1991 toyota front end not engaging when put in 4x4
05-11-2014, 05:09 PM
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1991 toyota front end not engaging when put in 4x4
So I just rebuilt my engine and put it back in, and now about a week later I just tried to use my 4x4 and the light doesn't come on in the cab and the front end doesn't lock in. When I put it in 4low I feel the truck gear down and get in the mode, but it stays in 2wd. It seems like my t - case is working but my front end won't kick in! I checked my fuses and my 4x4 cable that plugs into my front axle and it's all fine. My 4x4 worked great before, what happened? Help would be greatly appreciated, and pictures to help that even more appreciated! 
05-11-2014, 05:34 PM
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Sounds like you miss-routed a vacuum line or two. The front axle engages via vacuum, which is applied from two vacuum switches on the right-hand fender above the wheel. Check and make sure those are getting vacuum, and are switching.
If you park the truck with the engine off and shift the t-case between 2wd and 4wd with the key on, you should hear a relay click under the dash. If you stick your head out the window (or have a friend listen), you should also hear the vacuum switches cycle up on the right front fender.
If those are cycling, start the engine and check for proper vacuum switching at the ports on the vacuum switches. Keep tracing lines and wires until you find where the signal is getting lost.
Here's a diagram of the ADD (Automatic Differential Disconnect) system, which is where your problem is likely located.
http://ether3al.physic.al/93fsm-mobi...26addcontr.pdf
If you park the truck with the engine off and shift the t-case between 2wd and 4wd with the key on, you should hear a relay click under the dash. If you stick your head out the window (or have a friend listen), you should also hear the vacuum switches cycle up on the right front fender.
If those are cycling, start the engine and check for proper vacuum switching at the ports on the vacuum switches. Keep tracing lines and wires until you find where the signal is getting lost.
Here's a diagram of the ADD (Automatic Differential Disconnect) system, which is where your problem is likely located.
http://ether3al.physic.al/93fsm-mobi...26addcontr.pdf
Last edited by RJR; 05-11-2014 at 05:40 PM.
05-11-2014, 05:52 PM
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Sounds like you miss-routed a vacuum line or two. The front axle engages via vacuum, which is applied from two vacuum switches on the right-hand fender above the wheel. Check and make sure those are getting vacuum, and are switching.
If you park the truck with the engine off and shift the t-case between 2wd and 4wd with the key on, you should hear a relay click under the dash. If you stick your head out the window (or have a friend listen), you should also hear the vacuum switches cycle up on the right front fender.
If those are cycling, start the engine and check for proper vacuum switching at the ports on the vacuum switches. Keep tracing lines and wires until you find where the signal is getting lost.
Here's a diagram of the ADD (Automatic Differential Disconnect) system, which is where your problem is likely located.
http://ether3al.physic.al/93fsm-mobi...26addcontr.pdf
If you park the truck with the engine off and shift the t-case between 2wd and 4wd with the key on, you should hear a relay click under the dash. If you stick your head out the window (or have a friend listen), you should also hear the vacuum switches cycle up on the right front fender.
If those are cycling, start the engine and check for proper vacuum switching at the ports on the vacuum switches. Keep tracing lines and wires until you find where the signal is getting lost.
Here's a diagram of the ADD (Automatic Differential Disconnect) system, which is where your problem is likely located.
http://ether3al.physic.al/93fsm-mobi...26addcontr.pdf
05-11-2014, 09:07 PM
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