Eagle

Your procedure can't bleed air out of the bypass passage in the distribution block itself. The factory established the bleeding procedure in the FSM for a reason.

The rear height-sensing valve in my '88 MJ exploded in a panic stop, so I had no choice -- I had to eliminate it. I plugged the bottom (bypass) outlet on the distribution block and ran a single line from the "nose" outlet directly into the flex hose at the rear axle. I do have a problem with the rear wheels wanting to lock up. Not bad on level ground and dry pavement, but a couple of miles from home there's a 4-way stop intersection that's a steep downhill in the direction I usually travel. I have to be very careful about braking there, because there's even less weight on the rear wheels. Disclaimer: I'm 73 years old. I grew up and started driving decades before proportioning valves were invented, so adapting to rear wheel lockup is an automatic response for me. For you young guys who haven't driven anything that doesn't have brake proportioning, this could be an issue. An option other than using an XJ proportioning valve and hoping it's an improvement would be to use a Wilwood adjustable proportioning valve. That would allow you to dial in the amount of rear braking you're comfortable with, and to increase it if you have to haul a heavy load.

I answered this once, but I don't think I understood the question correctly, so let me try again: Look at the first two photos I posted -- the ones with the valve bodies resting on the cane chair seat. Both the XJ and the MJ units are set up with a bypass circuit that provides full braking power to the rear in the event of the front brake circuit losing pressure. So from that perspective there is no advantage to using an XJ proportioning valve. The advantage to using an XJ proportioning valve is that it allows you to eliminate the rear height-sensing proportioning valve, while still retaining some proportioning. The problem is, as Gene already noted, is that the XJ unit has a pre-set proportioning ratio, and it won't change when the bed of your MJ is loaded down. Of course, it also doesn't change when the back of an XJ is loaded down. On the other hand, I think the XJ curb weight distribution empty has a larger percentage on the rear wheels than the MJ, so the XJ proportioning valve probably allows more braking force to the rear wheels than the MJ height sensing valve does when running empty. So maybe it's not a bad swap. You wrote that you removed the height sensing valve and plugged the outlet to it. Which outlet did you plug -- the one at the "nose" of the distribution block, or the one at the forward bottom (the one labeled emergency bypass in my photos)? The one you need to plug is the one on the bottom. That way, the brake fluid still follows the normal path through the distribution block but, instead of going to the height-sensing valve, it goes directly to the rear wheels -- no proportioning. Which seems to be the answer to your question. If you are concerned about being able to have full power to the rear brakes if the fronts fail, if you eliminate the height sensing valve you will ALWAYS have full braking power to the rears. And that might become a problem if you mostly or always drive with the bed empty, because you might find the rear wheels locking up prematurely. The whole point of a proportioning valve is to prevent premature rear wheel lockup. So what I would say is, try driving it with the MJ distribution block and no height-sensing valve. If you find you're getting a lot of rear wheel lockup, then try an XJ proportioning valve.

I remember Frank well. Very sharp guy. Passed many emails back and forth with him picking his brain when I had questions. He always had the time and the answers. Is he still around? Profile updated August of 2016: http://www.zoominfo.com/p/Frank-Swygert/49380515

I have parts heaps -- an 87 4.0L auto, and an '89 4.0L auto. And I have the electrical manual and a soldering gun ... Frank Swygert, a retired AMC engineer who was active on the OLD old XJ bulletin board (the predecessor to NAXJA) had a 4.0L with AW4 running in an AMC Hornet. He controlled the tranny manually, with a 6-position rotary switch connected to the individual solenoids. Anything is possible.

Here's why just disconnecting the nut at the bypass outlet won't accomplish anything. The upper end of the bypass passgae falls between two O-rings on the shuttle plunger when it's in the normal position (centered, so the plunger on the brake warning switch is down). Unless the shuttle is tripped to slide to the right (in the photo), no brake fluid gets to it -- the flow in the normal rear brake circuit just goes past the end of the shuttle plunger and out the "nose" of the distribution block body. The only time brake fluid flows to/through that bypass circuit is when the front brakes fail, causing the shuttle plunger to be pushed to the right (again, as in the picture), which moves the last O-ring beyond the opening to the bypass passage and allows brake fluid to flow through that circuit. (See other photo, above.)

I have an '88 2.5L, 4-speed, 2WD Comanche. I'm undecided between replacing the 2.5L engine with a low-mileage rebuilt and a 5-speed, or converting to an AW4 with the engine job. I have the AW4 and a 2.5L bellhousing and torque converter. I don't have a 2.5L TCU. Is there a difference? Can I use a 4.0L TCU with a 2.5L?

You can do that. The XJ proportioning valve also has an internal by-pass (see the photo). The XJ proportioning valve won't be calibrated for an MJ, but it will work.

WRONG! Look at the photos. Unless you follow the factory procedure, the shuttle valve will not be tripped and there will be NO FLUID flowing through that fitting. Even if there were, if there's air in the line how is pushing fluid out of the distribution block at the start of the line going to bleed air out of the line? [Edit to add}You'll still have a good pedal if you do what you describe -- because you won't be doing anything to bleed the bypass circuit. Normal brakes will be good. BUT ... if there's air in the bypass line, if the front brakes fail you won't have rear brakes as your safety net, because air compresses and once the bypass circuit opens you'll be compressing the air in that line when you step on the pedal.

This is SOOOOO incorrect I don't even know where to begin. Brake proportioning in the MJ is handled by the rear height-sensing valve. That shuttle in the front distribution block is NOT in any way involved in proportioning. Look at the photo of the XJ proportioning valve. See the spring and plunger? That's what does the proportioning. The MJ doesn't have any of those parts. Look at the photo of the MJ distribution block in the normal position (the photo with the wicker background). See the O-rings on the shuttle plunger? See where the left-most O-ring is relative to the passage for the emergency circuit? In normal operation, that passage is completely shut off and isolated from the brake system. It only opens if the front brakes lose pressure and the shuttle moves to the right (in the photo orientation). The following is from the '88 MJ FSM:

Did you even look at the two photos? The horizontal shuttle valve in the front distribution block has nothing to do with brake proportioning. It serves two functions: First, when tripped in either direction it activates the brake system warning light (by pushing up the plunger in that plastic switch screwed into the top of the body). Second, when the front circuit fails, it opens up a bypass circuit to allow full brake force to the rear wheels. In the Cherokee unit, it does this my allowing fluid to bypass the proportioning plunger in the valve body. In the Comanche, it does it by routing brake fluid through that second line to the rear, which connects downstream of the height-sensing proportioning valve. If it didn't do that, all you would have if the front brakes failed would be the severely reduced rear brakes because the proportioning valve would be restricting the flow. The second line is not a return line nor is it a "signal" line. It's not a loop system -- it's a direct system with a bypass line that is actuated only when the front brake circuit loses pressure. I posted the bleeding procedure from the factory service manual in another thread just last night. I'll post an annotated photo of the MJ distribution block as soon as Photobucket comes back online so I can upload to it. [Edit: Photobucket is back up] MJ distribution block showing the shuttle valve in the tripped position, as it would be if the front brake circuit lost pressure. Compare with the photo above, which has the shuttle in the normal position.

I'm sure the above is intended to be helpful, but unfortunately it's completely wrong. The thing in the front that you refer to as a proportioning valve is not -- in the MJ that distribution block has NO proportioning function. The one in the XJ is a proportioning valve, but not in the MJ. I've posted photos of both units sliced in half to show the internals, and it's clear from the photos that the MJ unit has nothing that provides proportioning. The second line to the rear axle in the MJ is not a return line. Brake fluid doesn't circulate -- there's nothing to return, and nowhere to return to. The second line is an emergency bypass circuit. The primary line feeds the rear brakes through the height-sensing proportioning valve in the rear. Unless the truck is heavily loaded, this reduces the amount of braking to the rear wheels. If the front circuit fails and all you have is the rear, you want full power to the rear, so the second line is an emergency circuit that bypasses the rear proportioning valve and sends full power to the rear brakes. Here's the XJ proportioning valve: And here's the MJ distribution block: Your bleeding suggestion does not agree with the factory service manual, and will not bleed air out of the emergency line. Since air in the emergency line would result in not having functional rear brakes when you need them most (if the front brakes have failed), your suggestion is dangerous.

Agreed.

Similar procedure, but not a direct quote from the FSM. The FSM doesn't say anything about disconnecting the link rod at the rear axle.

I don't remember where. Here it is again: After bleeding the brake calipers and cylinders the rear brake by-pass line must also be bled. Open a front caliper bleed fitting and depress the brake pedal to the floor. This will shuttle the by-pass differential valve and allow fluid to flow through the by-pass line. The brake warning light on the instrument panel will illuminate when the ignition key is in the ON position. This signals the shuttling of the valve. Re-Bleed the rear brake cylinders with the front caliper bleed fitting open. After re-bleeding the rear brake cylinders the entire system must be bled again. Bleed the brake calipers and cylinders in the following sequence: 1st -- Right rear 2nd -- Left rear 3rd -- Right front 4th -- Left front

From FSM: After bleeding the brake calipers and cylinders the rear brake by-pass line must also be bled. Open a front caliper bleed fitting and depress the brake pedal to the floor. This will shuttle the by-pass differential valve and allow fluid to flow through the by-pass line. The brake warning light on the instrument panel will illuminate when the ignition key is in the ON position. This signals the shuttling of the valve. Re-Bleed the rear brake cylinders with the front caliper bleed fitting open. After re-bleeding the rear brake cylinders the entire system must be bled again. Bleed the brake calipers and cylinders in the following sequence: 1st -- Right rear 2nd -- Left rear 3rd -- Right front 4th -- Left front

We do. I transcribed it directly from the MJ FSM.

They do essentially zero prep work. And their paint is $#!&. I had my '88 Cherokee painted at MAACO because the original clearcoat was burning and peeling. The paint job looked ... "okay," maybe even "not bad" ... for about six months. After six months, the paint faded and went flat. And now there's rust popping through along edges where they sanded too energetically, and apparently didn't prime where they got to bare metal. I would never go there again. I think you can do better with a few spray cans.

The MJ never used a Dana 30 for the rear. The standard axle was a Dana 35. Are you sure that's not what you have?

Yes, the one on the lower block is the coolant temp sensor for the ECU.

Maybe, but I doubt you're really seeing an accurate check on your vehicle's odometer. Your vehicle's odometer records how far the vehicle actually travels -- a GPS only knows where the vehicle is relative to a set of satellites. Say you had a three-lane highway all to yourself for 100 miles. If you get in the center lane and drive, you'll go 100 miles. If you start in the right lane and drive the full distance weaving back and forth from the right lane to the left, you'll tavel more than 100 miles -- but a GPS will only know where you started and where you ended, so it'll show the same distance traveled. A GPS is okay (but not great) for checking a speedometer, but I don't think it's at all useful for verifying an odometer.

XJ springs and MJ springs are both 2-1/2" wide.

I often wonder the same thing. I owned a '99 WJ for a short while and the front VariLok worked well and was completely transparent. I think it would be a great match with a 242 transfer case, but I haven't had the money to try it.

Gotta luv thread necromancy. This thread is only five years old ... And in five years, the OP never came back to tell us if his buddy was able to diagnose and fix the problem.

Find some way to check the accuracy of your odometer before you do anything. It's not uncommon for manufacturers to set speedometers to read a couple of MPH faster than the actual speed, even if the odometer is correct. I have a 2000 Cherokee and I also have my late wife's 2000 Cherokee. Both are bone stock, and the speedometer on both reads 2 to 3 MPH fast when going past th0ose police radar trailers that display your speed as you approach. FWIW, I've looked at a lot of tire specs over the years, and all the 31-inch tires I've looked at are actually between 30.4 and 30.6 inches in diameter. If you want taller than that, you'll have to get 32s -- which will probably be around 31.5 inches.