AC switches back and forth from defrost to vent

[revhardSHO]

Like the title says, with the AC on the air will blow out the vents (bi-level) and then after a minute or so switch to the defrost vents, and then repeat the cycle. Id rather cool me and not the windshield if at all possible. Would this be a BCM issue?

 

[rangerj]

Check the blend air door actuator. rangerj

 

[projectSHO89]

None of the above.

The system defaults to the defrost when vacuum is lost or diminished.

Look for a vacuum leak from the intake supply manifold to the HVAC control head. There is a single small vacuum line that runs through the firewall with one of the heater core lines. If the airflow goes to the dash when the gas pedal is pressed, you can bet on the vacuum leak.

The blend door actuator controls the temperature mix of the airflow. The BCM controls the blower motor speed.

Steve

 

[revhardSHO]

None of the above.

The system defaults to the defrost when vacuum is lost or diminished.

Look for a vacuum leak from the intake supply manifold to the HVAC control head. There is a single small vacuum line that runs through the firewall with one of the heater core lines. If the airflow goes to the dash when the gas pedal is pressed, you can bet on the vacuum leak.

The blend door actuator controls the temperature mix of the airflow. The BCM controls the blower motor speed.

Steve

Thanks, I will do some troubleshooting tommorow. I suppose if I dont find a leak before the firewall I'll have to tear into the dash?

 

[rangerj]

Steves right, I pointed you towards the wrong vacuum acuator. The panel defrost door acuator, a vacuum operated acuator, sounds like it is reacting to variations in engine manifold vacuum.

At idle engine vacuum is high so the vacuum actuator holds the door. During acceleration the manifold vacuum drops and the doors closes to the default position, i.e. defrost. The "panel defrost" acuator should be to the left of the center register with a yellow vacuum line attached to it.

As Steve stated look for a vacuum leak. The leak can best be detected by shutting off the engine and using a vacuum gauge to check for vacuum loss while selectively blocking off vacuum hoses. If none of the hoses are leaking, then the vacuum actuator itself is most likely leaking internally due to a crack or pinhole in the diaphram, or the case.

There is also a vacuum resevoir near the firewall on the passenger side or near the center of the firewall that may not be holding a vacuum.
rangerj

Steve, thanks for keeping me honest.

 

[gtfastback]

I'm having a very similar problem on my 2000 Taurus SE...the A/C will only blow on the windshield and under the dash. I tried the above but none of it seems to be the cause. I did recently have a new cd player installed, could this be causing a problem?

 

[rangerj]

gt,

Did the tech who did the CD player have the HVAC controls out of the car? If so you may have a mis-routed vacuum line. The other possibility is a pinched vacuum line. A failed vacuum controler seems unlikely in a 2000 SE, but it is possible. Could the tech have left something disconnected?

You may want to go to the local library and look up the vacuum hose routing, hose colors, and location of the vacuum actuators and "doors" for your heat and A/C system. Then determine if everything is connected properly.

Then start looking for a pinched or punctured vacuum line to the actuator that controls the vent/defrost door.

The color codes for the vacuum hoses should be (but I am not sure for your model year) defrost door - yellow, panel/floor door blue, source vacuum -black, air inlet door - white, panel/floor door red.

Also check the electrical connections to the EATC if you have it, or the manual system if that is what is in your car.

If this started to happen just after you had the CD installed, why not have the CD installer look at it? Did they screw something up? If so, let them fix it.
Just some thoughts.

 

[gtfastback]

I was thinking it is very likley that it was something the tech may have done, however I am living out of town about 2 hours from the installer, so I was trying to figure out if it was something simple I could fix, rather than waiting for my next trip home. I will investigate into what you said though.

Thanks for the help, this seems to be a friendly forum

 

[Mike Gallion]

I had a similar problem with my 95 mtx (self induced) that had me chasing my tail. (bit of cursing as well) What I finally figured out was the YELLOW vacuum line had been displaced from the vacuum motor during an eatc changeout. I wound up removing the eatc again and at that time noticed the line had been pulled off the vacuum motor and was resting against the "******" on the vacuum motor. At idle I had proper operation but at speed it defaulted to defrost. Once I got that wiggled back into place everything worked like a champ.
Good luck!