GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 GMC_Truck 🢒 2000 🢒 Jimmy - 2WD 🢒 HVAC 🢒 HVAC Systems - Automatic 🢒 Heating Insufficient

Step

Action

Value(s)

Yes

No

1

Did you perform the Diagnostic System Check?

--

Go to Step 2

Go to Diagnostic System Check

2

Verify that the heating is insufficient.

Is the heating sufficient?

--

Go to Testing for Intermittent Conditions and Poor Connections in Wiring Systems

Go to Step 3

3

Inspect for the following conditions:

    • A leak in the cooling system
    • A low coolant level
    • A loose belt or a worn belt
    • A leaking heater hose or a kinked heater hose
    • The loss of the radiator cap pressure seal

Does the malfunction still exist?

--

Go to Step 4

Go to Step 29

4
  1. Adjust the heater controls to the following modes:
    2. • The HEATER mode
    • The blower speed control to the HI position
    • The full WARM mode
  2. With the ignition switch in the ON position, inspect the airflow out of the floor air outlet.
  3. Inspect the outlet attachment.

Is the airflow high?

--

Go to Step 5

Go to Step 8

5

Does the airflow change with different blower speeds?

--

Go to Step 6

Go to Step 12

6

Perform the System Performance Test. Refer to System Performance Test

Is the heater output temperature correct?

--

Go to Step 7

Go to Step 13

7
  1. Inspect the following areas of the vehicle for cold air leaks:
    2. • At the cowl
    • In the heater case
    • In the vents
  2. Correct any conditions as necessary.

Is the repair complete?

--

Go to Step 29

--

8

Perform the function test. Refer to System Performance Test

Is the repair complete?

--

Go to Step 29

Go to Step 9

9

Check the airflow from the vent outlets.

Is the airflow from the vent outlets high?

--

Go to Step 10

Go to Step 18

10

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

Is the defroster airflow correct?

--

Go to Step 11

Go to Step 19

11
  1. Remove the floor air outlet. Refer to Air Outlet Replacement - Floor.
  2. Inspect the outlet for any obstructions.
  3. Change the blower speeds.

Does the airflow change with different blower speeds?

--

Go to Step 10

Go to Step 12

12
  1. Inspect the following parts:
    2. • The motor blower switch
    • The blower motor resistor assembly
    • The blower motor wiring harness
  2. Repair any conditions as necessary.
  3. Perform the heater output temperature check. Refer to System Performance Test

Is the heater output temperature correct?

--

Go to Step 7

Go to Step 13

13
  1. Turn the temperature rotary knob to full HOT.
  2. Inspect the temperature cable indexing.

Is the temperature valve fully closed?

--

Go to Step 14

Go to Step 23

14
  1. With the temperature valve in the full WARM position, start the vehicle.
  2. Inspect the air temperature around the heater inlet and the heater outlet hoses.

Is the air temperature around both hoses at least at the specified value?

24°C (85°F)

Go to Step 15

Go to Step 24

15
  1. Remove the hoses at the heater core.
  2. Inspect the hoses for proper installation.

Were the hoses properly installed?

--

Go to Step 16

Go to Step 26

16
  1. Backflush the heater core.
  2. Drain and replace the coolant.
  3. Start the vehicle.
  4. Inspect the air temperature around both hoses.

Is the air temperature around both hoses at least at the specified value?

24°C (85°F)

Go to Step 17

Go to Step 24

17

Replace the heater core. Refer to Heater Core Replacement in Heater and Ventilation -- Non-A/C.

Is the repair complete?

--

Go to Step 29

--

18
  1. Turn the mode knob to the DEFROST mode.
  2. Inspect the airflow.

Is the defroster airflow not present or low?

--

Go to Step 19

Go to Step 11

19

Does the airflow change with different blower speeds?

--

Go to Step 20

Go to Step 12

20

Inspect for airflow obstructions at the blower inlet and plenum.

Did you find and correct a condition?

--

Go to Step 29

Go to Step 21

21
  1. With the blower in the HI mode, turn the temperature rotary knob from the full HOT position to the full COLD position.
  2. Listen for a change in the airflow.

Is a change in the airflow audible?

--

Go to Step 22

Go to Step 27

22

Check for an airflow obstruction between the blower and the system outlets.

Is the repair complete?

--

Go to Step 29

--

23
  1. Re-index the cable.
  2. Inspect the temperature valve adjustment, cables and linkage.

Did you find and correct a condition?

--

Go to Step 29

--

24

Check the thermostat for proper installation and seating.

Is the thermostat properly installed and seated?

--

Go to Step 28

Go to Step 25

25

Reinstall the thermostat.

Is the repair complete?

--

Go to Step 14

--

26

Properly reinstall the hoses.

Is the repair complete?

--

Go to Step 14

--

27

Inspect the following components:

    • The temperature valve
    • The control assembly
    • The blower motor wiring harness

Is the repair complete?

--

Go to Step 29

--

28

Replace the thermostat.

Is the repair complete?

--

Go to Step 29

--

29

Operate the system in order to verify the repair.

Did you correct the condition?

--

System OK

Go to Step 3