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JeepEV: Electric Vehicle Conversion Project
4x4 Modification

« July 12th, 2003 »


     The earlier model year Jeep Cherokees and Wranglers were somewhat cursed by a vacuum actuated front axle four wheel drive system. These systems use a manual hand-lever to shift the transfer case between two and four wheel drive output. The front axle, however, relies on a vacuum motor at the passenger-side axle shaft to connect/disconnect it for four wheel drive use. This vacuum system was very prone to failure due to vacuum leaks, broken lines, etc.. Plus the vacuum motor itself was prone to failure usually due to axle fluid seeping into it and ruining the diaphragms.

     My vacuum motor was long past its prime. It would often take forever to engage the front axle, and then would often get stuck engaged. I had two options, the first being to buy a very expensive manual cable conversion kit. Since I wasn't looking to spend over one hundred dollars for a kit, I opted for the second choice, making my own. So, like many Jeepers, I converted the existing vacuum shift motor myself. I first saw a good write-up about the process at: http://home.earthlink.net/~stevenschreiber/cabledis.htm and used it as a guide for converting my Cherokee's front axle vacuum motor to a manual cable actuated system.

     I did this not only because I wanted something more reliable, but also because I wanted to keep the number of vacuum-dependant devices on the Jeep to an absolute minimum. In an Electric Vehicle, the vacuum source is provided by an electric vacuum pump. Unlike an Internal Combustion Engine which creates a large volume of vacuum, most vacuum pumps used in EVs are small units which are mainly installed for the power brake booster. Thus, reducing the number of things which run off vacuum in the Jeep prevents my vacuum pump from having to run constantly, while ensuring that the power brake booster gets the volume of vacuum it needs to work most effectively.

     The first thing I did was remove the vacuum shift assembly from the front axle. I didn't need to remove any vacuum lines or components because I had already done so before I removed the Internal Combustion Engine. Once the vacuum shift motor was removed, I was able to pull the shift fork out of the axle housing for modifications. Before I did anything with the shift fork, I needed to take apart the vacuum motor and remove the diaphragm to prevent it from possibly jamming the motor up later. To do this, I pryed at the two halves of the vacuum motor until they separated. Once I had the diaphragms removed, I resealed the vacuum shift motor.

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The shift motor is removed from the axle. The rubber diaphrams
were completely oil soaked and ruined. No wonder it never worked right!

     The next step involved connecting a throttle/PTO style cable to the shift fork. To make this possible, I had to drill a hole in the side of the vacuum shift assembly housing, and another in the shift fork itself. This allowed a point of entry for the throttle cable at the housing, and a place to pass through and attach the cable to the shift fork. I installed a cable retainer in the new hole in the housing, and fed one end of the cable through it. Before attaching the cable to the shift fork, I installed a spring between the shift fork and the housing. I then passed the throttle cable through the spring before attaching it to the shift fork. I then reinstalled the vacuum shift assembly housing on the axle.

     The throttle cable I bought has a pull knob on the other end of it. I mounted the pull knob in the floor of the Jeep next to the transfer-case and transmission shifters. Then I made sure and tied up the cable along the frame underneath the Jeep to prevent it from interfering with moving parts.

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The throttle cable was attached to the shift arm and the return spring was installed.
In the process, the new black push/pull knob was mounted next to the transfercase lever


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