got a 90 cherokee 4wd 4.0 vac disco started off not being able to get it into 4wd went to JY and got a vac ball and still no go, end up being the shift fork anyway, its getting colder out and i been using heater but on acceleration or up a hill it goes from vents to defrost, I know there has been a million threads about this i searched but i checked every vacuum line from the engine to trans case into the cab everything, sprayed brake clean on them all no idle change what so ever, the only thing i dont know about is the cruse control as there is a vac line that runs from it to the brake switch in the cab but i dont see a supply line, anyway, what else can i check or replace to get this working, its getting to cold to just have defrost and no vents.

I am thinking maybe a internal leak in the brake booster or inside the vac disconnect but the jeep will stay in 4wd no problem, its just the heater that is screwy

 

the vacuum check valve is not working on the supply line to dash control

 

looks like this only allows suction in one direction cut the line by blower motor install

 

ok, so this goes on the black line from the vac ball that goes into firewall, because i dont have one there now, so maybe that is the problem, I know there are two other check valves but they seem to be working

 

yes that should cure the probjust make sure you install with the sucking side to the ball fixed mine

 

ok, so i got one and put it on small plastic line that goes into firewall, no go, tried both ways just maken sure i didnt mess it up, is there supposed to be a checkvalve on the main vac line that goes from the intake to the vac ball,

Free could you maybe take a picture of yours so i could compare , i am going to take pictures tomorrow after i go to the junk yard and maybe you guys can tell me whats up

 

could be the diaphram in the controll in dash get acces to a vacuum tester it looks like this and appl vacuum to the line that goes in by blower motor see if it hods vacuum dont go over 20 lbs on gauge

 

Check where the vac line goes from the passenger side down to the bumper if it goes under the battery, id check there.

 

The check valve should be on the main line right after it comes off the manifold.

 

ok, so i got her fixed, thanks for all the help guys, but it ended up being the vac ball itself, i went to junkyard and realized that i have a ball, not a egg, the new egg i got has 4 vac ports, hooked her up and bam no more problems, so i dont know if the old ball was to small or was it leaking internally, Thanks everyone, and free i actually tried to use that vac pump already, i have one for bleeding brakes and thats how i checked everything in the dash, no problems,
Thanks everyone, appreciate all help and comments