I have a 1999 F150, auto transmission with manual 4x4 shifter. When I put the manual selector in the 4H position the 4X4 dash light goes on but the 4X4 doesn't engage. There is no sound. Strangely, 4L does work. Any ideas about a possible cause would be much appreciated.

 

sounds like the front axle disconnect. check it for vacuum loss, and its supply lines.
when u say 4L works, you mean it engages low range, but do you also mean the front diff pulls?

 

Raise and support the front wheels off the ground. Truck in park Engine running engage 4Hi. get under the truck and try to turn the front driveshaft (the one between the t-case and the front differential). It should not turn. Engage 4Lo and try turning the front drive shaft. Again is should not turn. Place the t-case back in 2Hi. Try turning the front Drive shaft. It should turn.

Post your results. If your truck passes all these tests you have a front axle engagement problem.

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