ZJ Ring and Pinion swap

ZJ Ring and Pinion swap

I have been working on a 96 Jeep Grand Cherokee (ZJ) that came to me with 3.73 gears in the front and 3.55 gears in the rear. Obviously this did not work well with the Quadratrac full time four wheel system. So the previous owner had removed the rear drive shaft and was running around with front wheel drive. This solution worked surprisingly well except for the trouble with the back of the transfer case not being sealed.

The owner obtained a set of 3.73 gears for me to install. I decided to try to make use of as many of the existing parts as possible to reduce the overall expense. I also decided to use this as an experiment to learn how to change differential gears.

I was surprised to find that my automotive advisors: my dad and my millwright friend; had never set up differential gears before. They both knew the concept, but had never actually performed the task.

I began by removing the wheels and the brake calipers. Next I pulled the cover and drained the fluid. I pulled the pin retainer screw and then slid the center pin half way out so that I could release the C clips without dropping the spider gears. I slipped the axles out of the way. I then reinstalled the center pin and the retaining screw. Again this was to hold the spider gears in place.

I noticed that the bearing caps were marked with an H on each one. However one side had the H on its side. There was a matching mark on the housing flange to help keep track of the caps.

Since I do not have a housing spreader, I chose to use the “pry it out” method. It took a lot of force on the pry bar to get it to move the first time. And even when it was “loose” it still would not come out. I finally noticed that one of the axle shafts was not pulled far enough out and was catching the edge of the bearing. Once I slid it out, the carrier came out.

I was very careful to make sure the shims on each side stayed with the bearing races. I laid every thing out to make sure I kept them in order.

I put the carrier in the vise and loosened the retaining bolts. I then carefully tapped the old ring off. The new ring did not slip on at first. I decided to heat it to make it slip on easier. I also realized that if the bolts did not line up, I would have to hammer the new ring back off so I made some alignment pins to go in a couple of the bolt holes. I threaded some 5/16 rod to match the bolts holes in the ring and screwed them in two opposite holes.

As I set the ring up to test the alignment, it slipped right into place and stayed there. I guess the heat of handling it expanded it just enough to let it slip on. I put the bolts in and torqued them to 55 foot pounds.

I then removed the yoke and knocked the pinion out. The new pinion had a new bearing on it already so I decided to give it a try. I put the pinion in and crushed the sleeve to get 15 inch pounds preload on the pinion bearing. I then reinstalled the carrier using the original shims.

I put some Prussian Blue on the gears and rotated it around. I could not see any kind of pattern what so ever. I tried several times. I had read where there needed to be some resistance on the gears to make a mark, so I tried driving a wooden wedge between the gear the housing. I still did not see a mark in the dye.

I tried several more times and I noticed that the pinion was transferring marks to the unpainted teeth. The marks appeared on the toe of the gear. According to the manual, this meant that I needed to remove a shim form under the pinion. The trouble was, there were no shims under the pinion.

I checked the backlash using a dial indicator. The spec is 7 to 9 and I was reading not quite 10 so I decided it was close enough.

I waited until the next day and buttoned it all back up and took it for a test drive. As you might expect, it whined horribly. It only took a quarter of a mile test drive to realize this was not going to work.

I set the Jeep aside for a few days as more pressing projects to the service bay. Later, I put it back to try again.

I pulled the cover and axles again. I cleaned up the gear with brake cleaner and it was obvious where the pattern was. It was not on the toe as I had thought; it was very much on the heel. Well that was good news as I knew I could add pinion shims easily enough.

I pulled the carrier and the pinion shaft. I pressed the bearing off using my Dad’s press and my Harbor Freight bearing separator. I looked at the old pinion and saw there was a shim under the bearing. I tried pressing that bearing off but it refused to budge. I looked around and found another old pinion that had a shim on it.

I had no idea if it was the right one our not, but since it was the only one I had, I decided to give it a try. I slipped it on the pinion, heated the bearing and dropped it n place.

I slipped it in place with out the crush sleeve to check the gear pattern. I snugged up the yoke to take out the end play and then reinstalled the carrier.

I found that mixing the Prussian Blue with some of the gear oil and brake cleaner, I could make a mixture that smeared easily. I also found that by working the pinion back and forth across the due I could wipe off a spot. The clean spot looked just like the desired wipe in the manual so I decided to call it good.

I pulled the carrier and pinion again and installed the crush sleeve. I then set the preload to 15 inch pounds again. I put the carrier back in and tightened the bearing caps to 57 foot pounds.

I put the C clips back on the axles and glued the cover back with Black RTV. My tube of RTV is getting hard so I had to punch a new hole in the side of the tube to get to the good stuff.

I let it dry overnight before refilling it with oil. I am looking forward to test driving to see if it worked this time.

UPDATE: I filled it with gear oil and mounted the wheels and tires. A quick drive up the road let me know the rear is nice and quiet now.

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