CJ7 Transmission/Differential Problems

CJ7 Transmission/Differential Problems


Missouri Ozarks
1978 Jeep CJ-7 with T150 trans, Ford 302 engine conversion, and a lot of repairs needed!
I have a 1978 CJ7 with quite a few problems. I got this neglected beater and want to get it going and roadworthy again.

Although it has other problems, I am looking to resolve two drivetrain issues first.

1. The first to be noticed was that the transmission grinds and pops out of gear in 3rd gear. This has a T-150 transmission if that helps. It goes into all gears just fine and stays in gear under load. As soon as I let off, it grinds and pops out. This is worse in 4WD and engine braking situations are sure to make it pop out. Again, it goes into all gears fine and doesn't grind or pop out in any other gear. It will stay in gear if I pull down hard on the lever.

I changed the fluids in both the transfer case and transmission with synchromesh fluid. I was told to use GL4 gear lube but you can't seem to find this anymore and was told to use this instead. I was told DO NOT use GL5 lube! The lube inside both was dirty and milky so it was overdue for sure. This change might have made things a little smoother but I don't know. It certainly isn't worse.

I also just got done replacing the engine in this thing. All the mounts, etc. were loose. One motor mount bolt was missing, the bell housing bolts were about to fall out, as were the transmission to bell housing bolts. Could all these loose parts have caused wear in the transmission? This Jeep came to me converted over to a Ford 302 (5.0L) that was about to blow, which it did. I replaced it with the same engine as this is what the thing was setup to use already. The new engine is working well. I was hoping that tightening up everything during the engine replacement would fix the trans issue but I guess not...

I have been told this is likely not synchronizers as it goes into gear so well and doesn't pop out under load. People have told me it might be an "anti-reverse" thrust mechanism which is common to automatics but I am not sure about manuals. They have told me this is a cheap part and would be easy as pie to fix. Do I even his this in my trans? I looked for references and couldn't find any. I have also been told that the main shafts, etc. have lots of wear and play from everything being so loose and out of alignment and that the cost to rebuild this would exceed the cost of upgrading to a 4 speed. If it is likely an easy fix, I would like to tear it apart and keep using the 3 speed. If it is not, I would like to start working on getting a 4 speed and keep driving the 3 speed in its current condition until a new trans is ready to drop in.

I was also given an NP435 transmission from a 1986 Ford F-150. This came from a 4WD but has the 2WD bolt pattern on the back. I understand there are conversions for this unit to make it work in a Jeep. Would you suggest going this route or looking for something ready to go into a Jeep. Again, it was given to me and I would plan to do a basic rebuild if converting to a new shaft/adapter plate for the Jeep. Also, some have suggested I get a T18 as this is better. Is it that much better considering I have a good NP435 transmission that was given to me?

2. My other problem is related to the differentials.... I decided to change the lube in both diffs right after the above mentioned fluid changes and used GL5 lube in these. The rear diff looks like it has been recently rebuilt and had decent looking fluid in it. I am not sure if it was the correct fluid as it looked like a heavy oil and didn't smell at all like gear lube. It smelled more like ATF than anything. Either way, it now has GL5 inside.

Both diffs have blown pinion seals. I popped them both out last night and plan to purchase new today. I made sure to unclog the vents but I couldn't locate the rear vent.... Where is it? I felt around for a fitting to connect a hose and couldn't find anything that resembles a vent on the rear diff. What am I missing? I don't want to blow a new rear seal by having a clogged (or missing???) vent so I want to be sure it is ok. I have unclogged the front vent but but have greater concerns.

The front diff is a different story.... When I changed the lube, I had to scoop out the rusty, milky, sludgy gear lube that was in the front diff. It was so nasty and the only way I could tell it was actually gear lube was the odor. Otherwise I would have thought it was mud! The gears actually look to be in pretty good condition and seem to be tight with no play but I have other concerns (see below). There are not chipped or pitted teeth.

I refilled with fresh GL5 lube and planned to change it out again pretty quickly to "rinse" out the rest of the sludge. I didn't realize I had a bad pinion seal until this time but it began leaking like crazy with fresh lube. So, I pulled the front cover and drained all fluid, figuring this would be my excuse to change the fluid once again. It came out with a more normal consistency this time but was just as rust colored as the first "sludge" I drained from the thing.

I then went ahead and decided to pull the pinion seal. The first thing I noticed was the pinion shaft had some play. The play is in two directions. 1. The shaft has some wobble which I assume might be part of the reason for the front seal blowing. 2. The shaft also has some in and out play along the axis of the shaft when you pull/push on the shaft.

I suspect that the pinion bearings are bad for sure and understand that the entire axle assembly must be pulled apart to get to these so I might as well do other work at the same time. There is also a lot of play in the outer axle U joints for steering so I suspect they are bad. I can rotate the pinion shaft about 1/5 turn and this slop is due to the U joints, not the gears in the diff.

I understand there are quite a few axle upgrade kits for the front end. If I have to tear this apart, what should I look for and what else should I replace besides bearings/seals? I was told you can get upgraded axle shafts and U joints. Also, I have been told by several that non-serviceable U joints are better as the channels for greasing them reduce strength. Anyone have arguements for or against this and can you suggest anything?

I was thinking about just putting a new pinion seal in this and driving it for a while while keeping an eye out for new leaks and checking the fluid level frequently. I know that the pinion seal is likely to go again but they are only like $10 and I would plan on a major rebuild/upgrade later. Plus, this would give it another "rinsing" with fresh gear lube. Is this a bad idea? Am I risking a catastrophic failure of any kind and will this possibly make anything more costly in the future? Also, is this a safety issue if I decide to take it on the road?

As you can see, this beater has some issues. There are others but I want to get these two resolved first.

Please give me advice on these.


The T-150, or any transmission, may be subject to popping out of gear for various reasons with various repairs. For example if the clutch wasn’t line up properly when installed it can cause the problem; that is why it is important to use a clutch line up tool. The only way to know for sure is to tear it all down and start again; a lot of work for a peak and a poke at resolving the problem. Next if the main shaft end play inside the transmission is off it could be that too. You may be able to check end play with the transmission still installed through the floor cover. As this jeep has a motor transplant the pilot bushing comes into question but you may have resolved that issue when replacing the engine. The motor mounts are also a potential culprit. If the mounts have too much “slop” the engine will move/twist side to side particularly under load and release causing the tranny to pop out of gear from the movement. Along the same lines the transmission mount should also be check for the same reasons. The one piece to the puzzle that seems to often be mission is the torque stud. The torque stud attaches to the skid plate then bridges the transmission mount and fastens to a plate that sits between the bottom of the transmission and the top of the mount. The stud has rubber grommets that allow for some movement/twist but limits just how much the motor/transmission combo can slap from side to side under load and release. There are a few more reasons listed in the book that I can’t remember off hand but my first bet would have been the mounts and torque stud. It sounds like you replaced the mounts when you did the fresh engine so the search continues………
Thanks for the information. I think that the clutch is fine. We used the clutch line up tool as suggested. Also, the new motor is correctly fastened to the mounts so I think that is also OK. The pilot bushing was also replaced. I don't know if the old one was bad but the part is so cheap that why not?

I didn't check end shaft play but will also try to look at this as I think I can get to it without tearing it down. I don't recall anything unusual but who knows???

The transmission mount is a good idea and I plan to look into this as soon as the rain here lets up. This is the first thing I am going to actually check. Considering that most of the bolts were about to fall out of this thing, this is a good idea. Could the loose bolts have caused excessive wear on any parts?

Also, why would this only happen in 3rd gear? It also seems worse in 4WD.


I may have fixed the transmission but won't know until I get the driveline issues fixed. The mounting stud was quite loose and the bushings were free to move by hand. Anyway, I cranked this down and will see how it does once my pinion yoke comes in.



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