Quote:
Originally Posted by MetroMPG
I had the chance to talk to a transmission expert (works at a repair shop) at the EV association meeting last night.
I described engine off coasting with an automatic transmission. Not surprisingly, he strongly advised against doing it unless the manufacturer specifically says the vehicle can be flat towed. I asked, "even for short distances - say a hundred feet?" His reply: the damage is cumulative.
However he did offer two potential solutions:
1) One was significantly overfilling with transmission fluid (maybe he was joking about this...)
2) The most sensible option was adding an electric pump inline in the tranny oil cooling line return from the radiator. He says RV drivers who flat tow "non-flat-towable" cars sometimes do this, and power the pump from the RV's electrical system.
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The pump idea seems like a good one but it has to send the oil to the right place, which is the internal bearings. I'm not so sure that the oil cooler return line will feed any oil to the bearings. It would be a good idea to look at a manual to find out how the oil circuits work in the transmission you are interested in. I remember reading that there is sometimes a bolt that screws into the transmission case which leads to the main oil passage at the output of the pump. It is there to connect a gauge for pressure tests. I think that the pressure at this port is called "line pressure".
I think that the way to pump oil to this is to connect a hose to a pipe added to the side of the transmission pan, connect it to the inlet side of a pump, and connect the outlet side to a fitting screwed into the line pressure test port. It would be smart to add a filter before the pump to make sure the pump is protected from little pieces of gears and clutches. I don't think it is necessary to have much pressure from the pump. It should be good enough just to keep oil in the bearings. Maybe 5 PSI would be enough.
Aerohead, I'm sure the two modifications you mentioned are possible but I'm not sure why you would want to freewheel the torque converter. Maybe if you wanted to power something besides the wheels with your engine, you would want to eliminate the drag of the transmission oil pump. Unlocking the tail shaft is a good idea for coasting since it would minimize drag and protect the transmission when the engine is off. It could be done either internally or externally by using a synchronizer from a manual transmission.
Metromizer, you said "Bill figured out where to tap into the throttle body". You probably meant "valve body" instead of "throttle body", right? I have heard of a throttle valve in a transmission but not a throttle body.
One modification I would like is to have the torque converter clutch engage in all gears and to stay engaged even when using high loads. I don't understand why it couldn't be engaged whenever the transmission input shaft reaches a reasonable speed such as 1500 RPM. A lot of power is lost by letting the torque converter slip while climbing hills. Maybe the manufacturers are trying to extend the life of the clutch by using it only occasionally and only for light loads.
For flat towing with an automatic transmission, I did exactly what you suggested (keeping the engine idling and the transmission in neutral) when pulling one of my cars at highway speed to a new storage place using a tow bar. The transmission still worked fine when I finished the towing. The transmission oil pump should keep the oil circulating as long as the engine runs. Other ways of protecting the transmission are disconnecting the driveshaft or axle shafts and overfilling the transmission with fluid. If the fluid level is higher than the bearings then they should stay oiled.
I had one car that had both front and rear pumps in the transmission. It could safely be towed for a long distance with the drive wheels turning. The transmission was the two speed GM Powerglide which was made until about 1970.
If you want an introduction to automatic transmissions, you may like to check this web site.
What The Heck Is A Transmission Anyway?