GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 GMC_Truck 🢒 2000 🢒 Jimmy - 4WD 🢒 HVAC 🢒 Heating, Ventilation and Air Conditioning 🢒 Heating Insufficient

Step

Action

Value(s)

Yes

No

Important: The most likely causes of insufficient heating outside the heater system include the following conditions:

   • A low coolant level
   • A faulty thermostat
   • A partially or fully clogged heater core or hose
   • An obstruction of air circulation
   • An air leak past a faulty seal into the passenger compartment

1

Did you perform the Heater Output Temperature Check?

--

Go to Step 2

Go to Heater Output Temperature 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
    • A missing radiator cap pressure seal

Does the malfunction still exist?

--

Go to Step 3

Go to Heater Output Temperature Check

3
  1. Adjust the heater controls to the following modes:
    2. • The HEATER mode
    • The blower motor speed control to the HI position
    • The full HOT 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 motor speeds?

--

Go to Step 5

Go to Step 11

5

Perform the heater output temperature check. Refer to Heater Output Temperature Check .

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 Heater Output Temperature Check

--

7

Perform the function test. Refer to Function Test .

Is the repair complete?

--

System OK

Go to Step 8

8

Inspect 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 knob 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 Floor Air Outlet Replacement - Center .
  2. Inspect the outlet for any obstructions.
  3. Change the blower motor speeds.

Does the airflow change with different blower motor 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 Heater Output Temperature Check .

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 door position.

Is the temperature door fully seated to the right?

--

Go to Step 13

Go to Step 22

13
  1. With the coolant bypass valve in the full HOT 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 .

Is the repair complete?

--

Go to Heater Output Temperature 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 motor speeds?

--

Go to Step 19

Go to Step 11

19

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

Did you find and correct a condition?

--

Go to Heater Output Temperature Check

Go to Step 20

20
  1. With the blower motor 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

Inspect for an airflow obstruction between the blower motor and the system outlets.

Is the repair complete?

--

Go to Heater Output Temperature Check

--

22

Inspect the coolant bypass valve for binding or damage.

Did you find and correct a condition?

--

Go to Heater Output Temperature Check

--

23

Inspect 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 coolant bypass valve
    • The control assembly
    • The blower motor wiring harness

Is the repair complete?

--

Go to Heater Output Temperature Check

--

27

Replace the thermostat.

Is the repair complete?

--

Go to Heater Output Temperature Check

--