Interesting convo with local transmission shop

[meep]

I have vacuumed fluid from mine, but it's tricky and slow. the A750 transmission has a thermostat that cuts out the cooling loop AND cuts diverts to path-of-least-resistance loop through a heat exchanger in the heater core loop to warm the fluid. nifty little tidbit from some A750 info found online.

Anyway, it does make fluid extraction very slow.

 

[badtlc]

Originally Posted By: meep
I have vacuumed fluid from mine, but it's tricky and slow. the A750 transmission has a thermostat that cuts out the cooling loop AND cuts diverts to path-of-least-resistance loop through a heat exchanger in the heater core loop to warm the fluid. nifty little tidbit from some A750 info found online.

Anyway, it does make fluid extraction very slow.


That is what I currently do. I can pull out exactly 4 quarts with my pella pump. Takes about 10 minutes total time for the job if there isn't a leak in the pump system. I'm probably just going to keep doing that since it is so easy and apparently gets as much fluid out as any other option.

 

[Hokiefyd]

Originally Posted By: badtlc
No, he said ATF will flow just fine through the cooler lines but due to the transmission design, you will only have access to about 40% of the fluid. So, if you run 15 quarts through the cooler lines, when it is all said and done, he said you have still only replaced 40-50% of the capacity due to the design trapping the rest.


I can't quite wrap my head around this (either the original claim by your transmission guy or this clarification). If you're always pumping fluid through the system, then new fluid will replace old fluid (even in gravity drained areas, similar to oil "trapped" in a cylinder head valley) and the old fluid will eventually move to the sump.

If his claim is that you can't quickly get 100% of the fluid out, then I could agree with that. For example, if you have a 10 quart sump and you exchange 10 quarts through it, you'll still have a lot of old fluid, or at least old fluid that's mixed with new. Maybe his claim is that you'd have to pump 30 quarts of fluid through it before you'll get almost all of the old out (and this is more than the customer is willing to invest at the time). Perhaps that is true.

I'm not certain, by the way, how the Honda transmission would differ here. I realize that it's a different design, internally, from most transmissions, but you're still going to have areas where you have old fluid that has to be physically mixed then displaced by new in a gravity drained area.

Edit: I re-read the original claim, and it was that you could run all the fluid through it you wanted, and you'd still only get part of the old stuff out. That means that there is some fluid that will NEVER EVER be cycled through the system (or even filtered) and I'm not sure I subscribe to that.

 

[badtlc]

Originally Posted By: Hokiefyd
Edit: I re-read the original claim, and it was that you could run all the fluid through it you wanted, and you'd still only get part of the old stuff out. That means that there is some fluid that will NEVER EVER be cycled through the system (or even filtered) and I'm not sure I subscribe to that.

correct. these supposedly trapped areas only pass relatively small amounts of fluid in and out. So they are not completely sealed or anything but the flow between these areas is so small that just running a cooler line flush for a few minutes isn't near enough to effective replace this fluid. It wasn't that you'd never get this fluid flushed out but it takes a long time so you just replace what you can, let it mix for several thousand miles and then replace again.

 

[Kestas]

My fluid exchanges include shifting through all the detents as the fluid is being pumped out. I often see a shot of dark fluid come out when I shift. That should take care of some of the nooks and crannies.

It's like an oil change. You'll never get all the old fluid drained. There's still some left in the oil galleys.

 

[MolaKule]

The space where ATF might "dwell" and not get completely exchanged would be the actuators - the clutch pack pistons and band pistons.

If you look at their operation, they get pressurized and depressurized in short bursts, so fluid "flow" through these components is minimal.

One thing is for sure, replacing what fluid you can, such as pan drain and refills, replenishes the detergent/dispersants and sweeps and lifts particles that might slow valving or interfere with the clutch and band dynamic friction.

Most shops are good at dropping, rebuilding, and replacing transmissions but few really understand the ATF.

I work with a number of local transmission shops in fluid development and testing, and I can tell you that few transmission shops have techs that really understand lubrication and pressure relationships in the transmissions.

This is why I make presentations to these shops before a fluid testing regime is implemented.