Hey all--
Strange electrical problem here. '97 4x4 4.6L F-150. Whenever I am running the Air/AC/Heat in any of the settings (any speed...and panel, panel/floor, floor, floor/defrost..etc...) and I accelerate the truck, the climate control suddenly switches to defrost by itself. So I could have air blasting through the panel vents and about 1/2 second I start to give it some gas, the air stops blowing out of the panel vents and starts blasting through the defrost. The moment I let off the gas, the climate system goes back to normal. Step on gas...repeat.

-It only does this when the truck is under load. It won't happen when when I sit there and rev up the motor.

-It has been doing this for a while and has progessivly gotten worse. And now it's BAD.

-My truck seems much slower than it should be. Could be my imagination though...I know it's a big lug of a truck, but I really have to gun it to get somewhere.

-Cd's will go crazy on me sometimes when I step on the gas also. The sound will get fuzzy and sometimes the CD player just shuts down and says "No CD". This is with my factory 6-disc changer, which is re-located underneath the seats.

-Could this be part of the ignition system going bad??

Thanks for ANY ideas!!
-Eric-

 

I know you said this doesn't happen in nuetral, but what kind of voltage are you getting when this all takes place. I've seen some strange behavior when the power is low 9-10 volts, and also when its over 15 volts.

 

The voltage stays normal (according to the dash reading)....around 12-14. It never budges the whole time.

This really happens mostly on the highway...when the truck is under heavy load (aka...slow acceleration) in 4th or 5th gear in those low RPMS's.

---Eric---

 

I thought this was a vacuum line problem....check out your hoses and make sure you don't have any leaks.

Tony

 

TonyPTX said it. It is a vaccum leak.

The air distribution system is powered/redirected using vaccum lines and switches. The air system is setup so that in case of a serious vaccum leak, the air would default to the window defroster for safety reasons.

When under heavy load and high throttle, the vaccume drops in the lines. If you have a small leak, the drop in vaccum will be enough to drop below the threashold and the A/C or panel air will revert to default position. When you get off the gas and over the hill, the air will go back to the selected position after the vaccum builds back up.

You can take the front panels on the dash out to inspect all the vaccume lines going to the Air Selector switch (about 20$ if you have to change the switch) and also, there must be a vaccume collector tank in the engin compartment. (I don't know where it is in the F150).

I had the exact same problem in my Windstar when pulling my TT.

 

BRILLIANT!