GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 GMC_Truck 🢒 1998 🢒 Sonoma Pickup - 2WD 🢒 HVAC 🢒 HVAC Systems - Automatic 🢒 Heating Insufficient

Step

Action

Value(s)

Yes

No

1

Did you perform the Functional Check?

--

Go to Step 2

Go to Functional Check

2

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 3

Go to Functional Check

3
  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 4

Go to Step 7

4

Does the airflow change with different blower speeds?

--

Go to Step 5

Go to Step 11

5

Perform the System Performance Test. Refer to System Performance Test

Is the heater output temperature correct?

--

Go to Step 6

Go to Step 12

6
  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 Functional Check

--

7

Perform the function test. Refer to System Performance Test

Is the repair complete?

--

Go to Functional Check

Go to Step 8

8

Check the airflow from the vent outlets.

Is the airflow from the vent outlets high?

--

Go to Step 9

Go to Step 17

9

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

Is the defroster airflow correct?

--

Go to Step 10

Go to Step 18

10
  1. Remove the floor air outlet. Refer to Air Outlet Replacement - Floor (Rear - Utility) .
  2. Inspect the outlet for any obstructions.
  3. Change the blower speeds.

Does the airflow change with different blower speeds?

--

Go to Step 9

Go to Step 11

11
  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 6

Go to Step 12

12
  1. Turn the temperature rotary knob to full HOT.
  2. Inspect the temperature cable indexing.

Is the temperature valve fully closed?

--

Go to Step 13

Go to Step 22

13
  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 14

Go to Step 23

14
  1. Remove the hoses at the heater core.
  2. Inspect the hoses for proper installation.

Were the hoses properly installed?

--

Go to Step 15

Go to Step 25

15
  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 16

Go to Step 23

16

Replace the heater core. Refer to Heater Core Replacement (Pickup) or Heater Core Replacement (Utility) in Heater and Ventilation -- Non-A/C.

Is the repair complete?

--

Go to Functional Check

--

17
  1. Turn the mode knob to the DEFROST mode.
  2. Inspect the airflow.

Is the defroster airflow not present or low?

--

Go to Step 18

Go to Step 10

18

Does the airflow change with different blower speeds?

--

Go to Step 19

Go to Step 11

19

Inspect for airflow obstructions at the blower inlet and plenum.

Did you find and correct a condition?

--

Go to Functional Check

Go to Step 20

20
  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 21

Go to Step 26

21

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

Is the repair complete?

--

Go to Functional Check

--

22
  1. Re-index the cable.
  2. Inspect the temperature valve adjustment, cables and linkage.

Did you find and correct a condition?

--

Go to Functional Check

--

23

Check the thermostat for proper installation and seating.

Is the thermostat properly installed and seated?

--

Go to Step 27

Go to Step 24

24

Reinstall the thermostat.

Is the repair complete?

--

Go to Step 13

--

25

Properly reinstall the hoses.

Is the repair complete?

--

Go to Step 13

--

26

Inspect the following components:

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

Is the repair complete?

--

Go to Functional Check

--

27

Replace the thermostat.

Is the repair complete?

--

Go to Functional Check

--