Haunted HVAC

[3ZHP]

Looks like my Gremlin is back. Had this issue a few years ago and it went away.

The issue: driving the car on a hot day with the HVAC set to auto at 69° and the AC blows nice cold air. Then at any random time the center vents start blowing HOT air. This will continue until you stop the car and turn off the ignition for 5 or 10 minutes. This includes; getting gas, grabbing food, stopping at the store. After that, you start the car and everything is back to normal until the Gremlin comes back.

The quick cure, I found for it on the way home from the Mafia Family Reunion is, to press the manual buttons for Floor and Vent at the same time. Once these are set with the green light illuminated, I get cold air. This isn't as good as the Ice Cold Auto mode but, cold enough to be comfortable.

Any Ideas? :dunno :help

[Vas]

Re circulation button is pressed?

[3ZHP]

Doesn't matter, happens either way.

[ecrabb]

First, let me say I know very little about the cabin HVAC system in our cars. I've just looked at the service manual a little.

From my read of the manual, in "auto" mode, the system uses inputs from the temp setting you set on the dash, the interior temp sensor, the ambient temp sensor, a heater core temp sensor, an evaporator temp sensor, and possibly a solar sensor, then decides how much coolant to pulse through the heater core to mix warm air with the cold air from the A/C evaporator.

Basically, in auto mode, on a warm day, the heat and A/C are both "running". The A/C compressor is running to make cold air, and the heater core is used to warm the cold air to target the cabin temp you set in auto mode.

When you tap the snowflake button, it should be shutting off the pulse valve to the heater core, so you should get only cold A/C air, and it should be as ice cold as it ever gets.

Is it only the center vents that switch to blowing hot, or do they all change temp?

SC

[stephenkirsh]

Have you tried running it out of auto mode? I never use auto.

[3ZHP]

First, let me say I know very little about the cabin HVAC system in our cars. I've just looked at the service manual a little.

From my read of the manual, in "auto" mode, the system uses inputs from the temp setting you set on the dash, the interior temp sensor, the ambient temp sensor, a heater core temp sensor, an evaporator temp sensor, and possibly a solar sensor, then decides how much coolant to pulse through the heater core to mix warm air with the cold air from the A/C evaporator.

Basically, in auto mode, on a warm day, the heat and A/C are both "running". The A/C compressor is running to make cold air, and the heater core is used to warm the cold air to target the cabin temp you set in auto mode.

When you tap the snowflake button, it should be shutting off the pulse valve to the heater core, so you should get only cold A/C air, and it should be as ice cold as it ever gets.

Is it only the center vents that switch to blowing hot, or do they all change temp?

SC

Yes, it is only the center vent, that blows hot air. When this happens, the side vents are still cold but not enough volume to overcome the hot air from the center vent. Also if I roll the dial to the red zone, it gets even hotter.


Have you tried running it out of auto mode? I never use auto.

I've tried it in manual and that's how I found the temporary cure but, both the floor and dash vents have to be selected for it to work.

[jpalazzi87]

I had something similar happening at one point, it turned out to be a bad HVAC control head on the dash it was screwing with the door position, and the heater vavle, I also had a code in the HVAC system when I scanned it with my snapon verdict, I don't remember the code but it was a short to pwer code that was intermittent.

[Vas]

I actually noticed the same thing today happening on my car.