I Had My Jeep Towed For The Dumbest Reason

Folks, it’s time that I own up to an embarrassing misdiagnosis — one that led me to call a tow truck to have my 1991 Jeep Wrangler towed all the way home as my girlfriend (whom we’ll call Elise) and I rode in the truck’s cab. Yesterday, after months assuming the problem was a blow front U-joint, I asked Elise to jump behind the wheel of the broken Jeep and gently roll the vehicle backwards as I looked underneath trying to determine what was making that loud “clunk.” I then saw the issue, and boy did I feel dumb.

You’d think someone who has owned over a dozen old Jeeps, who helped engineer the current-generation Jeep Wrangler, and who has spent the last 15 years rebuilding engines, transmissions, transfer cases, driveshafts, and just generally wrenching like crazy would be able to figure out the cause of a banging noise under an old YJ. But nope, I screwed this one up.

It all happened back in January, when I took my 1991 Jeep Wrangler YJ off-roading with a friend, who brought along his lifted TJ. I was feeling fairly confident that day, so I took the Jeep up an absurdly steep grade that, from the bottom, looked impossible.

I would have pulled it off, too, were it not for the loose sand towards the top of the grade, which my Jeep sunk down into. I tried over and over, turning the front wheels back and forth to get grip on some new ground, backing up to gain momentum, applying more throttle, etc. — but every time, I just sank and spun up those 235 75R15 all-terrain Walmart tires. Here’s one short clip, which doesn’t do the grade justice, because it was gnarly:

I spent about 10 minutes on that grade, but I just ran out of grip, causing the Jeep to sink, at which point I ran out of ground clearance. After hammering on this Jeep trying to get it up the grade, I drove back onto the main trail and began hearing a clunking noise under the Jeep. I took a quick peak, but ignored it; I’d solve that later, I figured.

Fast forward a few hours, and after an incredible off-road trip, Griffin, owner of the TJ, and I hit the road and headed back home. I still heard the clunk, but it seemed mostly fine. Then, while on the highway going 60, things escalated. “CLUNK CLUNK CLUNK CLUNK CLUNK!!”

Loud and in rapid succession, these clunks were alarming and impossible to ignore. I pulled over onto the shoulder and looked under the Jeep; I didn’t see anything obvious.

I continued driving, cringing as whatever storm was brewing below my butt, thundered loudly into the cabin. I took the next exit, and came to a stop in a Costco parking lot. It was clutch, really, because Elise and I were hungry. We wolfed down a hotdog and slice of pizza, and then I returned to the Jeep and lifted the rear axle using the underhood factory floor jack:

To me, the noise sounded like it was coming from the drivetrain, and given that, in two-wheel drive, the front driveshaft doesn’t spin due to the YJ’s vacuum-powered front axle disconnect, I figured I’d turn the rear wheels and listen. I didn’t really hear much, but I did notice lots of end-play in those rear axle shafts. Hmm.

I jacked the front wheels, and I heard only a slight noise from the front left wheel — that’s gotta be it! A closer inspection showed that, indeed, my driver’s front axle u-joint was indeed toast, and was banging around a bit:

The weird thing, I thought, was that I’d driven a $600 Jeep XJ for an entire winter with a much worse front U-joint, and though it banged around, it wasn’t nearly as violent as the noise I had just heard under my YJ. I hopped back into the Jeep and drove a bit more — the noise sounded like it was coming from under my seat or towards the rear! Plus, it seemed to change based on the ride height, getting worse when hitting bumps.

“This has to be a sprung part of the vehicle,” I thought to myself. The front axle U-joint wasn’t sprung, so when the suspension went into jounce/rebound, it didn’t actually change the position of that component. Why would the sound change?

I checked the front driveshaft; it seemed fine. I checked the rear driveshaft — nothing looked or felt out of the ordinary. Damn.

I convinced myself that the rear Dana 35 was the issue, though not only is it unsprung, but normally an axle shaft failure would result in the shafts actually pulling out of the housing, taking the drums and wheels with them. This wasn’t happening with my YJ.

Anyway, I wasn’t sure, and the noise while driving was extremely disconcerting, so I called up AAA, and had the YJ towed about 45 minute to Van Nuys, with Elise and I hanging out with the jolly tow truck driver. It was a fun experience for Elise and me, actually; the cab was huge and comfortable, and it was nice talking with the gentleman about his job and his life in LA.

Anyway, fast forward to yesterday when I heard the noise quite prominently as I backed out of Elise’s driveway. I called her and asked if she’d come outside and hop behind the wheel as I looked underneath. She did, and that’s when I noticed this:

I saw the driveshaft close against the driveshaft speed sensor, and when the “nub” on the driveshaft spun around, it rammed right into the sensor and made a loud “bang!” that reverberated through the Jeep’s frame.

I simply grabbed the bracket upon which the sensor is mounted and bent it back. The problem was now solved.

How the heck had I missed this? Well, I think that, when I looked under the Jeep in the Costco parking lot, the sensor wasn’t actually touching the driveshaft, so I ignored it. Only when the nub spun around did that nub hit, and only when I hit a bump did the sensor make contact with the body of the driveshaft, leaving rub marks as shown above.

It was the simplest, easiest-to-fix problem ever. I had my Jeep towed for no reason. D’oh! I guess we all screw up every now and then!