2003 Eddie Bauer Expedition, HVAC issue
 

Over the last 2 days I've noticed my front vents will blow, then randomly turn off and the air will blow to another function. I believe it is the defrost, but not 100%. The blower motor doesn't shut off. This vehicle has the auto dual temp crontrol unit in it. My initial guess is that there is a vacuum leak somewhere, if there is a starting point where I should check first. Thank you!

If the airflow defaults to the defroster (verify!), it's a loss of vacuum.

EATC units are very susceptible to O-ring failure in the vacuum switching module. There are multiple write-ups around here on how to replace them if they are implicated as the problem.

That isn't the only cause, but it is one of the most common. It would be advisable to do some diagnostic work to see if the problem is elsewhere or not before tearing the EATC apart. Leaks in the line that runs past the battery is another common failure.

Has anyone been working under the hood just before this started happening ? If it's passing air up to the defrost outlets under acceleration then check the vacuum tube going to the hot water valve there on the passenger side of the firewall, just above the engine's valve cover. Then make sure there's no cracks in the vacuum supply line running into the firewall from the engine compartment. Finally, you may need to replace the vacuum control module there under the dash at the front of the center console.

Vacuum supply line where it passes through the firewall grommet....
https://cimg9.ibsrv.net/gimg/www.for...a2c86025d7.jpg



HVAC Vacuum Control Solenoid module ......
https://cimg6.ibsrv.net/gimg/www.for...bba6cba949.jpg



Hot Water Valve and vacuum line ....
https://cimg8.ibsrv.net/gimg/www.for...0a6c567a7e.jpg

Thank you for the pictures and the feedback!! I'll check the vacuum lines, if those seem to be fine I'll start looking inside the cab at the other possible issues. Thank you guys again!! I'll post a reply with what the issue was and the outcome.

Also look around and under the battery for any vacuum lines where battery corrosion might've washed down onto.

So it turned out to be the vacuum line that attached to the hot water valve. I changed the spark plugs and disconnected that line to access #3 & 4 spark plugs and I didn't reconnect the vacuum line. Talk about feeling stupid!!! Thank you all for taking the time to respond and offer possible solutions!!!

FWIW, that line should only have vacuum when the system was set to MAX A/C (recirculate).

At least it was an easy fix once you found it.