2002 Jimmy SLS 4x4 Heater controls don't seem to be working. I can't get the air flow directed to the proper outlets.eg. can't get full flow to defrost or foot outlet. Flow seems to want out at dash vents. When I close the vents, flow improves to defrost, but not full flow. How do I approach this problem?

Item Part Number Description 1 18A318 Vacuum Control Motor-, Panel/Defrost Door 2 18A478 Panel/Defrost Door (Full Vacuum Position) 3 - Defrost Air Flow 4 - Side Window Demister Air Flow 5 18B545 Temperature Blend Door (Full Heat Position) 6 19860 A/C Evaporator Core 7 19A813 Air Inlet Duct Door (Full Vacuum Position) 8 - Outside Air Inlet 9 18A318 Vacuum Control Motor-, Air Inlet Duct Door 10 - Recirculated Air Inlet 11 18527 Blower Motor 12 19A566 A/C Vacuum Reservoir Tank and Bracket 13 - Vacuum from the Engine Intake Manifold 14 19B888 A/C-Heater Function Selector Switch 15 - Vacuum to the 4x4 Hub Solenoids 16 18495 Water Diverter Valve (Full Vacuum Position) 17 18476 Heater Core 18 18A559 Floor/Panel Door (Full Vacuum Position) 19 - Floor Air Flow 20 18A318 Vacuum Control Motor-, Floor/Panel Door 21 - Panel Vent Air Flow

Sounds like the air blend door is not opening to let heat in.

Engine coolant is provided to the heater system by two 16 mm (5/8 inch inside diameter) heater hoses. With the engine idling at normal running temperature, set the heater-A/C controls as follows. Temperature control to full Heat, Mode control to Floor, Blower control to the highest speed setting. Using a test thermometer, check the air temperature coming from the center floor outlets and compare this reading to the Temperature Reference table.

If the floor outlet air temperature is insufficient, check that the cooling system is operating to specifications. Both heater hoses should be HOT to the touch (the coolant return hose should be slightly cooler than the supply hose). If the coolant return hose is much cooler than the supply hose, locate and repair the engine coolant flow obstruction in heater system.

POSSIBLE LOCATIONS OR CAUSE OF OBSTRUCTED COOLANT FLOW

• Pinched or kinked heater hoses.
• Improper heater hose routing.
• Plugged heater hoses or supply and return ports at cooling system connections.
• Plugged heater core.
• Air locked heater core.
• Restrictor in backwards

If coolant flow is verified and the heater floor outlet temperature is insufficient, a mechanical problem may exist.

POSSIBLE LOCATION OR CAUSE OF INSUFFICIENT HEAT

• Obstructed cowl air intake.
• Obstructed heater system outlets.
• Blend-air door not functioning properly.

TEMPERATURE CONTROL
If heater floor outlet temperature cannot be adjusted with the heater-A/C control temperature control lever, one of the following could require service:

• Blend-air door binding.
• Faulty blend-air door motor.
• Improper engine coolant temperature.
• Faulty heater-A/C control.

Check your heater core for good circulation. With engine running, heater on full hot, your inlet and outlet heater core coolant lines should both be hot. If one is much cooler, you may have a plugged, or partially plugged core (from corrosion, scale, rust, etc.), or an air pocket preventing circulation. Pull off the outlet line and see if a steady stream of coolant flows as it should. If you have good flow through your core, then it is possible a temperature door inside your heater case is not opening properly.

THERMOSTAT STUCK OPEN OR HEATER.TEMPERATURE CONTROL MODE SWITCH NOT GETTING VACUUM OR THERE IS A ELECTRICAL PROBLEM IN MODE SWITCH.

The air direction is vacum controlled. If you have 4X4 and ESOF ( switch in the cab) then you need to inspect the vacum lines that go to the hubs and possibly the hub seals. "default" position for air flow is the defrost position. If you DON'T have 4X4, and it's a diesel engine, inspect the vacum pump, If it's a gas engine then look at all the vacum lines from the intake manifold for a leak.

Check the fuel pressure regulator. Pull the vacuum hose to see if it has fuel in it. If it does that is your problem.

Could be the heater bypass valve is stuck or the heater core is blocked.
I'd check the heater core first.You may have trapped air in the cooling system or the heater core may be partially plugged up. Engine coolant is delivered to the heater core through two heater hoses. With the engine idling at normal operating temperature, set the temperature control knob in the full hot position, the mode control switch knob in the floor heat position, and the blower motor switch knob in the highest speed position. Using a test thermometer, check the temperature of the air being discharged at the HVAC housing floor outlets.
Temperature Reference
Ambient Air Temperature 15.5° C (60° F) 21.1° C (70° F) 26.6° C (80° F) 32.2° C (90° F)
Minimum Air Temperature at Floor Outlet 52.2° C (126° F) 56.1° C (133° F) 59.4° C (139° F) 62.2° C (144° F)
If the floor outlet air temperature is too low, Both of the heater hoses should be hot to the touch. The coolant return heater hose should be slightly cooler than the coolant supply heater hose. If the return hose is much cooler than the supply hose, locate and repair the engine coolant flow obstruction in the cooling system.
OBSTRUCTED COOLANT FLOW
Possible locations or causes of obstructed coolant flow: Pinched or kinked heater hoses. Improper heater hose routing. Plugged heater hoses or supply and return ports at the cooling system connections. A plugged heater core. If proper coolant flow through the cooling system is verified, and heater outlet air temperature is still low, a mechanical problem may exist.
MECHANICAL PROBLEMS
Possible locations or causes of insufficient heat: An obstructed cowl air intake. Obstructed heater system outlets. A blend door not functioning properly.
TEMPERATURE CONTROL
If the heater outlet air temperature cannot be adjusted with the temperature control knob on the A/C Heater control panel, the following could require service: The A/C Heater control. The blend door actuator.
The blend door. Improper engine coolant temperature

The heater core is possibly blocked or the temperature adjuster is not working.

First check the controls to be certain that the temperature aduster is working and the flaps/doors that direct the flow of hot air are working.

If they are..........

Check heater hose temp. If one is hot and one is cold, you are not getting flow through the heater core.

Bleed it to remove an air blockage.
If that does not work, flush the system.
If that does not work, replace the heater core.

Karl at topgunwon.com

if the inlet and outlet hoses are getting hot to the heater core then the problem is the air temperture control blend door actuator in the heating/A/C ducts, this door directs air flow either across the heater core or the AC evaporator or mixes the 2 air flows for a desired temp at the outlets, hence the name blend door, to change this part u must remove the dash assembly.

check fuel pump and make sure you have enough gas in it also