Before we get into troubleshooting heater problems, let's talk about the heater itself. The heater core is like a mini-radiator. It has inlet and outlet pipes and a finned copper or aluminum core through which hot coolant from the engine circulates. The heater core is located inside the HVAC plenum under the dash and usually on the passenger's side of the box.
When hot coolant from the engine flows through the heater core, the core gets hot and heats the air that's forced through it by the blower motor. Generally speaking, a heater should put out air that's at least 70 degrees hotter than the outside temperature. So if it's 30 degrees outside, your heater should put out at least 100 degrees F.
The hot coolant flows from the engine to the heater core through the "inlet" hose. After circulating through the core, the coolant returns to the water pump through the "outlet" or "return" hose. Both the inlet and outlet return hoses should feel hot when the engine is at normal operating temperature and the heater is on.
In the inlet hose you may find a "heater control valve" that blocks the flow of coolant to the heater when the heater isn't needed (when running your air conditioner on maximum, for example). Some older heater control valves are cable or vacuum operated, but most of the newer ones (when used) are electronic.
Most heater control valves are open all the time unless vacuum or power is applied. This means hot coolant circulates through the heater core even when the heater isn't being used. This is done for several reasons. One is safety. In the event the heater valve fails (doesn't close when vacuum is applied) you still have heat to defrost the windows for safe cold-weather driving.
Some heater control valves are "thermostatically controlled" with a capillary tube that extends into the heater air outlet. The capillary tube reacts to changes in the heater's output. This regulates the opening and closing of the control valve to maintain a more consistent level of heater performance.
Another method of regulating the heater's output is to control air flow through it. This is done two ways: by changing the blower speed and by opening and changing the position of the "blend doors" that route air through the heater core and A/C evaporator inside the HVAC housing.
On vehicles without factory air conditioning, sliding the heat selector control switch to the off position closes a door that blocks the flow of air into the heater core. Even if the blower is turned on, no heat will come out because the air inlet is closed. On vehicles with factory air conditioning, a second "blend" door is usually used to route some air through the heater core and some through the A/C evaporator, depending on the temperature range selected. Sliding or tuning the temperature range knob all the way to the cold position closes the blend door so all air is routed around the heater core and through the A/C for maximum cooling. Sliding or turning the temperature knob to a midway position between hot and cold opens the blend door so warm air is mixed with cold to take the chill off the A/C. A third air control door is used to reroute air to the defrosters.
On vehicles with automatic temperature control (ATC) systems, the ATC module figures out the best door positions and positions them accordingly to deliver heat or cooling. If the vehicle has a dual-zone system, there may be a separate heater core for each side. Or, the ATC system may use additional blend doors to route different amounts of heating or cooling to each side.
The blower motor, which blows air through both the heater and air conditioner on vehicles with factory A/C, is wired to a "resistor unit" (usually mounted on the outside of the heater box or plenum) to reduce circuit voltage so the motor will run at the desired speed. Power to the motor may be supplied through a separate relay hidden somewhere under the dash.
Blower Problems
If the blower motor doesn't work (no sound), it probably means the motor is bad and needs to be replaced unless debris has jammed the blower fan or there is an electrical fault such as a blown fuse, bad relay, switch or resistor, or loose wire.
A blown fuse, by the way, is a symptom not a cause. A blown heater fuse means the circuit overloaded for some reason. Replace the fuse with one of the same amp capacity (never one with a higher rating because doing so may allow the wires to get dangerously hot). If the new fuse blows as soon as the blower is turned on, you've confirmed an electrical short circuit in the wiring or motor that needs to be investigated further. If the fuse lasts awhile and then blows again, the motor is probably running hot due to worn brushes and/or bushings and needs to be replaced.
The motor itself can be checked by using a pair of jumper wires to see if it works. Connect one wire to ground and the other to a source of battery voltage. If the motor does nothing, it needs to be replaced.
Suppose you have a blower motor that runs (you can hear it) but no air comes out of the ducts. The problem here is a jammed or inoperative air flow control or blend door (this applies to defrosters that don't work, also). Try changing the temperature setting. If you don't hear movement from the doors inside the HVAC unit, you'll have to troubleshoot the control system. With ATC systems, you'll need a scan tool to access the self-diagnostics and/or to run door motor checks.
Doors also can be jammed by objects that have been placed on the dash and have disappeared down the defroster ducts. The cure here is to extract the object from the plenum by fishing through the heater outlet with a coat hanger or magnet - or removing the plenum.
With vacuum-controlled doors, the most common reasons for failure are leaky or loose vacuum hoses, or defective diaphragms in the little vacuum motors that move the doors. You can check for vacuum by starting the engine and disconnecting the small hose that goes to the vacuum motor that works one of the doors. If you feel vacuum or hear a hissing sound when trying different temperature settings, the vacuum source is okay and the problem is a bad vacuum motor. You also can test a vacuum motor by applying vacuum to it with a hand-held pump to see if it moves and holds vacuum (otherwise it should be replaced). If there's no vacuum, check for leaky vacuum hose connections, a defective temperature control switch, or a leaky vacuum reservoir under the dash or in the engine compartment.
Most defroster problems (no air to the windshield) are also caused by inoperative air flow control doors, or loose, deteriorated or damaged ducts.
Another often overlooked cause of poor blower performance, heating, cooling and defrosting is a plugged cabin air filter. These filters are used on many later-model vehicles and should be changed according to the maintenance schedule in the owner's manual. Filters that have activated charcoal to absorb odors should be replaced yearly. Dust filters should be replaced every two to three years as a rule. Cabin air filters can usually be found under or behind the glove box, or at the base of the windshield where air enters the HVAC system through the cowl duct.
Heater Woes
When no or low heater output is not due to a blower problem (plenty of air coming out of the ducts but the air isn't hot), the list of possible causes include:
The National Highway Traffic Safety Administration issued the following recall on 2010 model year Chevrolet Equinox and GMC Terrain SUVs to repair a fault with the defroster defogger system that affects visibility.
NHTSA CAMPAIGN ID Number: 09V489000
Component: VISIBILITY:DEFROSTER/DEFOGGER SYSTEM
Potential Number of Units Affected: 59031
Summary:
GENERAL MOTORS IS RECALLING CERTAIN MODEL YEAR 2010 CHEVROLET EQUINOX AND GMC TERRAIN VEHICLES FOR FAILING TO COMPLY WITH THE REQUIREMENTS OF FEDERAL MOTOR VEHICLE SAFETY STANDARD NO. 103, "WINDSHIELD DEFROSTING AND DEFOGGING SYSTEMS AND WITH FEDERAL MOTOR VEHICLE SAFETY STANDARD NO. 101, CONTROLS AND DISPLAYS". THE SOFTWARE IN THE CENTER INSTRUMENT PANEL CAN CAUSE THE HEATING, AIR CONDITIONING, DEFROST, AND RADIO CONTROLS, AS WELL AS THE PANEL ILLUMINATION TO BECOME INOPERATIVE.
Consequence:
DRIVING WITHOUT A FUNCTIONING DEFROST SYSTEM CAN DECREASE YOUR VISIBILITY UNDER CERTAIN DRIVING CONDITIONS AND COULD RESULT IN A CRASH WITHOUT WARNING
Remedy:
DEALERS WILL REPLACE THE COMPUTER MODULE IN THE CENTER INSTRUMENT PANEL FREE OF CHARGE. THE SAFETY RECALL IS EXPECTED TO BEGIN DURING JANUARY 2010. OWNERS MAY CONTACT CHEVROLET AT 1-800-630-2438, GMC AT 1-866-996-9463 OR AT GMOWNERCENTER.COM.
Notes:
GM SAFETY RECALL NO. 090298. OWNERS MAY ALSO CONTACT THE NATIONAL HIGHWAY TRAFFIC SAFETY ADMINISTRATION'S VEHICLE SAFETY HOTLINE AT 1-888-327-4236 (TTY 1-800-424-9153), OR GO TO SAFERCAR.GOV.
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