GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 GMC_Truck 🢒 1998 🢒 Jimmy - 2WD 🢒 HVAC 🢒 Heating, Ventilation and Air Conditioning 🢒 Defrosting Insufficient

Step

Action

Value(s)

Yes

No

1

Did you perform the Function Test?

--

Go to Step 2

Go to Function Test

2

Test the cooling system for the following conditions:

    • Cooling system leaks
    • Low coolant level
    • Loose or worn belt
    • Leaking or kinked heater hoses
    • Loss of radiator cap pressure seal
    • Repair the cooling system as necessary.

Is the repair complete?

--

Go to Step 3

--

3
  1. Adjust the heater controls to the following specifications:
    2. • HEATER mode
    • HIGH blower
    • Full WARM
  2. Start the engine.
  3. Inspect the airflow out of the floor air outlet.
  4. Inspect the outlet attachment.

Is the airflow high?

--

Go to Step 4

Go to Step 12

4

Inspect the blower speeds for airflow change.

Do the blower speeds change?

--

Go to Step 5

Go to Step 21

5

Perform the heater output temperature check.

Is the heater output temperature low?

--

Go to Step 21

Go to Step 6

6

Close the temperature valve.

Is the temperature valve closed?

--

Go to Step 7

--

7

Turn the temperature rotary knob to full HOT.

Is the temperature valve fully closed?

--

Go to Step 8

Go to Step 26

8
  1. Start the engine with the temperature valve on full WARM.
  2. Inspect the temperature of the heater inlet and outlet hoses.

Is the air temperature around both hoses equal to or greater than the specified value?

85°F

(24°C)

Go to Step 9

Go to Step 28

9
  1. Remove the hoses at the heater core.
  2. Inspect the hoses for proper installation.

Are the hoses properly installed?

--

Go to Step 10

Go to Step 25

10
  1. Backflush the heater core.
  2. Drain the heater core.
  3. Replace the coolant.
  4. Retest the hose temperature.

Are both hoses warm?

--

Go to Step 11

Go to Step 27

11

Replace the heater core.

Is the repair complete?

--

Go to Function Test

--

12

Perform the functional test. Refer to Function Test .

Did you find and correct the condition?

--

Go to Function Test

Go to Step 13

13

Test the air flow from the defroster and vent outlets.

Is the air flow high?

--

Go to Step 22

Go to Step 14

14
  1. Move the mode knob to DEFROST.
  2. Test the airflow.

Is the air flow high?

--

Go to Function Test

Go to Step 15

15

Test the blower speeds for airflow change.

Does the speed change?

--

Go to Step 16

Go to Step 17

16

Inspect the blower at the blower inlet and the plenum for airflow obstructions.

Are the blower inlet and the plenum free of obstructions?

--

Go to Step 17

Go to Step 31

17
  1. Turn the blower on HIGH.
  2. Turn the temperature rotary knob from full HOT to full COLD. Listen for an airflow change.

Did the airflow change?

--

Go to Step 18

Go to Step 19

18

Inspect the system for airflow obstructions between the blower and the system outlets.

Are any obstructions present?

--

Go to Function Test

Go to Step 31

19

Inspect the following components:

    • The temperature valve
    • The control assembly
    • The harness

Perform any repairs as necessary.

Did you find and repair a condition?

--

Go to Function Test

Go to Step 20

20
  1. Inspect the following components:
    2. • The blower switch
    • The resistor assembly
    • The wiring harness.
  2. Repair the components as necessary.

Is the procedure complete?

--

Go to Step 6

--

21
  1. Inspect the vehicle for cold air leaks at the following components:
    2. • The cowl
    • The heater case
    • The vents
  2. Inspect under the seat for obstructions.

Are the components clear of obstructions?

--

Go to Function Test

Go to Step 31

22

Move the defroster and/or vent valve to the HEATER mode.

Is the defroster airflow correct?

--

Go to Step 23

Go to Step 17

23
  1. Remove the floor air outlet.
  2. Inspect the outlet for obstructions.

    3. Does the air speed remain constant?

--

Go to Step 24

Go to Step 16

24
  1. Inspect the following components:
    2. • The blower switch
    • The resistor assembly
    • The wiring harness
  2. Repair any components as necessary.

Is the repair complete?

--

Go to Step 7

--

25

Properly install the hoses.

Is the installation complete?

--

Go to Step 1

--

26

Inspect the following items:

    • The temperature valve adjustment
    • The cables
    • The linkage

Is the repair complete?

--

Go to Function Test

--

27
  1. Retest the system.
  2. Verify the symptoms.

Is the repair complete?

--

Go to Function Test

--

28
  1. Inspect the thermostat for proper installation.
  2. Inspect for correct seating.

Are the installation and seating correct?

--

Go to Step 29

Go to Step 30

29

Replace the thermostat.

Is the repair complete?

--

Go to Function Test

--

30
  1. Re-install the thermostat.
  2. Retest the system.

Is the repair complete?

--

Go to Function Test

--

31

Remove the obstructions as necessary.

Is the repair complete?

--

Go to Function Test

--