4wd intermitant

i just picked up a 1990 jeep cherokee 4x4. just the rear tires work in 4 lo and in 4hi the only time 4hi works is when i have it to the mat. does this 4x4 system run off vacume? any ideas what may be causing this problem?

***edit*** i just found a good thread on how to trouble shoot the vacume control system. if anyone has anyother sugestions i'm more then happy to hear them.

i dont know i have this problem as well, i will never buy another jeep though cause ive had sooooo many problems with this one. i think its becasue of the part time 4x4.

The 4WD system itself doesnt work off of vacuum. You do have a vacuum actuated shift motor in the front axle housing that disconnects the passenger side axle shaft when not in 4X4. The shift motor is either engaged or not engaged. It either works or doesnt work and how much thottle you give it wouldnt matter.
How are you determining that you have no power going to the front axle in Low range? Are you saying you must have the accelerator floored to get any power to the front axle in 4 high? First off, do you have a fulltime selection on your transfer case or just parttime?

sorry to hijack, but someone explain to me how the part time 4x4 works cause it sucks.

Mustangman: either try searching the forum or start a new thread and provide more details please.

Phat: What Ray said (or asked in this case). How can you tell?

I suppose it's possible that a small vacuum leak may prevent full engagement of the front diff under idle or small load and then engage when the engine vacuum jumps under heavy load.

Nah!!

I suppose it's possible that a small vacuum leak may prevent full engagement of the front diff under idle or small load and then engage when the engine vacuum jumps under heavy load.

Nah!!

Very good point.
I havent looked into the system in detail but Im pretty sure that once the vacuum motor is activated and slides the sleeve over to lock the axles shafts together, it will stay in that position even if the vacuum is completely removed. It requires that vacuum be applied to slide the sleeve back to unlock the front axle shafts. Theres a vacuum switch that changes what side of the vacuum motor has vacuum applied to it to move the sleeve in either direction.
A good way to know if its a vacuum problem is whether or not he is getting a 4X4 indicator light. The indicator light switch is mounted on the vacuum motor and will come on only when the vacuum motor slides the sleeve over both shafts. No indicator and the vacuum motor isnt engaging.
The vacuum switch is on the side of the transfer case. This switch determines which side of the vacuum motor to send vacuum to to either engage or disengage the sleeve.

So my next question is, when does your 4X4 indicator light come on and go off?

hey guys,
i know its not working because if i put my back tires in a rut in the back alley when i'm in 4x4 the front tires won't spin in 4 lo or 4 hi. the indicator light works but i can't hear the hubs lock. the other day i was on a slope and i put it in reverse in 2wd. i didn't go anywhere, but when i put it in 4 hi i could back up the drive The confussing part is that the indicator light is on

hey guys,
i know its not working because if i put my back tires in a rut in the back alley when i'm in 4x4 the front tires won't spin in 4 lo or 4 hi. the indicator light works but i can't hear the hubs lock. the other day i was on a slope and i put it in reverse in 2wd. i didn't go anywhere, but when i put it in 4 hi i could back up the drive The confussing part is that the indicator light is on

I think everything is working normally. You have open differentials front and rear. If one of your front tires is off the ground or has no traction the other one wont spin. The only tire that receives power is the one with no traction. Same goes for the rear. If you get it crossed up in a ditch or rut, this could easily happen.

PS: The hubs on all XJs dont lock or unlock. All that you have up front is a untized bearing that supports the outer axle stub shafts.

really?? so what good is 4wd if your back tires are stuck and your front wheels don't turn?

***edit*** sorry i just re read what you said. but the front tires were out of the ruts just the back tires where in them. the only tire what was spinning was the rear passenger, niether of the front were moving

so are you saying the front wheels wont recieve power at all, because this is how it goes for me

What transfer case do you have? If its got a selection for fulltime, which is basically all wheeldrive, and you have it in fulltime, then what you describe is totally possible as only one wheel will receive power. That doesnt account for it happening in 4Lo though and it doesnt account for no wheels spinning.
If you are positive all of your tires had traction and none were spinning, you were in 4Hi or 4Lo and not N and you got a dash light indicator, Im at a loss.
Keep us informed on what the problem is because its a new one on me.

Youve obviously got someone walking around the vehicle to see that no tires are in the air spinning. Ask that persone to look at the front driveshaft to see if its spinning while you are in 4Lo and not moving when it should be moving.

i did that, had it in 4 lo cause it was back all the way, parking lot was shear ice, started doing donuts just like in regular 2wd, the light never comes on for me though.

no i'm not in neutral the back tires have lots of power they spin like mad but the front tires don't seem to have power. i don't have full time 4wd just part time. in 4hi it seems to work when i'm driving but when i'm stuck the front wheels don't do anything.

The XJ's don't have hubs that can be dis-engaged and they don't use vacuum to activate anything. They have mechanical linkage to the transfer case and the case engages a chain drive for the front driveline (very simple setup). The question is, is the chain broke, the linkage (shifter) broke, the front axle broke (in some fashion), you have a brain failure and it works as designed or you don't know what you have?

there is vacume going from a switch on the transfer case to the axle on the front left side... so u must be thinking about another year

The XJ's don't have hubs that can be dis-engaged and they don't use vacuum to activate anything. They have mechanical linkage to the transfer case and the case engages a chain drive for the front driveline (very simple setup). The question is, is the chain broke, the linkage (shifter) broke, the front axle broke (in some fashion), you have a brain failure and it works as designed or you don't know what you have?

Actually, both Ray and neon are right. The vacuum motor activates a sleeve that locks the front axle halves together and the trans case has a chain that engages the front driveshaft. It is possible that the chain is broken or the gears in the case have worn down. Although normally when this happens, you will not be able to move the shifter lever into the proper gear. I would check the linkage from the shifter to the transfer case first to ensure that it is not loose or broken and it is working correctly. I would then have someone check the transfer case to ensure it is working properly.

that makes sense. I would like to put it on a hoist and see what happens... i'll look more into it tomorrow. Thanx for the help i was hoping for an easy fix but it doesn't sound like a common problem

I did some more looking into this problem. i had the jeep on the hoist and put it in 4wd hi & lo, the indicator light was on but on the center shaft was turning not the from axle. i checked to see if i had vacume to the vacume motr adn i only have vacume on the rear vacume tube. i think thats normal but i'm not sure... can anyone check this for me? i'm thinking its the motor that needs replacing. any ideas?