My Truck Has A Dangerous Speedometer Leak And Yes You Read That Right – Gizmodo Australia


I just drove nearly 1,127 km in my 1985 Jeep J10 pickup, and encountered a failure so absurd, I’m still not sure I believe it: My speedometer is leaking buckets of oil onto my feet as I drive. While this may not seem like a big concern, it could actually be quite dangerous, which is why I will mend this seemingly impossible fault as soon as possible.

As the curator of a collection of old, cheap vehicles, I’ve come to expect certain mechanical failures. Bad wheel bearings, worn suspension parts, leaky transfer cases, worn out brake parts, compromised cooling system components — it’s all par for the course. But sometimes a car fails in a way I could never have anticipated. That’s what happened to my Jeep J10.

Shortly after rebuilding and installing its four-speed manual transmission, I noticed that the truck’s speedometer began bouncing. It seemed to become more erratic each day until, eventually, it reached a point where the gauge read over 161 km/h when the vehicle was slowly crawling through a parking lot. The speedometer is useless at this point and, while that would be a much bigger deal if the vehicle weren’t so overweight and underpowered, it’s still not great.

But not knowing how fast I’m driving isn’t even remotely my biggest problem.

Around the same time that the speedometer started acting up, I noticed dark fluid dripping from the base of the dash. I’ve been living with it for a few months now, allowing the oil to saturate my shoes and pants legs, figuring that whatever oil was in the speedometer would eventually run out and I’d just pull the gauge and rebuild the clearly broken speedo later. But the oil never stopped gushing. It was a seemingly endless geyser that, during my trip to North Carolina, became a significant problem.

On Monday, after pulling off the highway and driving slowly around truck stops on my way from Michigan to North Carolina, the amount of oil flowing from the bottom of the dash became too absurd to ignore. I recall looking down into the the footwell and seeing a continuous stream of oil dribbling off the pedals. “Where the hell is all this oil coming from?” I wondered, convinced that any lubrication in the speedometer itself should have been depleted by now.

I wasn’t sure what was going on, but what I was certain of was that those oil-saturated pedals were starting to become downright dangerous. On a few occasions during my trip, I found my left foot slipping off the clutch pedal as I tried downshifting to decelerate at a light.

The brake pedal, too, was so slick that getting a nice, firm press sometimes required extra focus. My shoes, the soles of which I was regularly dipping into oil that was rapidly collecting on the floor mat below, simply couldn’t get good traction on the rubber pedal pads, on which oil from behind the dash was dripping. Watch the video above to see my coworker, Jason, drive my J10 and express utter bafflement at the ridiculous safety hazard.

If you think I’m joking about the oil pooling on the floor mat, just look at this mess:

Clearly, based on the sheer volume, this fluid wasn’t just whatever American Motors used in the 1980s to lubricate the mechanisms inside their speedometer housings. “Maybe it’s brake fluid leaking from the back of the master cylinder pushrod?” I wondered. But a quick glance at where the master cylinder mounts to the firewall revealed no moisture. Plus, I monitor my fluid levels closely, and my brake fluid reservoir has shown the exact same fluid level for months.

My next thought was that the waterfall could have been flowing out of a leak in my engine oil pressure gauge, though I was certain my Jeep’s gauge is electrical, receiving its signal from a remote sender rather than via a direct oil feed. I crawled under the truck and confirmed this to be the case.

Ultimately, after some googling, and discussion with members of the Full Size Jeep Facebook page, I learned that my initial inkling had been right. The oil is indeed coming from the speedometer, though ultimately, its origin is actually my transfer case, which sits roughly three feet away. If that sounds absurd, it’s because it is.

The speedometer on this truck works in much the same way as it does on many old 4x4s: A small gear in the transfer case spins a cable, which reaches forward through the floor and into the cabin, and plugs into the back of the speedometer. As I understand, this cable isn’t meant to be continuously lubricated, but in my case, that’s happening because there’s likely a seal missing where the cable threads into the T-case. There’s also a chance that the vent hose that allows pressure to escape the transfer case as its internals heat up during use is clogged, leaving the case with only one other path to vent itself: the speedometer cable.

A cursory look at my vent line didn’t reveal a clog, so what might be happening here is that a missing seal is allowing oil to reach the cable, which is acting like an archimedes screw, funelling automatic transmission fluid through the sheath, all the way into the back of the speedometer behind my dash, and ultimately all over my pedals, shoes, and rubber floor mat.

That’s what multiple members of the Full Size Jeep Facebook think, saying the rotating cable is “wicking” oil all the way through its sheath, and into the speedometer. It’s a fascinating problem, and one that the hosts of the legendary radio show Car Talk apparently addressed at some point, as Car Talk forum-goer George_San_Jose1 mentions in his thread, saying:

Dave called and said oil was leaking onto his pants from underneath the dashboard. Tom and Ray said it was a leaky seal in the speedo-cable, allowing xmission fluid to come up the cable and drip onto Dave’s Dockers.

But doesn’t this violate the laws of physics? After all, the transmission is considerably below Dave’s leg. Can oil really flow uphill?

I’m guessing there must be some sort of Archimedes Screw effect pumping the oil up the speedo-cable as the shaft turns. But for this to happen, the speedo-shaft would have to be helically shaped, like a spring, and oriented in the right direction for pumping action, clockwise or counterclockwise, right?

Read the rest of the thread if you’re curious to hear others’ takes on this, but whether it’s simply pressure buildup in the transfer case due to a clogged vent or a missing seal allowing for the cable to act as an Archimedes screw — or some combination of the two — the point is that somehow, my Jeep has been filling its own speedometer with automatic transmission fluid from the transfer case. This has not only broken my speedometer, but, more alarmingly, recently it has made getting good grip on the brake and clutch pedals downright difficult.

As a temporary solution, my coworker Jason used rubber bands to tie paper towels to the shafts of the brake and clutch pedals. This seemed to do a decent job at preventing the light oil from flowing down onto the rubber step pads, though oil did still drop directly into them.

With this done, he and I drove to the car parts store here in Chapel Hill and sized up some O-rings. In the pouring rain, I installed a tiny O-ring that fits snugly over the cable, and I installed a larger one that sits between the cable housing and the transfer case.

Two or three small drops of ATF dribbled out of the cable when I removed it. Those little drops created a giant rainbow pattern.

I have yet to drive the truck far enough to know whether this fixed my issue, but I’ll keep an eye on those paper towels on my pedal shafts, on the base of my dashboard, and on my floor mat. I’ll also check on the paper towels that I’ve shoved behind my dashboard. If those are still wet after today’s drive to pick up Jason’s incredible little Changli electric car, which he bought for under $US1,250 ($1,875) (plus hefty customs and shipping costs) from the Chinese e-commerce website Alibaba, then I’ll look more closely at those vent lines. In any case, my interim solutions will keep the pedals dry, which is priority, because even though the likelihood of me crashing due to slick pedals is 0.000001 per cent, it’s not worth the risk.

Unfortunately, I’ll likely have to send my speedometer to a shop to have it professionally rebuilt, as I’ve heard these things aren’t exactly trivial to mend. And while that isn’t likely to be cheap, it’s worth it for my Jeep J10.

I love that truck, even if it surprised me with this truly baffling and potentially dangerous speedometer leak

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