The light on the dash means nothing really. IF you connected the vacuum to the wrong line it will light your dash up. or if you switch it around it will light up when in 2wd and go off in 4wd. You need to follow some basic trouble shooting. get a diagram of how the 4wd vacuum system is SUPPOSED to be.
Here is some info in the FAQ section under
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.
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.
This is a copy of the install instructions I got for my carb
There are two types of vacuum pull that you will need to understand. They are “ported”
vacuum and “manifold” vacuum. It is EXTREMELY important to know what type of
vacuum source each port is pulling, and to know and understand which vacuum source is
needed for each device. Example: Your distributor advance needs ported vacuum. The
ported vacuum pull advances your distributor timing as you accelerate. If by chance you
tie into manifold vacuum by mistake (and properly adjust your timing) you will actually
retard your timing as you accelerate. This is BAD by the way and you will be very
sluggish off the line and at WOT.
-Ported vacuum is drawn in when the throttle is opened up. This draw is increased as
more air is sucked into your air through cleaner and down through the venturis. Ported
vacuum is highest at W.O.T. and lowest (or non-existent) at idle. You will have almost
no vacuum pull at idle with ported vacuum unless you have moderately high idle speeds..
-Manifold vacuum is drawn when your throttle is closed or at idle to very low driving
speeds/RPM’s. This is everything under the carb. It is highest at idle and lowest at
Checking Vacuum Type
The easiest way to check vacuum ports and telling what type of vacuum they are drawing
is to start the engine with the vacuum ports capped off and uncap them one at a time.
Once you uncap the port, touch the opening with your finger (at idle) if it sucks on your
finger tip it is probably manifold vacuum. Increase to full throttle, the suction should
dissipate. If the vacuum is ported, it should have little to no suction at idle and should
start to draw respectable suction as acceleration is increased. Ported vacuum will reach
peak suction at W.O.T