Your car's heater is a life saver in cold weather.
Before attempting to repair your car heater, you should understand how
your heater system works. First, on this page is an explanation of how
the car heater system functions, further down the page are some
pointers on heater system trouble-shooting. To warm the passenger cabin
of your car, the heating system makes use of excess heat from the
engine internal combustion process. A car's
engine, in fact, when in operation produces so much excess heat that if the excess heat is not removed, it would destroy the
engine.
It is your car's cooling system that removes that dangerous excess
heat. Some of the excess heat is released through the exhaust. Most of
the excess heat is absorbed by a circulating liquid coolant, which is a
mix of water and antifreeze. The heated liquid coolant is carried from
the engine through hoses to the radiator, which transfers the heat
from the coolant to the outside air. That heat transfer lowers the
temperature of the liquid coolant, which is then circulated back to the
engine to absorb excess heat again. Whereas the radiator is located at
the car front grill, the unit that transfers heat to the passenger
cabin is located inside the dashboard. This unit is something of a
mini-radiator and is referred to as the heater core. Heated liquid
coolant circulates through tubes in the heater core and a heater fan
blowing across those tubes, as well as through little fins encasing the
tubes, directs warm air through heating vents into the passenger
cabin. Because your car's heating system works off of its cooling
system, heating system malfunctions are often caused by problems in the
cooling system. The heater core in your car is similar to the radiator
in the front of your car; in fact it looks like a small radiator. The
difference is the heater is mounted inside the car and air is blown
through the fins of the core. The heater hoses transfer engine coolant
from the engine to the heater core, this allows the heat from the
engine coolant to be utilized and warm the passenger compartment. When a
heater stops functioning determine what type of failure has occurred
to execute a repair. We have listed the most common problems below:
Heater Core
Troubleshooting Procedure
- Step 1: Check Engine Coolant Level
- Most cars have coolant reservoirs with opaque white plastic tanks
and measuring lines allowing you to see if the coolant is at the
recommended level. Coolant expands when heated, and for that reason,
most coolant reservoirs have a Full-Cold and a Full-Hot line. If there
is too little coolant in the reservoir, you should be alerted to the
possibility that there could be a leak somewhere in your car's cooling
system. Remove the reservoir cap and add coolant to the Full-Cold line,
if a very short time after you refill the coolant reservoir the
coolant level is low, there is a leak in your car's cooling system.
Engine coolant is used to heat the heater core and if the vehicle is
low on coolant the heater core will stop working. The heater cannot
produce heat due to the lack of heat from the engine coolant. Even if
the engine is not over heating the coolant level can still be low.
Inspect the engine coolant level in the coolant reservoir tank; coolant
level should be between the hot and cold marks. Always check the
coolant level when the vehicle is cold, preferably over night. Check
for engine coolant leaks and repair as needed, then refill with coolant
and recheck the system.
Radiator Cap
- Step 2: Inspect Heater Control Valve -
The heater control valve is used to shut off hot coolant from entering
the core when the heater is in the off position. A heater control
valve can be actuated by either a vacuum line or a cable from the
vacuum heater control valve to the heater core. This hose should be
warm; if not the heater control valve is stuck and needs to be replaced.
After the repair has been made refill the coolant level and recheck
system.
Heater Control Valve
- Step 3: Check Vacuum Feed -
Most heater systems are operated by engine vacuum. This vacuum is used
to actuate servos to move the temperature adjustment doors inside the
heater plenum. If engine vacuum is not present the heater will not
operate properly. To test for this condition remove the vacuum feed line
from one of the vacuum servos. With the vacuum feed line removed
insert a vacuum test gauge. If no vacuum is present, open the hood and
inspect all small vacuum lines from the engine to the firewall and
repair as needed. If a hissing noise is present under the dash when the
engine is running a vacuum leak exists and must be repaired to restore
heater function.
- Trouble Shoot Heater/Air Conditioner Blower Motor.
An electrical motor is used to push air through the heater and air
conditioner system. If this electric motor fails it will not force air
through the system. Ground a test light lead to a good ground source,
like a metal brace under the dash, seat mount bolt or under hood brace.
Turn the key to the on position; check the fuse in the heater or
blower motor fuse panel with a test light. Then lightly touch the
service port at the top of the fuse on both sides. If the test light
illuminates on both sides of the fuse, the fuse is ok and working
properly. If the fuse lights the test light on one side it is blown and
needs to be replaced.
Power Distribution Center
If
you are unsure of the location of the heater fuse consult a repair
manual. If the fuse condition is ok, position the blower speed to the
highest setting. Connect the ground end of the test light to a known
ground source. Then probe both ground and power feed wires of the fan
motor. The test light should illuminate on only one of the wires (fan
motor failed). If the test light illuminates on both wires the ground
source has shorted. If neither of the wires illuminate the power source
is shorted. To trouble shoot this style of problem a wiring schematic
is needed.
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