What exactly happens when towing automatic?

It says everywhere that transmission gets damaged when it is towed, that oil won't circulate and so on, but what exactly gets damaged in the trans?
If it has happened, then maybe it is only a matter of changing out a few parts and we're back on the road?
(I am thinking about TH350)

 

Without the engine running to drive the pump, there is no fluid being pumped into the clutch packs or bearings. Most automatics don't have roller bearings but instead have bronze or babit type bearings. So with the trany in neutral and being towed, the clutches will over heat and the bearings will gaul. Short tows shouldn't be too bad but for a tow-behind, I would drop the drive shaft.

 

Most of your late automatics don't have a rear pump, so the fluid doesn't circulate. I've towed plenty of them and never caused a problem, the one time I hauled one 150 miles I stopped a couple times, started the engine and cycled it through the gears. If I had to go, say, across the country, I'd take the driveshaft off. A few miles in town I wouldn't worry.

 

Lube oil is pumped from the return line of the oil cooler back through the gear train to lubricate the bearings, planetary gears and bushings of the transmission. Without the pump, there is no pressurized lube to reach these components and therefor they will run dry and fiery death of parts will ensue. Some automatics have these parts partially submerged in oil from the sump so they will last a little longer than others that have the gear train divorced entirely from the sump, (most GM transmissions), but they will all die eventually without this lube flow.

 

How far are do you need to tow it? If it's under 10-15 miles you should be OK if you keep the speed low. For a long distance tow pulling the drive shaft off is a must IMO.

 

If possible, let the engine idle in Neutral. Everything gets lubed, no damage. If itr won't run, pull the driveshaft. Some auto's are more forgiving than others. I towed a guy in a '66 Chev about 15 miles (against my advice!) and the P'glide fried. No drive in any gear, smoke out the dipstick tube, the whole enchilada.