What is this wart on my front axle? - JeepForum.com

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post #1 of 3 Old 06-24-2002, 12:37 AM Thread Starter
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2011 WK 
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What is this wart on my front axle?

When I pulled the front passenger tire while installing a lift kit I discovered a wart on the passenger side of the front axle. After looking in my chilton's manual I discover this is a "shift motor" and a 4x4 sensor. The shift motor has a couple of vacuum lines going to it and the 4x4 sensor has a couple of wires that controls the 4 wheel drive light (mine is always on ). The shift motor is suppose to couple the front axle when in four wheel drive. Now I'm confused

Doesn't the transfer case disable power to the front wheels when not in 4 wheel drive?

Isn't there a differential in the front axle that allows the wheels to turn at different speeds when cornering?

If so, then why is the axle on the passenger side split with a mechanical coupler used to connect it? It seems this is redundant?

2000 Jeep Grand Cherokee V8 Stock
93 YJ I6 hard top (really belongs to SPEED)
RE 4.5" Extreme Duty Lift, Advanced Adapters SYE, Tom Woods CV Drive Shaft, EA589 10" rims with 33" x 12.5" MT/Rs
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post #2 of 3 Old 06-24-2002, 07:23 AM
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That is indeed your vacuum shift motor. And it does couple the front axle when in 4wd. The deal is, instead of using hub lockouts, Jeep decided to go with hubs that are locked all the time and instead disconnect the passenger side axle shaft. So, your inner passenger shaft is actually two pieces, and what the vacuum motor does is slide a lockring to engage the two parts of the inner shaft.

You're right, the tcase does disengage power to the front axle. but because of the "always locked" hubs Jeep uses, if you didn't have the vacuum disconnect, your front driveshaft would spin all the time, even in 2wd.

FYI, the vacuum motor is often a cause for failure of 4wd on YJs. Some poeple replace it with a cable actuated motor, or defeat it all together by switching to 1-piece TJ inner shafts.


Robert Gorrell
1995.5 Isuzu Rodeo - SAS'd and SOA'd, sitting on 36x12.50 TSLs, D44 front, 14bff rear, locked by OX and Detriot, suspensions by Jeep and Chevy, high steer conversion, 5.13 gearing and Teralow in the case.
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post #3 of 3 Old 06-24-2002, 10:22 AM
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from what ive seen that front shaft spins all the time anyway but i think its just the drag.... if your 4wd light is always on it is most likely the sensor on top of the t-case... it has 4 vacuum lines on it, is about an inch in diameter and is a pain to replace if you have to lie under the jeep because of its position... they are around 14 bux to replace... thats what was wrong when my light wouldnt shut off... if you ever have to replace that actuator motor thats about 110 bux... i have done that too..

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