AOD - how does pump turn when TC empty?

[kishy]

I may be overthinking this, but that's why I'm asking...

Someone told me that draining a torque converter and then depending on the pump to refill it by running the engine risks damage to the TC and lubrication starvation elsewhere in the trans, because the transmission pump will not turn if the TC is itself empty of fluid.

Thinking about it, it kind of makes sense. The TC needs fluid in it to create a link between the engine and the input shaft. But, I did this on my 83 back when I changed the fluid, and although it did sound "weird" for the first couple seconds running, it worked fine after.

What am I missing here? Is it actually risky to drain the TC, given that there's no practical way to refill it while installed in the car other than to run the engine?

[DerekTheGreat]

NFC but that's probably why a flush is preferred eh? Disconnect input and return lines, input line is connected to drain, keep injecting new fluid until fluid appears clean at drain thus negating damage to TC and components?

[His Royal Ghostliness]

because the transmission pump will not turn if the TC is itself empty of fluid

The pump is powered via the torque converter's outer shell/housing - if the engine is turning so is the pump. That's why it's so important to make sure the converter is fully seated into the bellhousing before bolting the trans to the engine, if converter shell isn't lined up with the pump when you start cranking the bellhousing bolts the converter will push onto the pump and shit gets broken.

Also, flushing is actually less effective than drain/refill, as during flush you have new fluid mixing with the old one inside the converter. Eventually all the old fluid will be expelled, but it will take more fresh fluid to accomplish that than if you did drain/refill. For best results do train/refill AND pop the return line at the trans so no old fluid can find its way back into the case.

[kishy]

The pump is powered via the torque converter's outer shell/housing - if the engine is turning so is the pump.

Jackpot. That's exactly what I was looking for. I knew there was a "slushbox" output from TC to trans (via fluid coupling), and with the AOD there is also a direct drive shaft that effectively locks the crank to the trans, but I was thinking there was no way for the direct drive shaft inside to turn the pump.

If the pump is turned by the body of the TC, the mystery is solved.

Found an image here that seems to clarify it: https://www.cartechbooks.com/techtip...and-evolution/
Specifically https://www.cartechbooks.com/media/wysiwyg/6_38.jpg
Direct drive (lockup) in the centre, slushbox in the middle, and pump stator on the outside...

Good point about flushes, it does make sense...another thing I considered is that sediment (clutch material etc) may take longer to circulate as it isn't an evenly dissolved solution with the fluid, plus as fluid arrives back in the pan it doesn't necessarily push dirty fluid into the filter/pickup. So you could, say, hook up a bucket of new ATF to the cooler line going to the trans, and point the other line at a drain bucket, let the pump exchange fluid until it runs clean but there could still be a good quantity of old fluid and suspended material in the trans.

[His Royal Ghostliness]

So you could, say, hook up a bucket of new ATF to the cooler line going to the trans, and point the other line at a drain bucket, let the pump exchange fluid until it runs clean but there could still be a good quantity of old fluid and suspended material in the trans.

If you do that what will happen is you'll have a bucket full of clean ATF, about a gallow of dirty ATF in the other, and an empty transmission pan. Reason being, the transmission case is not sealed to the outside, you have a vent at the top of the case that will prevent the pump from pulling the vacuum you need to suck the fresh ATF from the new bucket. So as the pump is pulling fluid out of the pan, what will replace it is not fresh fluid from the bucket, but air entering via the vent.

[kishy]

If you do that what will happen is you'll have a bucket full of clean ATF, about a gallow of dirty ATF in the other, and an empty transmission pan. Reason being, the transmission case is not sealed to the outside, you have a vent at the top of the case that will prevent the pump from pulling the vacuum you need to suck the fresh ATF from the new bucket. So as the pump is pulling fluid out of the pan, what will replace it is not fresh fluid from the bucket, but air entering via the vent.

Right, I was envisioning a setup where the bucket is gravity feeding into the transmission cooler line. Like line attached to a spigot at the bottom of the bucket with an open vent on top.

Some things escape me, like transmission pumps, but that part I had some idea about

[His Royal Ghostliness]

Gravity feeding fresh fluid thru a 5/16" line will not match flow rate with the pump pushing dirty fluid out - you will end up with a dry pan. Now if you were to close the bucket off, hook up shop air to its top with pressure dialed way down, then you can accomplish what you're thinking of. Still, good luck matching flow rates in/out.

[kishy]

Perhaps I'll just stick with the old pan and converter drain, since we've established that the pump is in fact turning even when converter is empty

[jaywish]

Yes, be grateful that you have a TC drain plug.

[gadget73]

Considering the small amount of fluid that will remain its not worth worrying over. If the fluid is so bad that its a concern, the trans is probably nuked anyway so either way its not worth worrying over.

But yeah, basically there are 3 input shafts to the trans. Outermost is off the shell to drive the pump, middle is through the turbine to drive first and second gear, inner-most is also off the shell to drive 3 and 4. Thats how the AOD does lockup, though technically speaking it bypasses the converter rather than locking it up. Later transmissions have 2 shafts and some sort of magic voodoo that operates a clutch inside the converter to lock the turbine to the shell.

[turbo2256b]

One could run the line going to the cooler or out of the cooler into a bucket and pour new fluid into the dipstick tube. Have done this with a trans were the car went through a deep puddle and water got into the trans.

[His Royal Ghostliness]

Later transmissions have 2 shafts and some sort of magic voodoo that operates a clutch inside the converter to lock the turbine to the shell.

It's a piston that pushes on the clutch, electric solenoid in the valve body directs pressure to said piston. More pressure means better lockup in both feel and holding power, till you find out the damn piston has a crack in it cause it got overheated while running under heavy load with converter unlocked (E4ODs do that in 2nd gear, the temperature of the fluid exiting the converter can get scary high then).

One could run the line going to the cooler or out of the cooler into a bucket and pour new fluid into the dipstick tube. Have done this with a trans were the car went through a deep puddle and water got into the trans.

That's how DIY flush is done indeed, the disptick tube is plenty fat and thus if a proper funnel is used it's very easy to keep up with the flow rate of the old fluid being pumped out the into bucket without fear of running the pan dry. Still annoying tho, so draining the converter and pan are my preferred method, assuming converter has a drain in the first place.

[gadget73]

I installed drain plugs in both of my AOD's. Makes all this easy, drain the pan, drain the converter, then drop the pan without a bloodbath under the car. The one on the Mark VII I actually brazed in because I got the hole a teensy bit in the wrong spot and the plastic gasket was just into the edge of the script on the bottom. The large brass fillet around the plug gives no f's about that.

[kishy]

Yeah, I get the Dorman retrofit drain plugs and put them in with RTV and threadlocker. Not as permanent an installation but I'm up to 3 years on one of them with no leaks so it's doing the job.

Pan drop still necessary for the filter but it enables recovering the fluid to be used again if one wishes (like when I had just changed it and then found the pan gasket leaked), as well as pan drops without the aforementioned blood bath.

[His Royal Ghostliness]

Those drain plug kits, toss that plastic gasket-ring they come with right in the trash bin. Replace it with the proper copper washer, tighten the nut nice and good, and never ever worry about it leaking. Metal on metal contact, with or without o-ring sandwiched in-between, is the only seal I personally deem appropriate for an application that can get hot. Same applies to engine oil pan drain plugs as well, rubber-coated steel washer or aluminum-only washer is much better than the plastic seal many drains come with.