XJ trans tunnel cracks in metal?
 

New day new random crap the Jeep throws at me! Today I pulled out the rear seats to install an overland box system in the rear and I pulled back the carpet because I noticed it was a little wet and first I noticed a hole that was open and then I looked over and found 2 cracks running along the top of the trans tunnel in the rear. How do you even fix this/is it fixable? Also what are the implications of this happening, Im not sure if I've ever seen this in an XJ and kinda pissed off that I missed it when buying this jeep as I haven't done anything that should have done this in the time i've owned it.

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The rear shock mount is plug welded to the floor back there. I assume that can add some stress to that area, from the shocks pushing and pulling, causing those stress cracks. Or it could be from body flex considering it's a unibody. Also that latch where the rear seat buckles down to; years of slamming it down and vertical forces could be the cause for that stress crack. I think it's only 20 gauge metal; that's thin, not much strength.

I had the same cracks on my 2000 in just about the same areas; latch area for sure. I didn't do it at the time but it's a good idea to drill the ends of the crack with a 1/8 drill bit; nothing smaller but nothing too big is necessary. Drilling the ends will prevent the crack from trying to spread further.
I just body hammered the area down, so the cracks rested side by side then welded out the cracks from inside the cab. Grinded the welds, painted, and from underneath the body I seam sealed where the welds were. Let the seam sealer tack up a bit then touched it up with some undercoating. Then installed sound deadener on the entire floor, which was my initial project.....but saw those cracks and couldn't let that fly.

Does anyone know if this is due to fatigue from the shock absorbers or from flex during offroad?

Rear shocks don't actually attach to the floor, they attach to that rear crossmember running between the unibody rails and yea its still unibody soo its tied to the floor but id argue the front leaf spring mounts have more force applied near that spot/area over the rear shocks.

How does the rest of the jeep look? As far as use and fatigue to be considered?

Could something have been setting there under the seat over time and caused it like an improperly stored factory jack? Perhaps someone stuck something under the seat and folks sat back there not thinking anything of it?

Mine cracked similarly I wheel it, on 33 inch tires, rock crawl. I treat her well, dont hammer the crap out of it, but I noted two cracks on tunnel under the rear seat after a tough rock trip.

I weld so I used my MIG welder. Welded them up and they have held so far after several more trips!

I think it happened on a particularly difficult spot that I had to eventually back off and retry with a different approach , the rear diff hung up on a rock, my spotter and me didnt see it, I gave it more go , and the front wheels had grip, but the rear were off the ground and the rear diff was not going forward any do to the rock. I think that may have done it. By the way I did have center section frame rail stiffeners installed.

If you have an early XJ with the early style parking brake cable "L" brackets consider welding them along their edges to the floor at the same time. Factory uses spot welds, and these can tend to tear or bend the floor where the "L" bracket is mounted if you pull the brakes hard (which you need to do with large diameter tires!!!) I welded these parking brake brackets at the time I welded up the cracks in the floor. Might as well, they are right close to each other.

You really need to left the carpet and padding up from the inside before welding, that was actually the most annoying and time consuming part of the job. If you dont, you will get a fire going, the underlayment is very flammable to red hot metal.

I have been told that these cracks are somewhat common on XJ's

Now if you dont weld, or cant get around to it soon, I suggest you drill maybe a 1/4 inch diameter hole just PAST each end of each crack. The hole with its nice large diameter radius will resist the crack from growing. The crack ends are a sharp point on the molecular grain scale of the metal. That sharp point at the end of the crack tends to concentrate the stress to allow the crack to keep growing under load. But if you drill these holes known as Crack Arrestors, the large radius of the hole helps spread the stress out, thus reducing the chances the crack will keep growing.



To keep water ingress at bay you can put some caulking along the crack and the crack arrestor holes. Then monitor the cracks to assure they don't keep growing, if they do, then your fix is to get them welded up. Use a good urethane caulk, avoid silicone, as it can be a pain to remove later, and many silicones give off Acetic acid during cure, which can cause rust on the steel.

Good Luck!!!

My Cracks view from inside.... https://www.cherokeeforum.com/f2/cra...r-seat-267206/

Interesting that it happened with center section frame stiffeners installed.

Yeah, but upon close examine, the cracks are near the rear end of the center frame stiffeners. Those stiffeners are forward of the forward most leaf spring attachment. I suppose rear stiffeners are in order, but I need to do the front stiffeners next, already bought them. I need to do the front to beef up my front bumper mounts, (Got a winch) and so I can weld my trackbar drop bracket to the frame, as the bolts do come loose during wheeling. Needs to be welded to a reinforced frame. I am also looking at a regear project soon. Have 4.11 gears, will likely change to 4.88. Me I have been running 33 inch tires.

I know I would have seen these cracks during times of me under the jeep. I am under the jeep after every trip to blast off dirt and inspect. I am pretty sure they appeared right after that tough spot when rock crawling, center stiffeners installed for some time by then. But since they have been welded, they have not reoccurred so far, despite more wheeling trips. I will continue my inspections to assure I dont get cracks growing again. I did watch them grow once they started, I marked their ends and over use the cracks were growing, so I drilled some crack arrestor holes until I had the time to weld. The holes worked great, no more crack growth!!!

This is the insidious nature of crack growth, once a crack starts, do to that sharp pointed crack end, stress is concentrated at that tip of the crack and the crack will keep growing under less stress than it takes to start a new crack. That is why crack arresting holes should be drilled just past the ends of the cracks, this blunts the crack tip and can greatly reduce the chances the crack will keep going. The holes MUST be drilled past the end of the crack. Mindful that the crack can microscopically extend past the crack that you can see with the naked eye, so make sure the hole is drilled PAST the crack tip. If the microscopic crack is past through to the other side of the hole, then the crack arrestor hole will NOT work, as that sharp crack tip is still they past the hole.

Cracks are a thing with me, I took a course in Fracture Dynamics in school days, It really interested me.

Other than the rear floorpan above the trunk being slightly caved in nothing else I saw in terms of use/fatigue

My guess would be flex as people have stated there is a crossmember for the shocks

I've been fascinated for a while about whether unibody rail stiffeners actually reduce body flex. I can see them helping with preventing unibody rail cracking at high stress areas like the steering assembly and also reinforcing the rails, but it doesn't seem to me that they would help a bunch against body flex. For the body cracking issues, it feels to me like some kind of crossmember assembly linking high load attachment points would be more effective, e.g., having a crossmember in the underseat area, especially if it links to the leaf spring front mounting points.

I have considered a rear cross member , but this issue I run into is clearing the rear drive shaft. Even the catalytic converter will foul a cross member that is near but right behind the factory cross member

I have the seats out so we are considering making a square tube going across inside the cab, but I might also just not do that and install frame stiffeners later as I'm not really planning on doing any hardcore wheeling this minute.

Thats the thought, though my trans crossmember is much wider since it's an upgraded long arm lift, which is why its a little strange that its flexing like that enough to crack it with all the reinforcement. Plus the front and rear bumpers being there with alot of frame tie in you'd think that'd all really help keep the frame rails from twisting.

The unibody material is likely cold stamped and has high residual stresses that cause more rapid fatigue. Warming that area to 500-600C for about 30 minutes would relax the peak residual stresses. Probably would be easier to weld a crack than it would be to try to heat treat that area.

Out of curiosity what does cold stamped mean? I welded the cracks shut so that isn't an issue anymore but still just curious what that means.