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Unread 01-16-2010, 06:01 PM   #1
marvinb
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Vacuum switch

I have a 1991,completely stock yj. The vacuum actuator for front axle will not engage 4 wheel drive. i ve taken the actuator off the axle and it does work( started engine and cycled from 2hi to 4 hi to 4 lo) but wont engage the slide on the axle.. I dont have a hand vac tool but i am getting vacuum to the actuator.. The fork mounted to the stem on the actuator is not damaged or have any play in it and the slide collar on the axle that locks up the front end slides very very easily. what could be the problem? what kind of vacuum should i be getting at the actuator

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Unread 01-16-2010, 07:19 PM   #2
bernm8r
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Marvin,
It seems to me your not getting enough vacuum to slide your fork over. Or your control valve on your transfer case is not switching vacuum. If you ARE getting vacuum you SHOULD get it to switch from one side to the other when you engage your transfer case (short shifter) the axle does not care what gear your in or if it is low or hi 4WD. what your fork does when it gets the correct vacuum on the correct side is slid to engage the axle or to slde back and disengage it. Some people use the posi-lok system to by pass the vacuum setup. It is pricey and some people are not pleased with it. My setup is a home made vacuum control I setup inside the tub. I have vacuum coming into a 3 way ball valve that either sends vacuum to the 2WD or the 4WD side of the front axle. This eliminates the gummed up controller on the transfer case and the old used cracked lines that were the original. I have never lost 4wd while on the trail. granted I am NOT a hard core off road trail blazing type, but it has always worked for me. You can use the search for a diagram of the vacuum system to get a better idea of how it works, it very simple to understand. I wish you luck
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Unread 01-16-2010, 07:50 PM   #3
JeeperDon
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Another idea is the actuator may have a leak (it's a rubber diaphragm) and do fine with no work needs doing. Try that 'actuator off the axle' test again and exert force wit your hand to try to stop it moving.

To test if it's a t-case switch issue, just run a straight vacuum line from the motor to the axle, and swap it side to side to see if it does better.

Playing with both those two ideas should definitely tell you where the problem really is.
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Unread 01-16-2010, 07:53 PM   #4
bernm8r
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I found some more info under the FAQ
http://www.jeepforum.com/forum/f12/f...y-here-451154/


4WD Engagement problems:
This information applies only to stock Dana 30 front axle disconnect of the 87-95 YJ jeep models.

One of the most common questions asked by new jeep owners is, “Why isn’t my 4wd working?” Though there are several possible reasons why the 4wd system of the jeep may be malfunctioning, the most common answer lies in the vacuum disconnect system that these models use. To understand why, you first need to understand how this system works.

The heart of the system lies on the passenger side of the front axle, just behind the wheel. There is a small molded box on axle tube that houses the disconnect shift fork which is connected directly to the shift motor. The axle shaft on this side is split into two sections, an inner section that connects to the housing and the differential, and an outer shaft that runs from the housing to wheel hub. When 4wd is engaged via the transfer case selection lever, the shift fork in the housing slides a collar from the outer shaft onto the inner shaft, coupling the two together so that they spin as one piece. If this fork fails to move, the shafts are not joined.

The jeep uses and open differential on the front axle (as well as the rear), which means that power is sent to the wheel that has the least amount of resistance. Under normal circumstances, like dry and semi dry pavement, this isn’t a problem as both wheels usually have somewhat equal resistance and thus power is transmitted roughly equally. If stuck in the mud or snow however, it is possible for one wheel to have no resistance. Thus all of the power will be transmitted to that wheel, causing the one that has traction to remain still, while the one that doesn’t just spins and spins. You’re stuck. This characteristic of the differential also holds true if the two piece shaft isn’t coupled via the collar. Since the inner shaft has the least amount of resistance because it isn’t connected to anything, all of the power is applied to it, thus robbing the driver side of any movement.

To move the fork, Chrysler decided to use a vacuum powered motor. A vacuum source line is run from somewhere on the engine (the location varies and can come from the brake booster, the manifold, etc) to a vacuum switch on the transfer case. When 4wd is selected, the switch sends the vacuum along several lines to the shift motor located on the front axle. The vacuum pressure is applied to a diaphragm inside the motor, thus sucking it one way or the other, depending on whether the axle is engaging or disengaging. The diaphragm is connected to the end of the shaft that the shift fork is attached to, thus moving the fork along with it. The problem with this system generally lies in the vacuum lines themselves. Over time, the lines harden and crack and dry rot. The older the vehicle, the more likely this will happen. As cracks form, air leaks soon follow, lowering the amount of vacuum pressure that is in the lines. After a certain point, there is no longer enough pressure to move the diaphragm, thus causing leaving the fork motionless. More often then not this is problem with the system. Included below is a diagram of the 4wd vacuum system.






Other possible problems:
The shift motor itself has been known to fail in much the same manner as the vacuum lines. Cracks in the diaphragm as well as rust and corrosion of the casing all lead to air leaks. A new shift motor runs about $50-80 at www.quadratec.com depending on the year of the jeep.

Another common problem is the vacuum switch on the transfer case. Over time dirt can build up inside it, causing it to fail. This can be replaced for about $25.

How to know if it’s the vacuum system:
Usually the best indicator of this is the fact that the 4wd dash indicator light no longer works. The light is switched on and off by the same vacuum that moves the shift fork. If the light is not coming on and your front wheels aren’t turning under power, there is a good chance it’s the vacuum system.

The Posi-Lok (Posi-lock) System:
Often confused with a differential locker, the posi-lok kit is simply a manual system for moving the 4wd shift fork that replaces the vacuum system. A cable is attached to a handle inside the cab and routed through the firewall down to a modified housing that replaces the housing holding the shift motor on the axle. To engage 4wd, the user places the transfer case in 4 hi or low, and pulls the handle, moving the shift fork to couple the two piece axle shaft. This eliminates totally the need for any vacuum pressure to be present at either the case or the axle, making it a much more efficient system, less prone to failure.

Cost:
Though marketed at about $200 dollars, the posi-lok can often be found on sale for $180, and sometimes even $160. Ebay may yield even better results. Run searches in the Shopping sections of the more popular search engines and you’ll find many dealers and will be able to sort by lowest price first. Just be sure it is meant for Jeeps as there are many trucks/SUV’s out there that use a similar system and Posi-Lok offers different kits for most of them. At this time, Jeeps use the 9000 series. For the DIY people, there have been write-ups on making your own cable system using bicycle shifters and miscellaneous parts. One such write-up can be found here: http://home.earthlink.net/~stevenschreiber/cabledis.htm There may be others out there. Search for them on the net.


Installation Tips (Posi-lok Kit):
For the most part, the instructions included with the kit are self explanatory. The only tricky part is the removal of the shift fork from the motor housing. There are three e-clips that hold the fork to the shaft of the motor and can be very difficult to remove. I highly recommend purchasing a new fork (quadratec has them for ~$15, though there are two different models depending on the year of the jeep. 87-90 use part number 52430.00 and 91-95 use 52430.01) prior to installing the kit, as then the stock setup can be retained and rebuilt incase of problems with the system. A second fork also saves time and aggravation on the install, and will not show signs of wear as the old fork may. More often then not, the fork wears where it attaches to the motor shaft and will flop back and forth on the new shaft, causing problems during engagement. Better to have a new fork on hand before you find that this is the case for your jeep, forcing you to wait several days while you order a new one. Other then this, the installation is very straight forward and can be accomplished in a few hours, by yourself with only common hand tools. The only thing you will need that might not be common is a 5/8” drill bit, and a drill big enough to hold it (most cordless drills are limited to 3/8” chucks. The 5/8” bit will likely require a 1/2” chuck).
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Unread 01-17-2010, 07:26 AM   #5
marvinb
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Hey guys I really appreciate the input.. Im going to try running a different vacuum line to it and see what happens.
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Unread 01-17-2010, 09:39 AM   #6
jay-h
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Quote:
Originally Posted by marvinb View Post
Hey guys I really appreciate the input.. Im going to try running a different vacuum line to it and see what happens.
you can even use that as a semi-permanent fix if you don't mind switching the vacuum line under your hood when you are going to be using 4wd (it is NOT necessary to disconnect as soon as you get out of 4wd, indeed it's not truly necessary at all--if you're dealing with a snowstorm for example, leave the front axle engaged, and just use the t-case to go in and out of 4wd as necessary-- but some users report some driveshaft vibration from the front driveshaft at highway speeds.)
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Unread 01-17-2010, 04:42 PM   #7
marvinb
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Ok, still stumped... Bought a lentgh of rubber tubing to test actuator. Here is what i did.. Found a good vacuum source at the motor, took front axle actuator off and set it on the front fender, hooked the hose up to the vacuum source and to the actuator while motor was running.. The actuator moved quicker and resistance was much greater trying to hold the fork with my hand.. So i then reinstalled it on the axle, restarted motor, reconnected vacuum hose, and guess what. Nothing, still will not engage front axle. So now i jacked up front pass tire and tried again rotating tire so i could line up the splines and collar. Still nothing. Im stumped and im not gonna screw with it anymore.. Posi-lok time...
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Unread 01-18-2010, 05:12 AM   #8
JeeperDon
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Quote:
Originally Posted by marvinb View Post
Ok, still stumped... .. Posi-lok time...
Do what you want, people usually do, but that's just the quiters fix. 'I don't want to be bothered figuring it out, so just I'll just throw money at it.' A PL depends upon parts of your system working, and from what you wrote here you don't know if they do.

Does the coupler in the axle slide easily by hand to dis/engage the axle halves? If it dosent, a PL will get you nothing. Did that straight vacuum hose test work strongly on both sides with the unit in hand, or just one side?

This is one of the simplest systems on the Jeep to fix, especially when you have all the diagrams up earlier in this thread.
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Last edited by JeeperDon; 01-18-2010 at 11:53 AM..
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Unread 01-18-2010, 10:27 AM   #9
mrclean
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If the actuator is moving as you say, it should be moving the collar with no problem. Why do you believe it's not engaging? I'm not insulting you, but you're not judging by the 4WD light on the dash, are you? Many people have been fooled by this and wasted time trying to fix the actuator, when it was just a burned-out bulb or failed switch.
Another thing to check......there are small plastic shoes that clip on to the ends of the shift fork and ride in the slot of the shift collar......if these are missing, the engagement/disengagement will be sloppy.
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Unread 01-18-2010, 03:31 PM   #10
marvinb
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Guys i was aggravated yesterday cause i went through every single vacuum hose thoroughly, t-case switch, diaphragm, check valve, and i even ran a new hose from a very good vacuum source to the actuator and it WOULD NOT ENGAGE front axle... NO, im not going off of the idiot light. But, out of every thing i have read on any post ( maybe i didnt find the right one) i havent found one person say anything about check the actuator fork slop.. You experts know thats where the fork connects to the diaphragm push rod..So after a temporary fix the front axle engages when i shift t-case into four wheel drive and now all is well,now that i know what the problem is... Ordered a new shift fork for the actuator, will be here in a couple days...
And No I am not going to just throw money away, the only reason i said im done with it and goin to a posi lok is because i was aggravated a lil bit.
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Unread 01-18-2010, 03:41 PM   #11
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Quote:
Originally Posted by marvinb View Post
Guys i was aggravated yesterday cause i went through every single vacuum hose thoroughly, t-case switch, diaphragm, check valve, and i even ran a new hose from a very good vacuum source to the actuator and it WOULD NOT ENGAGE front axle... NO, im not going off of the idiot light. But, out of every thing i have read on any post ( maybe i didnt find the right one) i havent found one person say anything about check the actuator fork slop.. You experts know thats where the fork connects to the diaphragm push rod..So after a temporary fix the front axle engages when i shift t-case into four wheel drive and now all is well,now that i know what the problem is... Ordered a new shift fork for the actuator, will be here in a couple days...
And No I am not going to just throw money away, the only reason i said im done with it and goin to a posi lok is because i was aggravated a lil bit.
Congratulations on figuring it out, and sticking with it!

IMO, the CAD system is just fine. Our 2 YJs have been trouble free so far, and I expect that I will attempt to fix the system properly when/if the time comes.

Also, thanks for posting what the remedy was for yours. You end may up helping out others who might read this thread later on down the line.

Max
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No one here knows what they are talking about. You should try Pirate 4x4 they will be happy to help you.

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Unread 01-18-2010, 03:47 PM   #12
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Silly question. Did you try pushing the engagement ring by hand over? Does it move freely? When you put it back together did you line up the fork with the groove?
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Unread 01-18-2010, 03:52 PM   #13
marvinb
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Mean max I thank you for your comments. I had a hell of a time, but the problem was staring me in the face and didnt realize it.. And hopefully it will help someone else..
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Unread 01-18-2010, 03:55 PM   #14
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Mean max I thank you for your comments. I had a hell of a time, but the problem was staring me in the face and didnt realize it.. And hopefully it will help someone else..
You have no idea how many skinned knuckles & thrown tools have resulted in my overlooking the obvious. It just happens.

Good job!

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Unread 01-29-2010, 04:59 PM   #15
shansel88
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First, let me say hello to all. Ihave an issue with this same issue but my light is stuck on and the 4wd is activated. Can this be the same situation? I was thinking of getting a vacuum tester and seeing if I had, or how much, vacuum I have going to either side of the shifter. Does this sound right? Maybe it just broke locked in 4WD??
Thanks
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