91 F150 4x4 problems

Relays shot?

Could be a bad actuator too because there is a feed back loop in the system.

----------------- The SnoMan

Where is the relay located?

Where is this actuator located?

I would check the actuator on Tcase under truck because they as a rule do not last the life of the truck and wiill need to be changed at least once in its lifetime. Given trucks age, that would be first thing to check after you have confirmed that the fuse for it is good.

----------------- The SnoMan

Thanks for the ideas. I am new to 4 wheel drives, but a fairly decent shade tree mechanic. I will be under the truck today and will let you know. Do you know of any other forums that may be of help or is this the best place to ask questions? I have been successful in the past with this site. I also have a Ford Windstar that this site helped me to perform repairs.

You might try

If you have a volt meter, check for voltage at actuatior when 4x4 is selected.

----------------- The SnoMan

If it was an Explorer I'd say it's almost certainly the servo motor on the transfer case. I imagine it's pretty much the same on the F150. Sometimes it will become unstuck if you tap it slightly with a small hammer while someone is pushing the 4X4 button. On the Explorers of this era simply disassembling the motor and gears, cleaning it real good, and relubing will usually take care of the problem.

I agree with SnoMan's link to the Ford Truck Forum. I don't much care for their search engine though. And it's real fussy about having animations and ads turned on.