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It is clear that cars with automatic transmissions cannot be towed with any drive wheels on the ground, engine off, in neutral, because the torque converter powers the lubrication pump for the transmission.

Why is it considered safe for a manual transmission? Doesn't the transmission need to be lubricated and cooled, just like an automatic? What about the differential or all the other parts of the car that move while being towed? Don't they need to be properly lubricated too? What is the technical reason behind this?

Edit #1

I thought that for a manual transmission in neutral, the output shaft does not spin the layshaft, which in turn does not spin the crankshaft, otherwise the entire powertrain would be 'closed' or 'connected', so how is it possible that splash lubrication is occurring at the bottom of the sump?

Why was the method for supplying lubrication to an automatic transmission engineered differently compared to a manual transmission?

  • 'because the torque converter powers the lubrication pump for the transmission' This isn't true for all transmissions some do run the pump off the tailshaft – draksia Apr 18 '17 at 15:49
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    Remember, for a manual, 4 wheel drive car, towing is not considered safe. You always lift. /subaruowner – Rory Alsop Apr 19 '17 at 7:31
  • That's just not true. Some automatics can be towed without mileage limit and some sticks can't be towed at all. It's all in the peculiarities of a given design, not in some grand idea. – Agent_L Apr 19 '17 at 14:56
  • @RoryAlsop - Well if its AWD and all 4 wheels are on the ground then I believe it should be safe. – Narcotixs Apr 19 '17 at 15:11
  • If it's in neutral what would be the problem – WILDBILL Feb 7 '18 at 18:06
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Manual transmissions (most, but not all) lubricate not through a pump, but through the action of the gears and secondarily through the level of the transmission fluid itself. In some transmissions, the lower gears in the transmission, which touch the pool of fluid at the bottom of the transmission, transfers fluid through contact to the upper gears. In this situation, since the output to the drive axles is still attached to the gears, even when the transmission is in neutral, the fluid still flows.

EDIT: Checking through many different transmission layouts, it also become apparent the level of the fluid is high enough to be right at the level of the main shaft in the transmission. In some transmissions where the lay shaft (as @Narcotixs pointed out) doesn't have gears actually turning, the amount of transmission fluid in the gear box allows for the lubrication of any moving parts.

This representative image (sorry, best I could find on Google) shows the location of the fill plug in relationship to the main shaft. Most manual transmissions will have this same relationship between the fill plug and the main shaft.

Manual transmission fill plug location.

/EDIT

The differential is much the same as the ring gear sits in the gear oil and the oil flows through differential. The differential doesn't care so long as the tires are moving. It will behave the same either way, whether there is power applied from the engine, or the tires are just rolling along.

That being said, putting a car up onto a tow dolly or trailer is a much "safer" way to transport a vehicle. It will be much more secure, especially if you're looking to transport the vehicle for any longer length of time.

  • I'm not sure what the British English for "tow dolly" is, but it's definitely safer to tow for any significant distance with a rigid towbar than with a rope - especially if the drivers aren't experienced in towing, and after a long stretch at constant cruising speed the towing driver hits the brakes too hard for the towed driver to react fast enough! – alephzero Apr 18 '17 at 12:55
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    Aside from the splash lubrication, there is no load on the gearbox and differential when towing, so the gears don't need the oil supply to maintain the oil film under load, and handle the extra cooling required. Even towing a "dry" manual transmission probably wouldn't do much damage, with modern accurately cut and hardened gears. – alephzero Apr 18 '17 at 13:00
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    @alephzero while we don't routinely use tow dolly it should make sense to most people with some engineering/mechanical experience. – Chris H Apr 18 '17 at 14:10
  • @alephzero Also, reader must note that in vacuum servo brakes it is much more harder to break when the engine is not rotating, thus the breaking action of the vehicle towing is much more effective than the vehicle being towed. – ricardomenzer Apr 18 '17 at 15:10
  • @alephzero towing a car with a "dry" transmission may not harm the gears but would probably damage the bearings... – Solar Mike Apr 18 '17 at 17:29
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Because, in a manual gearbox (those that I am aware of) the method of lubrication is by splash as the layshaft sits in the oil, so there is no engine driven pump connected to the input shaft.

Same argument for the diff - splash lubrication.

  • Good simple answer, and accurate in most cases. – mongo Apr 20 '17 at 14:27

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