rear diff thingy on mj

i was going to change my diff fluid in my new 89 mj. and i ran into somthing attached to the diff. what is this this thing and do i need it?

Proportioning valve for the brakes? Just my guess.

Yeah I'd say that's a brake proportioning valve. Never saw them on anything except 80's vintage Chevy trucks (my Suburban has one). I'm surprised it isn't leaking brake fluid.

Confirmed -- it's the load-sensing proportioning valve. Empty, the back of an MJ is lighter than an XJ so this valve reduuces the amount of braking to alleviate locking the rear wheels first and spinning out. As load drops the bed, the arm pushes up on the valve to allow more pressure to the rear brakes.

Eagle said:

Confirmed -- it's the load-sensing proportioning valve. Empty, the back of an MJ is lighter than an XJ so this valve reduuces the amount of braking to alleviate locking the rear wheels first and spinning out. As load drops the bed, the arm pushes up on the valve to allow more pressure to the rear brakes.

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Good idea.

That thing has some goofy sized fittings on it...beware if you ever have to replace a hard line going to it. I ended up eliminating it from the circuit altogether (it helped the entire brake system).
Not suggesting that you do the same if yours isn't giving you a problem, but if you do pull it, you have to cap off the return line from it back to the prop valve under the MC.
Jeff

When the system works it's a great idea...when it dumps, unless you can still get the stock fittings from AZ or the dealer, it's a royal pain.

Sarge

So are these on all MJ or is it a option??

All MJ's have 'em. Like Sarge said, if yours isn't leaking, let a sleeping dog lie. There's actually a cleaning/adjusment procedure for them, but I wouldn't fool with it. I didn't notice that it did anything one way or the other (the little arm fell off mine once and I never realized it until I was under it one day), but I did notice a definate difference with it gone. I haven't had mine in slippery conditions since I've pulled it, so I don't know how it's gonna react with full pressure going to the rears all the time. I know they lock solid on gravel along with the fronts, but have yet to experience rain or snow at speed.
Jeff