Blogs Car Info Our Show Deals Mechanics Files Vehicle Donation

Transmission oil from speedo-cable flows uphill, violates laws of physics?

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?

My question: Wouldn’t the manufacturer orient the speedo-cable helix so that as the cable spun, it would push the fluid back down into the transmission, instead of pumping it up the cable? Help! Help! This has been bothering my poor brain!

I believe most speedometer cables of old have small grooves that act as a helical screw which causes a small amount of transmission fluid to travel up the cable. I know that most of the time when I removed a speedometer cable from the speedometer a small amount of fluid was at the top of the cable. I believe the fix is a new “O” ring on the transmission end of the cable.

I think you are on the right track. It is designed to funnel oil upward to keep the whole conduit lubricated. If you have a push mower, think about the engine brake cable. The little groves funnel it up.

Missileman and smallengine are correct. The old cables are twisted multistrand cables oriented to pull lubricant upward while turning just like augers.

Interesting … hmmm, ok, I see how the spinning of the cable draws the xmission fluid upward … but that brings forth another question. If that’s the case – that indeed the speedo-cable is supposed to draw xmission fluid from the transmission upward for proper lubrication of the cable – then how will replacing the seal at the xmission help? Isn’t the seal at the xmission designed to prevent xmission fluid from moving up the cable? Or is it a different seal that Tom and Ray are talking about? Maybe there is a seal at the top of the speedo-cable, where it plugs into the speedometer? Is it that upper seal – where the cable plugs into the speedometer – that needs to be replaced? Or is it the seal at the xmission that needs to be replaced, but that this lower seal is designed – when it is working properly – to only allow a small amount of xmission fluid through it, and when broken it allows too much?

There should be a seal at the top of the cable.

The very fact that the caller has tranny oil dripping on his pant legs is evidence that the Archemedes screw principle is a part of the system.

I don’t think that the transmission fluid lubricates the speedometer cable. I always thought there was some sort of graphite that was used to lubricate the cable. I remember installing a cable inside the housing some years back and there was some sort of lubricant I put on the cable as I pushed it through the housing. When I was a kid, I had a speedometer on my bicycle and it was driven through a cable from the front wheel. The bicycle speedometer cable was much like the one on a car. I think the seal is at the transmission end to prevent transmission fluid from going up the cable.

The more I think about this, the more I realize that Triedaq might be correct. I’ve removed speedo cables at the tranny end for other servicing (instrument cluster removal) and I don’t recall any fluid coming out.

They talked today on NPR’s Science Friday about this experiment you can do at home where you spin a hard boiled egg it a puddle of milk. Apparently the milk clings to the egg and rises up the egg as it spins. Maybe it’s the same effect that would pull xmission fluid up the speedo-cable if the xmission seal leaks. You can see the vdo at

A paper towel placed over a spill will draw water up against gravity. This at first appears to be doing work with no source of energy. The hidden source of energy is the room’s ambient temperature which energizes the water molecules, causing them to move up the towel. This is the same energy that causes water to evaporate.

Not sure why people are disagreeing. It’s basic physics.

I believe Triedaq is right as well. and I believe you have a point about the orientation of the helix. The manufacturers could have, but they didn’t.

Transmission fluid should not be entering the cable housing at all.

You mean no one in this thread is familiar with capillary action?

Cap action is involved of course, but there’s a spinning wire component too. I think that is the more interesting part.

“Cap action is involved of course, but there’s a spinning wire component too.”

Nonsense. Speedometer cables are not lubricated by transmission fluid by capillary action, Archimedean screwpumps, or by any other means.

In my early days, a squeeking or fluctuating speedometer reading indicated a dry cable that began to bind in the flex-tube. The fix was always to disconnect the cable behind the speedometer, pull out the inside cable, wipe some light grease on it and replace. The lower end of the cable had a square ferrule that plugged into a mating recepeticle in the tranny. The spiraled inner cable was never exposed to the tranny fluid unless the lower o-ring failed…that is when you suffered “oil-on-the-knee”.

@Mechaniker …ok, but then what is leaking on Dave’s pants leg if it isn’t transmission fluid?

@GeorgrSanJose–It is transmission fluid leaking on Dave’s pants leg. The o-ring seal is defective where the speedometer cable connects to the transmission and the fluid then works its way up the cable.

When the seal is bad, a channel is open. Do this experiment at home. Cut a bicycle brake cable at both ends. Strip back enough like a 1/2 inch to clamp into a drill. Put the other end in oil. Spin. Do you wind up emptying the oil? Maybe. Do you get a drop or so at the top of the drill? Everytime. It is just physics get over it

What a coincidence. I’m fixing my bike this weekend. I’m driving to the bike shop this afternoon in fact. Good idea. I’ll do that experiment.

Question: I hav eno doubt the oil will move up the cable under the right conditions. But do you think the oil will moves up the bicycle cable independent of the direction the drill is spinning? i.e. if I do the experiment with the drill in “forward” and the oil moves up the cable, will the oil also move up the cable if I put the drill in reverse?