1. #1
    bravo4's Avatar
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    Front wheel drive not engaging

    I just replaced the Vacuum shift motor on my 89 YJ but still no four wheel drive.I have been tracing the vacuum lines and they seem OK,I pulled the shift cover off and checked the lines to the transfer case.I was told the switch might be bad ,but I can't find a switch any ideas as where to start next.

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    jfrabat's Avatar
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    Re: Front wheel drive not engaging

    Is it not engaging, or is the light not coming on?

    '94 YJ 2.5L, Sperwinch EPi 9.0, RE 4" Std Lift, Adv. Ad. SYE, Woody Shafts, Alloy Axles, ARB Lockers, 4.88 gears, 62mm TB & Spacer, 165A Alternator, Griffin 3" Aluminum Rad, Taurus e-fan w/ DC Controller, Borla Cat-Back, Flowmaster hiflow cat, PD Ignition, 19# injectors, Adj. FPR (42PSI), MOPAR cam, Safari Snorkel, 33" BFG KM2, 15" AR wheels, Xenon H4 lights, KC Daylighters & Slimlites, Sony sound, Pavement Ends Targa Top, Hanson Front Bumper, JCR Offroad rear, AtoZ Rockers, Rhino lined Tub.

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    Re: Front wheel drive not engaging

    Quote Originally Posted by jfrabat View Post
    Is it not engaging, or is the light not coming on?
    The light does not come on either.I took the Jeep to a steep grass bank and all I get is the back tires spinning.I thought right after I put it back together and engaged the 4x4 I heard it kick in, and It felt in the steering like it had but it did not last.

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    Re: Front wheel drive not engaging

    The switch is located where the vacuum lines attach to the transfer case. You can get a replacement at parts stores.

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    Re: Front wheel drive not engaging

    Do this; put the Jeep up in Jack Stands, and engage 4WD; put it in 1st and see if the front DS rotates (MAKE SURE THE JEEP IS SAFELY IN THE AIR WITH NO CHANCE FOR IT TO FALL!). If the front DS is rotating, the TC is in 4WD. If so, your problem is in the vaccum servo in the front axle. If it does not, then the problem is in the TC (my first bet would be linkage adjustment).

    '94 YJ 2.5L, Sperwinch EPi 9.0, RE 4" Std Lift, Adv. Ad. SYE, Woody Shafts, Alloy Axles, ARB Lockers, 4.88 gears, 62mm TB & Spacer, 165A Alternator, Griffin 3" Aluminum Rad, Taurus e-fan w/ DC Controller, Borla Cat-Back, Flowmaster hiflow cat, PD Ignition, 19# injectors, Adj. FPR (42PSI), MOPAR cam, Safari Snorkel, 33" BFG KM2, 15" AR wheels, Xenon H4 lights, KC Daylighters & Slimlites, Sony sound, Pavement Ends Targa Top, Hanson Front Bumper, JCR Offroad rear, AtoZ Rockers, Rhino lined Tub.

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    Re: Front wheel drive not engaging

    Thanks for all the input, I was starting to get frustrated I will try these suggestions .

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    Re: Front wheel drive not engaging

    I do not think there are linkage adjustments on a 89 yj.

    if it is not the CAD system, then it will most likely be a worn shift fork in the t-case. as for the jacking up the jeep and stuff, the front drive shaft most times visually will spin regardless 2 or 4wd due to lack of resistance. In the past, I've put some masking tape around the front drive shaft and then loop it around the frame, if there is anough force to tear the masking tape then the t-case is fine. if the tape does not tear, t-case is suspect.

    I mention this because it is just simply not safe to be under a suspended vehicle, running in gear and try grabbing the front driveshaft with your hand
    It used to be a jeep Thing,
    But all that stuff broke

    -Johnny-


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    Re: Front wheel drive not engaging

    To test the vacuum switch on the transfer case, pull the vacuum harness from the front axle. With the jeep running, have a friend shift from 2wd to 4wd and back while you feel for vacuum in the harness at the front axle. Vacuum should be strong in one hose for 2wd and the other hose for 4wd.

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    Re: Front wheel drive not engaging

    which one is which? mine were pulled out so i just stuck them in a hole hoping it didn't matter.
    I love my old jeep.

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    Re: Front wheel drive not engaging

    If the 4wd light comes on in 2wd and off in 4wd, you have them reversed.

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    Re: Front wheel drive not engaging

    So far I have found no vacuum at the shift motor,next step will be to replace the switch.

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    Re: Front wheel drive not engaging

    Good idea. Also trace the red vacuum line from the intake manifold to the transfer case and make sure you have adequate vacuum to the transfer case.

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    Re: Front wheel drive not engaging

    Thanks for the red line advice.The motor has been switched and there are some vacuum lines not connected to anything and I can not find where they go.The 4x4 was working when I got the jeep so I had assumed they had them hooked it up the right way.I have been thinking about just rigging the thing up to be always engaged.

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    Re: Front wheel drive not engaging

    Quote Originally Posted by bravo4 View Post
    I have been thinking about just rigging the thing up to be always engaged.
    That's what I did. I didn't want to replace the vac motor because they are prone to failure, and I didn't want to drop $215 on the PosiLok. So I crawled under there and flipped that shift fork housing right around, and now the front axle is always engaged.

    You will get a little extra vibration when you're cruising at high speed. This is because the front driveshaft is not balanced from the factory on YJ's. However, I have it on good authority (from Jeep techs and other Jeepers) that doing this will cause no damage and is perfectly safe.

    Doing this also has the added benefit of allowing you to shift into 4WD on-the-fly, i.e. while moving. The transfer case can do it, the shift collar can't. So if the shift collar is always engaged, then there's nothing to grind
    '95 Wrangler Rio Grande - 2.5L 5-speed - 2" Black Diamond Lift - Carr Rota Lightbar - 4x PIAA Daylighters - Cragar 397 Series Soft 8's - 31" BFG A/T's - 4.0L Throttle Body - 3" Custom Intake w/ Speed by Spectre Conical Filter

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    Re: Front wheel drive not engaging

    Quote Originally Posted by Sine Deviance View Post
    You will get a little extra vibration when you're cruising at high speed. This is because the front driveshaft is not balanced from the factory on YJ's. However, I have it on good authority (from Jeep techs and other Jeepers) that doing this will cause no damage and is perfectly safe.
    TJ's use the same transfer case, and the axle does not use the vac disconnect. I run a solid axle on my YJ as well, so my front DS is always turning, and I have had no problems so far (even drivong from Panama City, Panama, to San José, Costa Rica, a 600 mile road trip each way). I did change the front DS (got me a Tom Woods one) because the stocker was bent, so I have no vibrations on mine.

    '94 YJ 2.5L, Sperwinch EPi 9.0, RE 4" Std Lift, Adv. Ad. SYE, Woody Shafts, Alloy Axles, ARB Lockers, 4.88 gears, 62mm TB & Spacer, 165A Alternator, Griffin 3" Aluminum Rad, Taurus e-fan w/ DC Controller, Borla Cat-Back, Flowmaster hiflow cat, PD Ignition, 19# injectors, Adj. FPR (42PSI), MOPAR cam, Safari Snorkel, 33" BFG KM2, 15" AR wheels, Xenon H4 lights, KC Daylighters & Slimlites, Sony sound, Pavement Ends Targa Top, Hanson Front Bumper, JCR Offroad rear, AtoZ Rockers, Rhino lined Tub.

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    Re: Front wheel drive not engaging

    Quote Originally Posted by jfrabat View Post
    TJ's use the same transfer case, and the axle does not use the vac disconnect.
    TJ's have a balanced front driveline from factory to make up for the fact that they do not use a disconnect collar. I wonder why Jeep didn't just do the same for the YJ's? It's a design that is much stronger and much less prone to failure. I guess they learned their lesson with the YJ setup.

    Quote Originally Posted by jfrabat View Post
    I run a solid axle on my YJ as well, so my front DS is always turning, and I have had no problems so far (even drivong from Panama City, Panama, to San José, Costa Rica, a 600 mile road trip each way). I did change the front DS (got me a Tom Woods one) because the stocker was bent, so I have no vibrations on mine.
    Wow, how did it get bent in the first place? Your new driveline is balanced isn't it?
    '95 Wrangler Rio Grande - 2.5L 5-speed - 2" Black Diamond Lift - Carr Rota Lightbar - 4x PIAA Daylighters - Cragar 397 Series Soft 8's - 31" BFG A/T's - 4.0L Throttle Body - 3" Custom Intake w/ Speed by Spectre Conical Filter

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