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I tried to put my truck in 4WD today and the "4WD Engaged" light never came on. So I put it in park then tried it, same thing. So I drove it around the block put the truck in neutral and same thing, then I finally came in here to ask you guys. Last night it worked fine, I put the truck in 4wd in Park and the light came on. Help! BTW I have an '89 Dodge Ram Charger, thnx
 

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I've got a 90 RC and experienced the same thing about a year ago during the winter. A couple of things may be happening. 1) Your transfer case vac switch may be getting weak, leaking. 2) Check all vac source lines for leaks, possible contact with exhaust, etc? 3) Round vac. resivoir may be leaking. 4) The vac actuator motor on the front rear may be getting weak, (diaphram getting bad), or finally, 5) The elec switch that engages when you shift to 4 wd may be getting bad or the elec connection may be dirty to the switch or the switch may just need another 1/4 to 1/2 a turn in to fully engage when in 4 wd to keep the dash lit. Of course there's always the possibility the 4 wd dash light bulb may be loose. Are you sure it's actually in and out of 4 wd when the light is off / on?
Best way to tell is to jack up the left front of the truck and spin the wheel while someone else shifts the shifter to 4 high, with the trans in NEUTRAL, (There may or may not be enough vac in the vac resivoir to do this without the motor running, depending on whether you have a vac leak or not.) It will be real easy to tell if the slipper collar that engages the axle is being engaged by the vac motor and whether the light is telling the truth about what's going on, or not.

Good luck.
Dave
 

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there's some good information on the CAD (Central Axle Disconnect) system on the FAQ board too.
 

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First don't rely on a light bulb to tell you if you are in 4wd. The bulb could be burned out or it may have a bad ground or other problem that could cause it not to light up.

You need to try the truck out and make sure the front drivetrain is engaging. If it isn't, then follow the suggestions others have mentioned.

Ed
 
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This is going to sound really stupid, but bear with me, your post doesn't really say what you did.

With the CAD system the truck has to be moved forward a few feet before the axles actually engage. When disengaging a CAD you must shift the transfer case to 2wd then back up a few feet to disengage the axles.
 

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In addition to the things the others mentioned, like vacuum leaks, also check to make sure the vent line is open to the atmosphere (not the transfer case vent, but the CAD vent line). If the system can't get air in, it can't move the disconnect lever.
 

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I have the vacuum schematics and a troubleshooting flow chart from a '90 FSM if you would like, I can scan them and send them to you.
 
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