 Loose connection(s) at the  Insure all connections are mated properly.
control panel.

2. Controller knob is
rotating beyond its
defined positions.

3. Discharge and/or
inlet air systems
aren’t functioning
properly. Blower is
operating properly.

 Failed control panel.

 Replace control panel.

 Knob is damaged.

 Replace knob.

 Control device is
damaged.

 Internal stop has been broken. If control
device can be rotated through more than
designed positions, replace control device.

 Loss of power supply.

 Examine the chassis’s HVAC circuit’s
protection device for failure (i.e. fuse or
circuit breaker). Separate source from the
blower motor circuit.

 Open circuit between
 Verify vehicle voltage at each connection.
vehicle ignition and control
Perform continuity test between the control
panel. Open circuit
panel and shutter actuator(s) connections.
between ground source
Repair or replace harness if necessary.
and control panel.
 Incorrect circuit(s).

 Reference the “Expected Voltages Matrix” section. Correct the circuit(s) if
possible or replace harness.

 Faulty shutter actuator.

 Replace the actuator if needed.

 Faulty ventilation mode, or
recirculated air push
button switch.

 Replace the switch or control panel if
needed.

11

Electrical/Electronic System
Diagnostics Guide (Continued)
(Reference the appropriate electrical schematic first and follow “POSSIBLE
CAUSES” in sequence)
PROBLEM

4. Blower does not
operate at any
speeds.

5. Blower does not
operate at all speeds.

POSSIBLE CAUSES

REMEDY

 Loss of power source.

 Examine the chassis’s HVAC circuit’s
protection device for failure (i.e. fuse or
breaker).

 Open circuit in
harness between the
circuit protection and
control panel.

 Verify vehicle voltage at each blower switch
connection. Perform continuity test between
connections. Repair or replace harness if
necessary.

 Faulty blower switch.

 With vehicle ignition ON, rotate blower
switch through all of its positions, check for
voltage at all terminals. If no voltage is
measured, replace switch or control panel if
needed.

 Open circuit in
harness between the
control panel and
blower resistor.

 Rotate the blower switch to LOW speed.
Verify vehicle voltage at LOW speed
connection of the blower resistor. Perform
continuity test between connections. Repair
or replace harness if necessary.

 Faulty blower resistor.

 Measure the voltage at the connection that
feeds the motor lead. If no voltage is
measured, replace blower resistor.

 Faulty blower motor.

 Rotate the blower switch to HIGH speed.
Check for voltage at the motor connection.
If no voltage is measured, replace blower
motor. Perform continuity test between
connections if necessary.
 With vehicle ignition ON, rotate blower
switch to its inoperative speed position.
Check for voltage at the controller’s speed
setting terminal. If no voltage is measured,
replace switch or control panel if needed.

 Faulty blower switch.

 Open circuit in
harness between the
control panel and
blower resistor.

 Verify vehicle voltage at each speed
connection on the blower resistor. Perform
continuity test between connections. Repair
or replace harness if necessary.

 Faulty blower resistor.

 Measure the voltage at the connection that
feeds the motor lead. If no voltage is
measured, replace blower resistor.
Reference the “Expected Voltages Matrix” section.

12

Electrical/Electronic System
Diagnostics Guide (Continued)
(Reference the appropriate electrical schematic first and follow “POSSIBLE
CAUSES” in sequence)
PROBLEM

POSSIBLE CAUSES

REMEDY

6. A/C Clutch does not
operate.

 Blower switch isn’t
activated.

 AC switch receives its power source from
the blower switch. Turn blower switch to
any speed selection. Verify blower
operation.

7. A/C Clutch does not
operate. Blower is
operating properly.

 AC switch isn’t
activated.

 Verify AC switch is depressed and
operating correctly.

 Faulty AC switch.

 Metal Control Panel’s AC switch requires a
“Compressor Activated” signal from the
blower switch. Make sure the switch is
receiving this signal from the blower
switch. If not then replace the blower
switch. Otherwise measure voltage at the
control panel’s “AC Signal” terminal. If no
voltage is measured on the correct circuit,
replace switch or control panel if needed.

 Open circuit between
A/C switch and A/C
thermostat, or
thermostat to clutch.

 Check primary connections at the control
panel. Perform continuity test between AC
switch connection and A/C thermostat.
Repair or replace harness if necessary.

 Defective A/C
thermostat.

 Place a jumper across terminals of
thermostat, if the clutch engages then
replace thermostat.

 Faulty A/C pressure
switch (make certain
adequate refrigerant is
contained in system).

 Place a jumper across terminals of
vehicle’s harness mating connector, if the
clutch engages then replace the switch.

 Faulty A/C Clutch.

 With the engine OFF apply a separate
12V+ supply directly to clutch terminals
and listen for clutch engagement. Replace
clutch if there is no engagement.

 Faulty chassis circuitry.

If voltage is read at pressure switch, and the
clutch is working as noted above, the
problem is originating in the chassis wiring.
Refer to the chassis manufacturer service
manual.

13

Electrical/Electronic System
Expected Voltages - Matrix
Plastic or Metal Control Panel Integrated To Plastic Tooled
And Form Molded Air Distribution Assemblies.
Estimated Voltage Readings, Per Ventilation Mode, At The Control Panel Connection
SHUTTER ACTUATORS
Ventilation Mode

Fresh / Recirculation
Inlet Air

Defrost - Floor /
Panel

Defrost / Floor

BLK
0
0
0
0
0
0
0

YEL

WHT

GRN

Fresh Air »
Recirculation Air »
Panel »
Panel / Floor »
Floor »
Defrost / Floor »
Defrost »

Grd
No Signal
NA
NA
NA
NA
NA

RED BLK
11-13
0
11-13
0
11-13
0
11-13
0
11-13
0
11-13
0
11-13
0

NA
NA
0
5-7
11-13
11-13
11-13

RED BLK
11-13
0
11-13
0
11-13
0
11-13
0
11-13
0
11-13
0
11-13
0

NA
NA
11-13
11-13
11-13
5-7
0

RED
11-13
11-13
11-13
11-13
11-13
11-13
11-13

Metal Control Panels Integrated To Metal Air Distribution Assemblies.
Estimated Voltage Readings, Per Ventilation Mode, At The Control Panel Connection
SHUTTER ACTUATORS
Fresh / Recirculation
Defrost - Floor /
Defrost / Floor
Ventilation Mode
Inlet Air
Panel
BLK
YEL
RED BLK WHT RED BLK GRN
RED
0
11-13
0
NA
11-13
0
NA
11-13
Fresh Air
Grd
0
No Signal 11-13
0
NA
11-13
0
NA
11-13
Recirculation Air
0
NA
11-13
0
11-13
0
11-13
Panel
0
0
0
NA
11-13
0
11-13
0
11-13
Panel / Floor
5-7
0
0
NA
11-13
0
0
11-13
Floor
11-13 11-13
5-7
Defrost / Floor
0
NA
11-13
0
11-13 11-13
0
11-13
11-13
Defrost
0
NA
11-13
0
11-13 11-13
0
11-13
11-13
Estimated Hot Water Valve Actuator Voltage Readings
Per Potentiometer Position
Potentiometer Position
Full Cool (Far Left)
Full Heat (Far Right)

BLK
0
0

YEL

RED

11-13 11-13
0
11-13

Estimated Blower Motor Voltage Readings Per Blower Speed
Plastic Control Panel
Metal Control Panel
Blower Speed RED BLK
Blower Speed
RED BLK
Off
Off
0
0
0
0
Low
Low
2-4
0
2-5
0
Medium / Low
Medium
5-7
0
6-10
0
Medium / High 8-10
High
0
11-13
0
High 11-13
0

Notes:
1) Voltages per vehicle will vary depending on voltage regulation and voltage loss due to
resistance of the wire.
2) Always test with voltage meter referencing vehicle ground and not wire harness ground.
3) A limiting resistor is wired in series with the shutter actuator's control signal wire in order to
reduce the rotation of the air flow shutter to desired position. Voltage readings at the shutter
actuator will be less but follows the same voltage logic as the control panel voltage
readings.

14

Air Flow Systems
Blower Motor Assemblies
One of the most basic controls for maintaining a comfortable passenger compartment is the
blower motor assembly. This is a fan that forces air through the evaporator and the heater core
and into the passenger compartment.
In most cases, the blower motor speed is adjustable. By changing the switch position on the
control panel, the operator can adjust the speed of the blower motor from low to high, in three or
four steps.
The slower the fan speed, the longer the air sits along the evaporator and heater coils. This
removes more heat, and reduces the temperature of the air being discharged from the vents, in
A/C cooling mode. Conversely, in the heat mode, slow fan speeds increase the discharge air
temperature.
In addition, cooler air holds less moisture than warm air. The longer the air stays near the
evaporator fins, the more moisture will condense on the fins and the drier the air coming into the
passenger compartment. As more condensation builds up on the evaporator fins, it begins to run
off, and drains out of the HVAC module through the evaporator condensate drain tube.
This also helps to purify the air to some degree. Dust and pollen particles that come in contact
with the wet evaporator are pulled out of the air stream and wash out through the condensate
drain.

Plastic and Metal Blower Motor Assemblies

15

Air Flow System
Air Distribution Systems
Plastic Injection Molded Air Distribution Assembly - Air Flow Logic

Air Distribution Assembly

Blower Motor
Assembly

Electric Shutter Actuators

Blower Speed Resistor

Plastic Injection Molded Air Distribution Assembly
16

Air Flow System
Air Distribution Systems (Continued)
Plastic Injection Molded Air Distribution Assembly – Shutter Logic