Rear diff wouldn't lock

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Dec 22, 2003
Messing around on some muddy roads today, and figured I would try out the different combinations of locked/unlocked diffs. '99 with center and rear diff. Center diff locks when pressing button in 4H (only did from a stop, can you do this while moving?) or automatically in 4L. Try rear, when in 4H just flashes never stays lit, tried several times. Go into 4L and first 10 times flashes only, then once it actually stays on. Try to then disenage and re-engage, never get rear to lock again. Muddy road, going straight. Speed range from crawl to 20Mph. Is there a sequence I need to follow to get rear locked everytime? SHould rear lock only in 4L?
Rear should only lock in 4L.
You more than likely will need wheel spin or drive something other than a straight line to engage.
Yes the rear diff is hard to lock unless one wheel is spinning or you turn sharp and go back and forth till it locks
Mine is a 99 100. Rear locks ONLY in 4L on the 99 100's. Not sure about the rest of the 100's. It will flash only if in 4H.
Can center diff be locked while moving? When is appropriate to use the center diff, if not offroad? Will try to swerve and get rear to lock. Really muddy road so should have spun tires.
> When is appropriate to use the center diff, if not offroad?

When you bust a drive shaft while driving off-road. W/o a center diff-lock you are stuck.

My personal opinions on any locking of differentials is to stop the vehicle. I can look in my owners manual for you tonight if you like.

Use the center lock in any case where either your front or rear wheels (any of them) are spinning e.g. snow. If you are locked in center and you still have a wheel spinning at the rear, you can then lock the rear. But you have to be in 4L first prior to locking the rear. Not a good idea to lock the rear if you are traversing a muddy/slick slope at an angle i.e. not straight up/down. Your rear has a tendency to slip out from under you since both wheels must turn therefore reducing friction to the surface. Our trucks do not have the option of locking the front (from factory). In my case, snow has been the only case to lock the center if not off road.

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