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Discussion Starter #1 (Edited)
General diagnosis of the Heating Ventilating and Air Conditioning system is pretty easy when it comes to how everything operates as a liquid and electrical system. I'll be covering the electrical; blower motor and fan, heating controls, and blower motor resistor along with the liquid system; coolant and heater core.

Most of this information is going to be quite general and can be applied to nearly any vehicle that you have.

Blower Motor Inoperable

  • Blower Motor seized
  • Heating switch failure
  • Wiring failure
  • Blower Motor Resistor failure*
*Blower Motor Resistor failure will usually allow the fan to run only on high as there is very little resistance across this part of the circuit. However, in some rare cases (just mine recently) the blower motor will not work in any setting due to a very corroded blower motor resistor.

Blower Motor works only on High

  • Blower Motor Resistor failure
  • Wiring failure
Only Defrost Vents blow air

  • Vacuum leak in HVAC system
Generally the hardest part is tracing down a vacuum leak in an HVAC system. However, these will more than likely be a rare occurrence given the high quality vacuum hoses used in the Dakota's HVAC system.

Blower Motor spins fast, but hardly any air comes out of vents

  • Debris between fresh air duct (between hood and windshield under cowl)
  • Debris between blower motor fan and A/C condensor or heater core
  • Recirculation door partially open

Low Heat/No Heat Driving or Parked

  • Low Coolant level
  • Clogged Heater Core
  • Collapsed Heater Core Hoses
Heat comes in bursts while accelerating/Heat is warm/hot only when driving

  • Thermostat stuck open
  • Low Coolant level
 
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