91 Toyota 4x4 truck won't lock in the auto locking front differential

've got a 1991 Toyota 4x4 truck with a 3.0 V-6 and auto locking front differential that is supposed to lock in when the transfer case is put into four wheel drive. The transfer case shifts into four wheel drive but I do not get the 4x4 light on the dash and the differential does not lock in. I've checked the vacuum hoses and the wiring and everything appears to be connected. Has anybody got any tips on how to go about troubleshooting this?

Reply to
Handyman
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See if this helps--

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folks have permanently locked the ADD and installed manual orautomatic hubs because of the complexity of the ADD system.

Reply to
Anyolmouse

Here is a PDF file on the ADD Control System

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whichis on this page in case the PDF file doesn't load.http://www.ncttora.com/fsm/1993/suspensionandaxle.html There wasn'tmuch change between the '91 and '93 model years so the info should begood.

Reply to
Anyolmouse

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