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The temperature cannot be changed on the hvac system. It always blows hot air, even when the a/c is on.

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The change of the temperature is done by the temperature control The temperature control is a simple variable resistor that sends a signal to the blend door actuator module. If the shaft of the control does not rotate smoothly, then the control is probably bad.

A system which automatically combines cooled air conditioned air and warm heated air in precise amounts is going to be of little use if either the heater or a/c system aren't working properly. Similarly, if vacuum controlled doors inside the heater box aren't working properly with the old manual a/c system, they probably won't work right with the new eatc system.

So before beginning this project, we'll need a baseline to figure out if additional parts need to be ordered:

  • Turn the knobs on the front of the control unit and make sure that air gets discharged from the proper vents, if it doesn't you may need some new vacuum controlled door actuators or a new plenum assembly. A little bit of leakage to the floor when the controls are set to windshield defrost or dashboard a/c vents is acceptable. If air all the air always blows to the dashboard vents regardless of the position of the knobs, then look for disconnected vacuum hoses.
  • Flip the knob between a/c and max a/c and observe the recirculation door under the passenger's dash near the kick panel with a flashlight. You should see it open and close.
  • If the blower motor only works on "high" that's fine, as you're not going to use the old blower resistor or it's connector anyways. If you can't get any air to flow from the dashboard vents when you turn the knobs, that's a problem that you should correct before proceeding though.
  • Run the engine for a while to sufficently heat the coolant so the temperature guage on the instrument cluster is in the middle of it's travel range. Now set the control knob to a/c, fan speed on high and turn the temperature control knob between full hot and full cold. You should notice a change in the volume of air discharged from the vents as you turn the temperature knob since you're intermittenly forcing air through an obstruction to air flow called the heater core. And you should be able to get vent discharge temperatures on the full cold setting in the 40-45 degree range even on a 90 degree day in the summer. If not then the check refrigerant low & high side pressures, temperature deltas, and a/c compressor clutch cycling times to get an idea of the condition of the a/c system components and refrigerant charge level. If all is within spec, then disconnect the heater core hoses and temporarily install an inline coupler between the two hoses to bypass the heater core. If vent discharge temperatures are now within the desired range, the blend door is directing air through the heater core when it shouldn't likely due to the actuator. To check for sure, open the glovebox door and use a flashlight to look at the top of the actuator shaft, if it doesn't move when you turn the temperature control knob, you've confirmed that the blend door actuator is inoperative. A bad blend door actuator is not a problem as we'll be replacing it with it's eatc counterpart anyways, but this does prohibit us from knowing if the blend door itself is damaged or if an obstruction such as a pen or pencil is lodged in the plenum. Similarly, if you set the system to full hot and don't get hot air, watch the actuator shaft movement with a flashlight. And if there is a large temperature difference between the two heater core tubes, the core is clogged. And if you get a sweet smelling mist that's greenish in color out of the dashboard vents, you've got a leaking heater core. But a clogged or leaking heater core is fine, as it's not hard to replace once the dashboard is removed from the car anyways.
  • The interior temperature sensor is a resistive thermal device that converts temperature data into an electrical signal useable by the eatc control circuitry. More information on this sensor is avaliable by clicking here.

    the temperature cannot be changed - interior1-vi.jpg


Thanks for contacting Fixya

Posted on Jun 19, 2009


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