Just bought this truck yesterday (short bed regular cab 4.2L AT w/floor shift for 4WD, has H9 limited slip differential, 102,000 miles). Noticed a little thump from the rear when braking or taking off (little test drive around town), cheap enough so didn't worry, will just fix whatever needs it. Later, I got hard thump when turning right, then noticed at freeway speed the "LOW ENGAGED" dash light flickers, then once in a while (hitting a bump or decelerating) there's a click like a solenoid engaging comes from the dash, the "4X4" dash light comes on, and lots of noise comes from underneath (can't really tell if it's front or rear, just something in the drive train). I read about limited slip, friction modifier additives, etc., so today I changed ALL the fluids, took it out to some dirt and drove it around in figure eights in 4H, 4L, etc. It's been sitting a long time so I figured it wouldn't hurt to try & free things up. So, what I see now happens consistently is that in 2H, if I brake fairly hard or turn right sharply, I get the click, the 4X4 dash light, and the drive train clatter. When I turn sharply to the left, or accelerate hard, the light goes off, sound clears up. If I drive in tight circles, turning either left or right, forward or reverse, everything's smooth unless I rock it enough (or hit a bump) to cause that click/4X4 light deal - while that's on, there's the big thump periodically again - rock it the other way, click/light goes off, all is smooth again.

Sorry if this has been answered, looked quite a bit before posting (mostly Googling, lot of hits on this site but not this specific set of symptoms). Any advice or suggestions will be greatly appreciated . . .

 

If you have the electronic 4x4, (push button), then check the wiring and the switch on the dash. Sounds like something is moving and causing it to close the circuit to turn on the 4x4. Could be loose wiring, something else loose crossing the terminals of the switch. Could be just a bad switch. But check the wiring first. If that checks out, check the 4x4 motor on the t-case. Make sure it isn't engaging with the movement you described. pull it out and move it around with the ignition on. See if the movement is engaging it.

 

Or if it's a manual shift 4X4 check the lockout (not sure why Ford calls it that) switch on the front of the transfer case. It is sorta above the front driveshaft. You might try unplugging it and driving it to see what happens.

 

Thanks to both you guys, yes it is a manual shift which is why I was confused searching for an answer, kept seeing references to interior switches that I don't have. I'm heading straight for that lockout box, will let you know what I find.

 

So, we unplugged the lockout switch per your suggestion Dave, no change in any symptoms. I say no change, except that I now realize we don't have 4WD (checked by nailing the accelerator in soft dirt, no scratch marks on front wheels in either 4H or 4L). Should've done that at the dealer but no dirt for miles - (my foreman says on pavement you check by flooring it with your foot on the brake, says brake won't hold truck if 4WD is working - you guys agree with that?). Anyway, we have the noise and thump when the 4X4 light's on with the manual shifter in 2H, no noise or thump in 4H or 4L (though with the gizmo unplugged we had no dash light in 4L, only 4H . . .). My guy says the thump is coming from the front, probably the other racket too. Kind of strange, no? If everything's mechanical with the manual shifter, we're thinking the 4WD sensor in the transmission case is just a prox switch that sees some arm in position, turns on the relay for the dash light. So, even though it seems like the problem is some electrical actuator operating at the wrong time, maybe it's a broken piece of linkage flopping around in there . . . doesn't happen right away, have to be driving for a little while - first the "LOW ENGAGED" light starts to flicker, then comes on steady, then finally the "4X4" starts to click on and off, accompanied by the racket or thumping starting and stopping.

I have a 15 day mechanical warranty from the dealer but he's 300 miles away, would like to have some idea what it's going to take to fix it before I call him to negotiate repair or maybe return/refund. Thanks again for thinking about my problem . . .

 

An easy way to tell if it's actually in 4x4 is just to slowly turn tight circles on pavement . If it's in 4x4 you'll feel it binding.
The lockout switch on the front of the transfer case tells the GEM (Generic Electronic Module) when the transfer case is in 4x4 so it knows to send a signal to send vacuum to the front axle center disconnect. On the front center of the front diff you'll see a plastic cover over a vacuum servo. The vacuum servo moves a shift fork to lock the right side axle to the differential. The shift fork and collar that it moves have been known to have problems. You should be able to manually move the shift lever at the servo and feel if the axle disconnect is working by turning the axles.
If the lockout switch is bad the front axle disconnect wont lock and you'll have no 4x4. The switches get moisture in them and that ruins them. If that's the case you usually can just replace the switch and change the oil in the transfer case.