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Heater Systems

<table border="0" height="1" width="100%"> <tbody> <tr> <td align="left" height="1" valign="middle" width="22%">
</td> <td align="left" height="1" valign="top" width="78%">This is a basic description of the heater system, it's operation and trouble shooting. Please keep in mind that I will be focusing on the heat controls found in most early General Motors vehicles. A basic heater system will consist of the engine's cooling system, (radiator, engine fan, water pump, thermostat, related hoses, and coolant,) with the addition of a few other components:</td> </tr> </tbody> </table> Heater Core

<table> <tbody><tr> <td width="85%">The heater core is actually a heat exchanger. (Think about the heater core as a mini radiator.) Hot coolant flows via the heater hose, from the water pump, to the heater control valve, (if equipped,) then through the heater core and back to the engine, then repeats this cycle. Heat transfer occurs when the blower motor, (fan) forces cooler air across the heater core as the coolant passes through the rows of the core.</td> <td align="right" width="16%">
</td> </tr></tbody></table> Heater Control Valve

<table border="0" width="100%"> <tbody> <tr> <td width="15%"></td> <td width="85%">The heater control valve controls the flow of coolant in to the heater core. Some older control valves are cable operated, and some actually regulate the amount of flow. Newer models are vacuum operated, and are normally open, until a vacuum signal from the control panel causes it to close, shutting off the flow of coolant through the core. </td> </tr> </tbody> </table> Blower Motor

<table width="691"> <tbody><tr> <td width="507">The blower Motor is a variable speed DC motor, the more voltage the motor receives, the faster the motor speed. Voltage is controlled through a fan selector switch and blower resistor. When the ignition is in the "On" and "Accessory" positions, voltage is supplied to the heater/and/or a/c mode switch. </td> <td width="33%"></td> </tr></tbody></table> <table border="0" width="100%"> <tbody><tr> <td width="19%"></td> <td width="81%">When the mode control switch is placed in any position other than "Off" voltage is supplied to the blower switch. (Note: some cars have power to the blower switch whenever ignition is turned on, causing the fan to run continually, regardless of mode switch selection.) </td> </tr> </tbody></table> Blower Speed Control

<table border="0" width="100%"> <tbody> <tr> <td width="16%">
</td> <td width="69%">When the blower switch is in the "Low", "Med", or ("Med1-Med2") positions, voltage is supplied to the blower motor through a resistor, thus dropping voltage and varies the motor speed according to speed setting. When the switch is placed in the "Hi" position, full battery voltage is supplied to blower, (sometimes through a High Blower Relay, depending on year and design) causing the fan to run at full speed.</td> <td align="center" width="15%">
</td> </tr> </tbody> </table> Distribution

<table> <tbody><tr> <td width="84%">The heater core and blower motor are generally housed together in a plenum mounted to the vehicle's outside firewall, however, some designs incorporate the core and the motor inside the distribution box..</td> <td width="16%"></td> </tr></tbody></table> <table border="0" width="100%"> <tbody> <tr> <td width="17%"></td> <td width="83%"> The distribution box is usually mounted to the firewall, behind the dashboard inside the passenger compartment. The distribution box contains the blend (Hot/Cold) and mode (Heat/ac/recirculation/defrost) doors. These doors are responsible for diverting and regulating air flow to the floor, dash and windshield vents, (defroster.)</td> </tr> </tbody> </table>
<table border="0" width="100%"> <tbody><tr> <td width="100%"> Diagnosis & Repair

The obvious should be checked first:
Coolant level.
Make sure you have a thermostat installed and is correct temperature range.
Make sure system holds pressure.

Most problems can categorized as follows:
</td> </tr> </tbody></table> <table border="1" bordercolor="#008080" width="100%"> <tbody> <tr> <td width="26%">
</td> <td width="45%">
</td> <td width="28%">
</td> </tr> <tr> <td width="26%">Blower Motor Inoperative</td> <td width="45%">
  • Blown Fuse
  • Loose or corroded electrical connection
  • Faulty ground
  • Faulty switch
  • Faulty motor
  • Faulty resistor
</td> <td width="28%">
  • Replace fuse
  • Clean and tighten
  • Clean and tighten
  • Replace Switch
  • Replace Motor
  • Replace resistor
</td> </tr> <tr> <td width="26%">Blower motor runs at one speed only</td> <td width="45%">
  • Faulty switch
  • Faulty resistor
</td> <td width="28%">
  • Replace switch
  • Replace resistor
</td> </tr> <tr> <td width="26%">Blower motor runs, but no air flow</td> <td width="45%">
  • Intake air blocked
  • Fan scroll not fastened to the motor, or broken
</td> <td width="28%">
  • Clean debris from intake
  • Tighten or replace
</td> </tr> <tr> <td width="26%">Heater will not heat</td> <td width="45%">
  • Coolant not reaching high enough temperature
  • Heater core restricted
  • Air in cooling system
  • Blend air door misadjusted or binding
  • Check for restricted heater control valve
</td> <td width="28%">
  • Check thermostat
  • Flush or replace
  • Purge air
  • Adjust and inspect
  • Replace valve.
</td> </tr> <tr> <td width="26%">Heater will not defrost</td> <td width="45%">
  • Control cable adjustment incorrect
  • Defroster duct hose torn
</td> <td width="28%">
  • Adjust
  • Replace
</td> </tr> </tbody> </table>
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