Ac/heater Does Not Work!

changarter · February 27, 2008

Hello everybody,

I have a 1999 GMC Z71 1/2 ton p/u and my air conditioner and heater both do not work. No cold air or hot air but still blows air (even at different speeds). I already replaced the ac/heater controller on the dash and did not help. Has anyone ran into this type of problem and know of any solutions. Please help me out, summer is around the corner and I live in Phoenix, AZ. Thanks. Randy.

jrrod2004 · February 27, 2008

This may help out:

Service Information

1999 GMC Truck GMC K Pickup - 4WD | Escalade, Pickup (Classic), Suburban, Tahoe, Yukon (VIN C/K) Service Manual | Document ID: 427298 Heating Insufficient <A href="http://gsi.xw.gm.com/si/showDoc.do?docSyskey=427298&pubCellSyskey=49442&pubObjSyskey=427298&from=sm&cellId=49442#ss1-427298">

Diagnostic Aids

The most likely causes of insufficient heating outside the heater system include a low coolant level, a faulty thermostat, a partially or fully clogged heater core or hose, an obstruction to the air circulation, or an air leak past a faulty seal into the passenger compartment. For the proper temperature verification, use the Tech II.

Step

Action

Value(s)

Yes

No

1

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

• The proper engine temperature thermostat operation.

Does the malfunction still exist?

--

Go to Step 2

System OK

2

<LI type=1>Adjust the heater controls to the following modes:

• The HEATER mode

• The blower speed control to the HI position

• The full WARM mode

<LI type=1>With the ignition switch in the ON position, inspect the airflow out of the floor air outlet.

Inspect the outlet attachment.

Is the airflow high?

--

Go to Step 3

Go to Step 6

3

Does the airflow change with different blower speeds?

--

Go to Step 4

Go to Step 10

4

Perform the heater output temperature check. Refer to Heater Output Temperature Check in Heater and Ventilation (Non-A/C).

Is the heater output temperature correct?

--

Go to Step 5

Go to Step 11

5

<LI type=1>Inspect the following areas of the vehicle for cold air leaks:

• At the cowl

• In the heater case

• In the vents

<LI type=1>Inspect under the seat for any obstructions.

Correct any conditions as necessary.

Is the repair complete?

--

System OK

--

6

Perform the function test. Refer to Functional Check .

Is the repair complete?

--

System OK

Go to Step 7

7

Check the airflow from the vent outlets.

Is the airflow from the vent outlets high?

--

Go to Step 8

Go to Step 16

8

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

Is the defroster airflow correct?

--

Go to Step 9

Go to Step 17

9

<LI type=1>Remove the floor air outlet. Refer to

Floor Air Outlet Replacement in Heater and Ventilation (Non-A/C). <LI type=1>Inspect for any obstructions.
Change the blower speeds.

Does the airflow change with different blower speeds?

Go to Step 8

Go to Step 10

10

<LI type=1>Inspect the following parts:

• The motor blower switch

• The blower motor resistor assembly

• The blower motor wiring harness

<LI type=1>Repair any conditions as necessary.

Perform the heater output temperature check. Refer to Heater Output Temperature Check in Heater and Ventilation (Non-A/C).

Is the heater output temperature correct?

--

Go to Step 5

Go to Step 11

11

<LI type=1>Turn the temperature rotary knob to full HOT.
Inspect the temperature cable indexing.

Is the temperature valve fully closed?

--

Go to Step 12

Go to Step 21

12

<LI type=1>With the temperature valve in the full WARM position, start the vehicle.
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 13

Go to Step 22

13

<LI type=1>Remove the hoses at the heater core.
Inspect the hoses for proper installation.

Were the hoses properly installed?

--

Go to Step 14

Go to Step 24

14

<LI type=1>Backflush the heater core. <LI type=1>Drain and replace the coolant. <LI type=1>Start the vehicle (Retest at less than ot equal to 30 psi).
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 15

Go to Step 22

15

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

Is the repair complete?

--

System OK

--

16

<LI type=1>Turn the mode knob to the DEFROST mode.
Inspect the airflow.

Is the defroster airflow not present or low?

--

Go to Step 17

Go to Step 9

17

Does the airflow change with different blower speeds?

--

Go to Step 18

Go to Step 10

18

Inspect for airflow obstructions at the blower inlet and plenum.

Did you find and correct a condition?

--

System OK

Go to Step 19

19

<LI type=1>With the blower in the HI mode, turn the temperature rotary knob from the full HOT position to the full COLD position.
Listen for a change in the airflow.

Is a change in the airflow audible?

--

Go to Step 20

Go to Step 25

20

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

Is the repair complete?

--

System OK

--

21