A/C?

[BOSS]

Today I jumped in my truck and since it was 100F outside turned the A/C on but it was only blowing air out of the defrost vents. I turned the A/C back off and back on MAX A/C and all the other settings and it was still blowing out of the defrost vents. Anyone got an idea on what it could be, the air blows really cold but I just dont understand why it could be bolwing out of the defrost vents.

[TxPSD9673]

a short? mines startin to act weird too but mines 12 years old lol

[alabamafrog]

Most every vehicle I have ever worked on uses vacuum to control the air vents. As a failsafe if vacuum is lost then the vents default to defrost so you can see to drive. As a rule of thumb if only the defrost works then you should look for a vacuum problem. Since diesels don’t create vacuum they have a vacuum pump. You don’t have any info about the truck but since the post is in the 99-03 section then I assume it is a 99-03 which use electric vacuum pumps. May be a blown fuse or a bad vacuum leak or a bad vacuum pump. You should be able to hear the pump if you turn the key on but don’t start the engine and move the vent controls around. Mine is mounted near the passenger fender.

[BOSS]

Cool I will do the process of elimination. Yeah my truck is an 02 CC 4x4 Shortbed.

[Hank]

Check the vac. lines at the front hubs. Real common to be loose, off or leaking here. Good luck. Probably nothing major. You just have to trace everything out if the pump is working.

[alabamafrog]

The 4x4 vacuum system only operates for about 1 minute after moving the dash switch then it shuts off. If the hubs are leaking the truck will switch to defrost when the dash switch is moved and then after a minute or two the vents will go back to normal. It is a common misconception that there is always vacuum at the hubs when in fact the vacuum is only a pulse that latches and unlatches the hubs and last less than a minute and is either 6” or 15” depending on whether your locking in or unlocking. There is a vacuum solenoid/regulator on the passenger fender that controls the 4x4, I suppose it could be bad and causing the hubs to always have vacuum but then they would not lock or unlock properly. I have the wire unplugged to mine so I can use 4x4 Low to back trailers without locking the hubs so I can turn sharp.

[Hank]

Wow! More complicated than I thought. I've never had a problem with mine, just remember the defrost activation could be a hub (vac.) leak. You're pretty smart for a frog!
I've got so much to learn.

[alabamafrog]

Originally Posted by Hank

Wow! More complicated than I thought. I've never had a problem with mine, just remember the defrost activation could be a hub (vac.) leak. You're pretty smart for a frog!
I've got so much to learn.

Nothing to do with smart, just had my butt kicked by my truck more times than I can count and I always try to remember a good butt kicking to prevent it from happening again.

[Hank]

Originally Posted by alabamafrog

Nothing to do with smart, just had my butt kicked by my truck more times than I can count and I always try to remember a good butt kicking to prevent it from happening again.

I heard that Frog. That's one of the benefits of the org. Maybe one of us can avoid having our butts kicked by our trucks since your truck kicked yours. Thanks. I have read a lot of good posts by you. Keep it up!

[bcype]

My truck has the same problem. Air will only blow through the defrost vents. Do any of you know which fuse the vacuum pump runs off of. I check the owners manual but did not find it. I also checked my shift on the fly and it appears (light comes on the dash) to work. If it is not the pump what else might it be?