Towing and auto trans

Belgian1979vette

Belgian1979vette

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I know that one cannot tow an automatic equiped vehicle with the drive wheels on the ground, but can anyone here explain exactly why this is so. I have some very basic knowledge about the auto trans, but that is basically it.

Thx
 
Belgian1979vette said:
I know that one cannot tow an automatic equiped vehicle with the drive wheels on the ground, but can anyone here explain exactly why this is so. I have some very basic knowledge about the auto trans, but that is basically it.

Thx
Click to expand...

That is a very olde tyme thing from the daze of the 4 speed hydromatics of daze gone past....

they had two pressure pumps, I am told, and so if the driveshaft was spinning the rear pump would be supplying pressures to somewhere.....

over the last 50? years now, the things have only a front pump, direct engine driven...so not a issue.....

obviously you put it in neutral.....duh.....
 
With no rear pump and in neutral, the rear shaft and all the rear planetary gears are being driven with no lube pressure. So they overheat. If you're going to tow any distance you need to pull the driveshaft. I wouldn't recommend any long distance stuff, but it is possible to leave the engine idling in neutral to keep it all under lube pressure.
Even a manual trans will overheat if towed a long distance. The rear half is spinning and the front half and the countershaft is not. I've seen manual trannys that have been towed with the driveline not removed. It was ugly. And expensive.
 
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Tim, really?? I thought those output shafts were free to rotate with no pressure on the output clutches/clutch packs....

I retract my statement....


:eek:h:
 
mrvette said:
Tim, really?? I thought those output shafts were free to rotate with no pressure on the output clutches/clutch packs....

I retract my statement....


:eek:h:
Click to expand...


That is correct- there is NO pressure on the clutches and they are free. But the output carrier is splined to the output shaft and the rear planetary runs inside that, then the clutch pack has lugs that run in the OD of the carrier, so they spin with it.

Kinda like the foot bone is connected to the ankle bone connected to the leg bone connected to the ....
 
I used to teach auto trannies at the GM Training Center - here is the GM rule on towing automatics:

"35/35"

You can tow an automatic for no more than 35 miles at no more than 35 miles per hour.

An automatic transmission, unlike a manual, gets no splash oiling - all bearings and bushings are pressure fed from the engine-driven front pump. If the engine is not running, the tranny gets no lubrication. When towing, the tranny output shaft and rear drum turns from driveshaft input, and none of those rear bearings are getting any oil. If you exceed the 35/35 rule, you will destroy the bushings/bearings in the rear of the tranny. Anything less than 35/35 is okay and perfectly safe.

Lars
 
How about manual trasmissions?
lars said:
I used to teach auto trannies at the GM Training Center - here is the GM rule on towing automatics:

"35/35"

You can tow an automatic for no more than 35 miles at no more than 35 miles per hour.

An automatic transmission, unlike a manual, gets no splash oiling - all bearings and bushings are pressure fed from the engine-driven front pump. If the engine is not running, the tranny gets no lubrication. When towing, the tranny output shaft and rear drum turns from driveshaft input, and none of those rear bearings are getting any oil. If you exceed the 35/35 rule, you will destroy the bushings/bearings in the rear of the tranny. Anything less than 35/35 is okay and perfectly safe.

Lars
Click to expand...
 
As noted above, manual trannies don't have pressure oiling or a front pump - they lube themsleves from splash oiling and have no constraints on towing.

Lars
 
Is the output shaft submerged enough in the lube to get good splashing? As pointed out in post #3 above, without the input and clustershaft turning, would there be adequate lubrication?
I see a lot of RVs pulling small cars, I'm assuming they're not all eating trannys.
 
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Looking at this picture, I would be concerned about towing one for any distance or speed. Looks to me like the output shaft will be turning all the way to the input with nothing else. The gears run on the shaft and wouldn't get any fresh lube, seems to me like it'd cook it.

2349dc275e1da8b.jpg
 
GM has never had a limitation on towing any manual transmission car without the engine running.

Lars
 
Actually, in spite of their management, the engineers at GM are pretty damned sharp. I have worked with many of them over the years, and, yes, they know what they're doing.
 
Stupid followup question. I have not taken apart a manual trans in over 20 years so forgive me.
Is the clustershaft always engaged with the input shaft, even with the car out of gear?
If not, I would think there would be some rotation of the output shaft gears and clustershaft gears during towing. And this would provide some lubrication.
 
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