the Max AC switch, but on a 1500 truck Pin 7 is not used.

The Blend (temperature) door control is powered directly from +12V via Pin 5.

The rest of the circuit is powered from a switching transistor controlled on Pin 3 by the Blower switch. Pin 3 is open when the Blower is running at any speed, and is +12V when the Blower is off. When the Blower is on, current flows through the 750 ohm resistor to the base-emitter of the transistor, switching it on. The output (collector) of the transistor is Switched +12V power.

I was not able to find datasheets on the transistors on the PCB. The part numbers are N723 (a PNP transistor), N728 (a PNP transistor), and N724 (an NPN transistor). It is possible that all of the transistors on the PCB are Darlington types, which have higher gains than standard transistors.

The purpose of Switched +12V is to prevent the AC compressor from running when the blower is not running. Without the blower the evaporator and AC lines could ice up. It is a mystery why GM / AC Delco decided to disable all other functions such as Mode and Recirculation doors when the blower is not running.

Blend Door Control

As previously noted, if the blower is OFF, the Blend (temperature) door is still active.

The value of the Blend door potentiometer is guessed to be 10K ohms (same as inside the actuator). The output goes through an 82K ohm resistor. This produces a variable voltage of 0V to +12V on Pin 8 to actuate the Blend door servo.

Mode Door Control

The Mode door control is enabled by Switched +12V when the Blower is on at any speed setting. To the right of the Switched +12V transistor in the schematic is the Mode door potentiometer (guessed to be 10K ohms). The output goes through a 3K ohm resistor at Pin 9. This produces a variable voltage of 0V to +12V on Pin 8 to actuate the Mode door servo.

Mode Door Override

The Mode door actuator directs airflow to the floor when the Blower is off. Below and to the left of the Switched +12V transistor is a voltage divider consisting of a 1.5K ohm and a 3K ohm resistor. The output of the voltage divider is +8V, and it goes to the emitter of a PNP transistor (an N726) located below the Mode door potentiometer on the schematic. This transistor switches 8V to Pin 9 because, when the Blower is off, Switched +12V drops to 0V.

The transistor is switched on by current flow from Switched +12V (now 0V), through a 22K ohm resistor, to the emitter-base junction of the transistor, then into the voltage divider. Current flow through the emitter-base junction causes the +8V from voltage divider to be switched to the Mode door actuator. There is a capacitor across the emitter-collector that provides filtering and noise suppression.

Recirculate Door Control

The Recirc door control is found at the center-left of the schematic. It is enabled by Switched +12V when the Blower is on at any speed setting. If the Blower is off, the Recirc door defaults to the fresh air position. The Recirc door actuator is closed when a ground is applied at Pin 4 of the Climate Control PCB. When Pin 4 is open the actuator opens the door allowing fresh air to flow into the cabin.

The functions of the Recirc door are controlled by detent switches that are part of the Blend and Mode dials, and the AC Switch. When Switched +12V is present the Resirc door is closed by one of the following two conditions: ((Recirc switch depressed) or (not Defog, plus Max AC, plus AC on)).

AC Compressor Control

The AC Compressor control is found at the lower- right of the schematic. Pin 10 outputs +12V to drive the AC Relay. The relay is driven by Switched +12V when the Blower is on at any speed setting, and (the AC switch is depressed, or the Mode switch is in one of the defog settings).

Blower Motor Control

The drawing below shows the Blower Motor wiring diagram. Note the OFF switch position sends +12V to the Control PCB to control the Switched +12V, disabling the most of the PCB functions in the OFF position. Also note that the connection diagram for the HI motor speed relay in this diagram is correct.


Many electrical diagrams on the Internet show the Blower relay contacts wired backwards, which would cause the HI speed to work incorrectly.

The Blower Switch is Delco PN 15-72275.


HVAC Controller Connections

Below is the connection diagram. The PCB connections are on the left, and the Blower switch is on the right.

Wire harness colors are as I found them on inspection. They may vary from the GM designated colors. The wire colors GM used may vary, or colors may have changed due to fading over time, I am not sure which is the case here.



External Circuits


Energizing the AC Compressor Clutch
A Schematic is shown Below:


The AC Relay is located external to the controller. The coil of the AC relay is powered from the ECU, located on the left side of the engine bay. The High Pressure, Cycling, and Lo pressure refrigerant switches also go to the ECU. When all conditions are met, the ECU provides power to the AC relay coil, and the AC compressor clutch is energized. The switches provide diagnostics for the ECU, and increase idle speed when the AC relay is energized, as well as compressor clutch control.



This procedure applies only to versions of the 1500 which use pressure switches. Newer vehicles may use pressure transducers. If your vehicle uses transducer(s), refer to the applicable manufacturer’s test procedures.

Unplug the low pressure switch and the cycling switches. Use paper clips or wires to jumper across the switch connectors on the wire harness. Key on engine off, compressor clutch should "click". If no click, check the wiring harness for open wires, functioning high pressure switch, and correct voltages.

With the truck running, A/C turned on. Check the Dark Blue wire at the low pressure switch for +V with the switch Disconnected. Check for ground on the black wire. With the switch plugged in BOTH wires should show ground. If the blue wire still shows +volts, refrigerant is low or the low pressure switch is bad.

With key on, disconnect the A/C pressure sensor next to the accumulator and check for 12 volts on the GRN/BLK wire. Check for ground on the BLK/WHT wire.


With the key on, engine off, momentarily jump the A/C clutch relay from pin 30 to pin 87, the Compressor clutch should "click". If no click, unplug the clutch connector, jumper pin 30 to pin 87, check for power at the green wire at the compressor. Check the black wire for ground.

Forcing the AC relay to engage bypasses the control head, and will make the compressor run continuously. If the compressor runs with the relay forced on, the Heater AC Control Unit may be bad, or the pressure switches may not be closed.

Use a voltmeter, check terminal #85 in the socket (should be the lower right hand corner if standing on the drivers side looking down on the relay/fuse center) changes voltage when the AC button is pressed. The engine should be running. The Heater AC Control Unit switches terminal #85 to complete the circuit for the coil in the relay. With AC on from the Control Unit, #85 should have +12V, and #86 is ground for the relay coil.

Applying power to terminal #30 with the relay unplugged will engage the compressor clutch. Check for power at #87; it is the power for the clutch circuit. This is what is switched to #30 to power the clutch circuit.


The pin diagram is the relay socket (not the relay) to show terminal locations.

Heater AC Control Unit Repair

The Control Unit is a high failure rate item. The most common failure mode is an open Switched +12V power transistor. An open circuit in this transistor effectively disables the Control Unit. The interesting thing is … that is also the designed behavior when the blower switch is OFF. With either the blower off, or a typical failure, almost all functions are disabled. The reasoning is that the compressor should not run with the blower off, because it could cause freeze up and icing.

OK … the Blower / Compressor part makes sense, but the rest of it is stupid. It must have never crossed the design engineer’s mind that the customer might want to direct air flow someplace other than the floor, or have control over the recirculation function with the blower off. An analysis of the circuit offers an easy way to overcome these design deficiencies, and still be compatible with an unmodified HVAC Control unit.

You can do this mod even if there is no malfunction. Here is how it’s done:

Heater / AC Control Unit Circuit Modification


Pull the AIR BAG FUSE and wait ten minutes before proceeding.

Remove the Dash Bezel:
1) Pop the dashboard bezel loose by opening the drink holder, and gently tugging around the edges of the bezel.
2) Set the parking brake. Turn the ignition switch to run (no need to start the engine). Lower the steering wheel down to its lowest position. Press the brake pedal, and shift the gearshift into Lo.
3) Gently wiggle the bezel to free it from the top of the dashboard. Be careful not to damage the Hazard Flasher Switch.
4) Unplug the Light Switch and release / remove any other wires and switches.

Remove the Heater / AC Control from the dash:


1) If the vehicle is equipped with a CD Player, remove it by pressing down on the release tabs on both sides and pull it out. Unplug the harness from the back.
2) Remove the Heater AC Control by pressing the release tabs inward with a screwdriver as you pull it towards you.
3) After you get it out, extend the wire harness by tugging it out of the cubby hole behind the unit. Unplug the connectors by operating the releases as shown in the pictures below.

If the unit is old, the plastic body may be very brittle. If so you may end up breaking off some or all of the tabs / guide pins. No matter how careful you are, it may break. If so, you can opt to jury-rig it back together with screws, or you can buy another brand-new unit on EBay for about $100.


PCB removal:
This step is not usually necessary, but if you wish to remove the +12V Switching transistor, this shows how to do it.

The PCB is a flexible Mylar circuit board that is attached to the body by a nylon retainer clip, and the lamps that are clipped onto the body. There is no electrical or electronics inside of the body; it is just a mechanical piece to provide support for the knobs, buttons, and components.


1) Remove the six lamps by turning them counter-clockwise and pulling them out.
2) Remove the nylon retainer clip by removing the screws into the stand-offs.
3) Gently pry around the edges of the PCB with a small screwdriver or other flat object to release it from the plastic pegs that align it to the body.
4) Gently pull the PCB away from the body until it stops against the AC and Recirc buttons. The switches are attached to the buttons by slots in the sides of the buttons.
5) Rotate the PCB to release it from the buttons and pull it the rest of the way out.

Note the position of the rotary shafts and knobs. You will need to align the flats during reassembly.

+12V Switching Transistor Removal:
This step is not mandatory. In most cases the failure mode is an open transistor. If this is the case, you can just as well leave the transistor on the PCB, and jumper across it.


Flexible Mylar circuit boards are very delicate. Applying too much heat from a soldering iron can destroy the lands on the PCB, and/or damage the Mylar substrate. Use a low wattage or temperature controlled soldering iron.

You can control the temperature of a standard low wattage soldering iron by using a plug-in lamp dimmer, available at Home depot or IKEA. Turn up the temperature only enough to make the solder flow.

Now you can get to the Switched +12V transistor. The photos below show its location:



Reassemble the Heater AC Control Unit:
No instructions should be required, just reverse disassembly. Do not install it in the vehicle yet; there is still work to be done.

Bridging The Transistor:
In most cases the failure mode is an open transistor. If this is the case, you can just as well leave the transistor on the PCB, and jumper across it, as is shown here.

Locate the Emitter and Collector (E-C) pads on the etch side of the circuit board. Solder a jumper across the E-C pads.


I used solder wick (available at Radio Shack) to bridge the lands across the transistor E-C. Solder wick is very easy to solder, and makes a low resistance connection. I used office scissors to trim the end of the solder wick. Tin the end and tack-solder it to one of the solder pads on the circuit board. Trim it to length, and tack it at the other end. Then carefully add more solder to make a secure low resistance connection. Clean off the flux with some alcohol and a Q tip.

Disable the AC Compressor when the blower is off:
The AC Relay has a cycling switch in series with it. All you have to do is break the wiring and add another interlock that opens up when the Blower is off. A normally closed (N.C.) relay is what you need. The Blower switch has a contact that outputs +12V when the Blower is off; use that wire (DK GRN) to drive the coil of your Blower Interlock Relay. When the Blower is off, the relay is energized, the contact opens, and the Compressor is disabled.

A suitable relay is available at most auto parts stores: Echlin Ignition Parts, ECH AR143. It is a SPDT automotive relay. Pins 30-87 are the N.C. contacts; Pins 85-86 are the relay coil. Cost $11.19 at NAPA Auto.

There is one unused terminal on the relay, 87a (N.O.). Put a piece of shrink-fit or an insulated spade lug on it to prevent accidental shorts. I mounted my relay on the firewall between the wiper motor and the brake master cylinder. A short wire with a ring terminal grounds the relay coil.

Cut the (GRN/BLK) wire at the cycling switch connector, and crimp on a male-female pair of insulated spade lugs. Crimp M-F spade lugs on the pair of wires from the N.C. contacts of the relay. That way if you want to disconnect your interlock relay, you can unplug the wires, and reconnect the Cycling switch wire with the M-F spade lugs.

Tap into the Blower Switch harness at the Blower Off (DK GRN, Switch Pin D to HVAC Module Pin 3) wire with a quick-tap and a female spade lug. Put a male spade lug on the wire to the relay coil and plug it in.


Some schematics on the internet show two interlock switches, LO Pressure and Cycling, in series with the coil on the AC Relay. This is incorrect. Both switches are actually wired into the ECU. Use the Cycling Switch, not the LO Pressure Switch, to wire your interlock. If you wire your interlock in series with the LO Pressure Switch, it will not work.

Here is a Schematic:



Reinstall the Heater AC Control Unit:
Test the system, and you are good-to-go.

With these changes to the system, you should have all functions of the ventilation system even with the Blower Off. Except that the AC Compressor will not kick in when the blower is off.

If you install an unmodified Heater AC Control Unit, the system will work the same as a factory HVAC system. No need to uninstall the relay.

Here is a parts list that can be used if you want to undertake a similar project:

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Original Poster
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Gawain Tomlinson
Firing on 1 cylinder [L1]
Firing on 1 cylinder [L1]
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Year: 1998
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Extra Info: King Cab 1500 5.7L
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Re: How To Repair GMC CK Series Trucks HVAC Control Unit

Postby div4scpro » Jul 17 2014, 9:26pm

if the response to my question is already posted, I apologize. Having replaced 4 control units and 2 switches, your article is a godsend. My question is that having bridged the transistor, known problem in this case since one leg burned completely through, if you stop there and don't install the additional relay would that be satisfactory as long as you never turn engage the A/C button without the blower run ning?
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Joined: Jul 17 2014, 8:55pm
Year: 1997
Make/Model: Chevrolet Silverado
Extra Info: 4x4 Z71 350
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