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2009 Toyota Venza

Recently noticed that when the front defrost mode is select (which sets fan to max and directs air to the windshield), the entire HVAC system shuts off (i.e. blower motor stopped, HVAC controls stopped working, HVAC screen went blank). Only way to restore HVAC that I've discovered is to turn the key off, then back on.

  • This HVAC shutdown is repeatable within 1-2 seconds any time the fan goes to max (i.e. selecting defrost, or manually setting the fan speed to max, or setting the automatic climate control to max hot or max cold) AND when the engine is ON.
  • If the key is ON but engine is OFF, the problem does not occur.
  • Turning the engine OFF, then key back ON restores HVAC system functionality.
  • If fan is on MAX, starting engine (key ON, engine ON) will cause HVAC system to shut down in 1-2 seconds.
  • There are no DTCs.

Diagnostic advice?

Two factors that may or may not be relevant, but which precede the observation of this issue:

  1. The blower motor was making some noise a few months ago. Sounded like a bad bearing and some fine copper colored dust was observed near the motor shaft. I lubricated the bearings as much as possible with mineral oil and reinstalled it. The noise has not returned so far and the fan works at all commanded speeds. Also note that I do not believe there is a resister module and that this fan works with PWM control.

  2. The alternator died and was replaced also a few months ago. The replacement is an aftermarket unit. A battery tester shows the charging system is ok, ripple is ok, battery is ok.

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This is just a shot in the dark, as I have no special knowledge of the Toyota Venza.

Access the blower motor and make sure it is spinning easily and freely by hand. If it's dragging due to a bad bearing, the current required to run it at max speed may be overloading the air conditioning module and forcing it to shut down. If the blower is not spinning freely, replace the blower motor.

If the blower spins freely, access the air conditioning module and give it a good long shot with a freezing spray, hitting the board components themselves if possible, not just the outer case. Then immediately turn the A/C to max and note how long it takes to shut down. If it still shuts down in 1-2 seconds, I give up and have no further suggestions. But if it runs significantly longer than 1-2 seconds, you may have a failing air conditioning module that needs to be replaced.

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  • And just a note about engine on vs engine off: The vehicle voltage is notably higher with the engine running. If something is overheating or overloaded, it will happen quicker with the engine running.
    – MTA
    Commented May 1 at 1:37
  • Thanks for your suggestions. Verified that the blower motor is still spinning freely by hand. AC module is enclosed in a case and board components are not accessible. The most recent change was the alternator, and one worry is that I've heard aftermarket units can cause many strange electrical issues. Not sure what else to check other than charging test with battery tester. 14V unloaded, 13.7V loaded (2500rpm), 39mV ripple, OK status. It's all just shots in the dark.
    – adatum
    Commented May 1 at 15:47
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    @adatum You suspect the alternator, and you sound like someone who has a benchtop power supply (0-30VDC, 0-30A?). TEST: Start car, idle a few minutes to recharge batt, disconnect alternator wire at batt and carefully insulate/make safe the wire. Connect power supply to battery and slowly raise volts and amps until you have a steady 13.7 volts. Now turn amps to max. Volts won't change but this will allow power supply to power A/C at 13.7V when it comes on. Turn on A/C. Still 1-2 sec to dead? If so, the alternator is not causing it.
    – MTA
    Commented May 1 at 20:33
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    @adatum Module innards not accessible? If the module can be removed / installed with reasonable promptness, pop it out and stick it in the freezer for 4 hours. When ready, wrap in a dry towel, run it to the car and install it as quickly as possible. Start engine and A/C. If A/C runs much longer than 1-2 seconds, suggest you replace module.
    – MTA
    Commented May 1 at 20:34
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    @adatum Condensation shouldn't be a big concern. Cars are built to be robust and survive all reasonable environmental conditions. You would be duplicating conditions such as a car full of teen hockey players sitting in a freezing cold car waiting for Dad to come out and drive them to hockey practice. The interior of the car is at 100% RH within minutes due to breathing. Condensation will be on everything, yet nothing will fail from this treatment. Anyway, it's your car and your call!
    – MTA
    Commented May 2 at 22:09

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