A couple of days ago I was pulling some brush, put my truck in 4lo backed down the hill and almost didn't get back! I didn't check the 4wd light before I backed up but it never came on. finally got out of there drove back to the barn and suddenly the light pops on. Since then I have not been able to get it into 4wd. My guess is that the front axle disconnect is not working properly.
I was just wondering if some of you guys may have gone through this before and may know where to look first or should I just scrap the thing and convert to a manual shift cable.

Thanks for any advice you may have
pacmule

 

more than likely its vacuum and yea i'd take avantage that it quit working and get posi lock but before you do check the pump and all the lines and also do you dash vents and cruise work? if not its vacuum unless its not the same on 98's

 

ditto...check the vacuum lines first and foremost...both my trucks (96 and 98) have come loose before and done the same thing...I lost 4wd, cruise, and vent controls...

 

I'll bet that "brush" you were involved with popped a vacumn line off under there.

 

I got a little time to check some things today, vacuum lines were intact. I pulled them off and fired up the truck and one had suction but not the other, but I do not know if is enough. will try to check more tommorrow.
pacmule

 

OK since u have a vac system

one has suction, the other should be pushing air to keep the collar in place. I assume u tested this with the truck on correct? Possibly what happened is that you got oil somewhere in the lines... by the T case, check the connections and check all lines for oil stains or buildup. This could cause ur vac malfunction. Also check to make sure that EVERY SINGLE line is hooked up to where it goes, if so then the problem might be in ur actuator itself, but since one line is not blowin air, its a vac problem..

TIP for the vac lines, zip tie them with the itty bitty zipties and they wont get loose as easily.. worked for me..

Rick

 

I believe that the 4WD system ony has vacuum, no pressurized lines. One of the lines at the front has vacuum when you are in 2WD to pull the collar off of the splined axle. When you switch to 4WD the other line gets the vacuum while the original goes neutral. This pulls the collar to engage the splines. The best way to check is to put it in 4WD and see if you have vacuum to one of the lines. Then put it in 2WD and see if you have vacuum in the other line. If you are missing vacuum you need to trace the lines back to the Tcase and look for leaking connections. As stated above, if oil of some sort got in the lines it could affect their performance. You can try disconnecting the lines and blowing a shot of compressed air through to try and clean them out.

 

The way it works is while one line has vacuum, the other vents.
Otherwise it would be like working against a closed valve.

 

Makes sence

i stand corrected, thanks BIll!

Rick

 

O.K., I took another look at it. pulled off the vacuum lines, started truck and it works just as Crimedog said, 2wd one sucks, 4wd the other sucks. All the lines look fine not really even dirty or oily around them. removed the bolts and shift fork assembly. reinstalled vacuum lines,repeated test with assembly hanging there from the diff. vent line, fork moves back and forth when shifted. Checked collar and could not slide it over with my fingers, seemed stuck. gently pried over with bar and was able to slide back and forth. if I snapped it over too hard to the right (2wd) would stick again. reassemled while in 4wd. rt. wheel still jacked up, shifts in and out of 4wd as it should. drop off jacks test drive around property a few minutes later and it has quit working again. I believe the fork doesn't have the power to shift the collar, but I don't know if that is from a weak shifter or a sticky collar.

pacmule

 

You could have a small vacuum leak somewhere that isn't allowing you to build enough vacuum. Simple way to tell is if there is still vacuum when pulling apart a hose several minutes after shutdown. My system holds vacuum for several days without the engine running.

 

my 97 did the same thing in 4 lo only it would come out of 4wd under a load the dealer didn't beleive me until another truck had the same problem. it was just a linkage adjustment on the shifter it wasn't letting 4 lo fully engage in the t- case so it would kick in and out at the worst possible times.