My truck is a 98 dodge ram and it does the same thing, I noticed the problem last week. Also when i'm driving or accelerating I can here a high pitch sucking noise, almost sounds like a quit turbo spooling out but with no blow off. Could these problems be linked? I thought the noise my be a vacuum leak.

 

mojo- after reading your post i took the truck for a drive... The heater doesnt blow heat (it blows warm air but not hot). I will be calling my local dealer.. is it something i can do the work? i really dont like paying 60$ per hour if i can do it.

 

Ok, is the 4wd engaged by a vacuum line?(could they be linked?) because i was out wheeling at one of my favorite spots and i cant get 4wd to engage??[:@]

 

It is a vacuum leak. The hissing or sucking sound is the vacuum leak. Check the elbow under the battery tray going to the cruise control servo. Also check the vacuum line going through the firewall on the passenger side. Mine was the hose on the firewall as well as like 10 other people I work with. Everytime I'd pull it out of the firewall it would check out fine, but when I pushed it back in it would start switching the a/c back and forth. I finally figured out that it was on the cab side of the firewall and everytime I'd pull the plastic piece out on the engine side it would seal the crack in the grommet. Then when i would push it in it would push the cracked piece back out and let it leak.

 

good call junkman, thats where mine was. right on the cruise control. I just fixed mine yesterday. whenever I would accelerate up a hill it would feel like I had the heat on. sucked during summer without any AC