I used 4WD hi to pull a trailer out of a grassy area where I was slipping. I then drove down a dirt driveway, and then pulled onto the highway and went a 100 yards or so to where I could do a U-turn into a turnout. As I was doing that turn, I felt the 4WD still engaged, and so put it back to 2WD. Ever since then, I can't get into into 4WD hi or low, and the 4WD light on the dash doesn't come on when I try. Usually for 4-lo, I feel it clunk as it engages, but now there's nothing.

I'm a total novice, though I've changed out an exhaust manifold, change my oil, fuel filter and transmission filter/fluid, and have changed out two fuel lift pumps myself. I'm a goil, so I'm pretty proud of myself for that!

So, what's going on here, and is this anything near what I can deal with myself, or is it a rebuild situation? I try to do what I can to save $$, and right now we're not pulling a paycheck.

I did a quick search for 4WD problems, and someone had a similar situation with an '02, and they were talking about the CAD...ours is a '99, and ironically the cruise control quit working even though the dash light comes on, AND our air conditioning has gone funky....when we use the switch to change vents, it doesn't work. We have cold air, but no matter what you do, everything is coming through the defrost vents. So, is this a vacuum leak problem? If so, how to fix it?

 

Yes sir, Vacuum leak sounds like a good place to start. The vent will default to defrost for safety reasons. Your cruise is also controlled by vacuum as well. Your transfer case is probably going into it's position, however there is almost like a vacuum relay on top of the T-case. This needs vacuum to run your 4wd properly.

Basically I would start at the vacuum pump and trace hoses from there. There are quite a few vacuum lines under the hood, so anything is possible. If old age is the culprit a lot of times the little connections and "t's" will crack. If you don't find anything under the hood, you can follow the lines from the pump, underneath the truck to that relay on the T-case. Then there are also lines that run from that relay, under the tranny, along the passenger frame rail and into your front axle.

Your 4WD light will not illuminate if your CAD unit is not working properly. The light sensor is on the left side of the CAD unit. Once vacuum is sent to the CAD for 4wd (I forget which hose), the CAD fork slides over to the left and locks the centre axle to the outer axle. If there is proper vacuum, the fork slides far enough to make contact with the 4WD light sensor, turning it on.

Forgot to mention, back with my 99 Cab and Chassis it was having 4wd drive problems. Front axle would not engage, so basically no 4wd. Light would also not come on. All my venting and cruise worked, so I knew it was 4wd only related. I ended up pressurizing all 4wd hoses (with a very small amount of air) and having my co-worker listen for leaks. Ended up finding it very easy that way. I'm leaning towards your's is in the engine bay however, since nothing vacuum related works for you.

Good Luck!!

 

Thanks so much, Dinner! Great information and advice. Can't wait to find the leak :->

 

also check the switch on the transfer case. should be just above the linkage arm.

 

Yeah, vacuum leak all around. If that doesn't fix it try taking off the tubes from the CAD motor (brass looking flying saucer shaped thingy on the right rear of the front axle) and running the engine and check for vacuum. One hose should be pulling vacuum at all times. The right one pulls when in 2WD, the left in 4WD. Probably time for a new CAD motor anyway if it hasn't been changed in the last 5 years or so (rockauto.com or amazon)....