Flush means to push fluid through under pressure.

Drain means to pull the plug and let gravity and atmospheric pressure do the job.

 

Yes but when you are flushing, normally you are cycling through new fluid not reusing the old fluid. The use of the word flush in the procedure is slightly confusing but all is good now. Unfortunately with the new change, a by the book transmission filter change would require the dealer or having the official diagnostic device. Sucks for those wanting to do simple filter maintenance at home especially with how it could affect the warranty.

 

I wonder if a dealer (or independent shop) would do the flush/cleaning procedure as a stand-alone procedure. It should only take 1/2 hour or so and the car doesn't need to be put on a lift (could probably be done in the parking lot - easy money). You could then drive home and do the filter change. As long as driving the car after the flush doesn't cause worse problems this could be a compromise.

Ultimately, it would be best if a reasonably priced aftermarket scan tool came out with a flush/cleaning procedure. There are already some that can do certain bi-directional procedures.

 

The “Flush” is to dislodge contaminants from the internal part of DTC and let the canister filer catch contaminants before it’s changed. Don’t see any reason you couldn’t drive it 1 or 500 miles before replacing filter.

 

Yes, that is the purpose of the hydraulic system flush. How does the "dislodged" debris in the lines and valves make its way to the filter? I believe the lines and valves are downstream of the filter. So I speculate that the dislodged debris is first returned to the transmission fluid pan, before being sucked up by the internal filter and then passing through the external canister filter, before going back to the valves. If that is how it works (IF) then what you say makes sense to me. It might even help to drive the car to make sure all the debris is sucked up and caught by the external canister filter (I am assuming any "dislodged" debris in the lines and valves is already to small to get caught by the internal filter. If any of the particles are ferrous metal, there are two magnets in the fluid pan to catch them. And it all that is true, the only reason for not doing it is if one thinks that the dislodged debris might "fill" the external canister filter to "capacity". I/m not saying that happens.

 

doesn’t matter how it works. The GM procedure doc say it so the canister filter can capture debris.

if it was to dislodge and drop to dct pan, that wouldn’t make sense. They don’t drain fluid with the canister filter change. Why would they want to dislodge stationary particles smaller than what filter can capture?

 

While I agree, what is important is if GM agrees with respect to warranty coverage.

 

I don't think we have enough information to say that the hydraulic system flush returns contaminated fluid to the pan. It is quite possible that the fluid is redirected to the canister filter without passing through the sump filter.

 

You may have missed the distinction I made between the internal filter and the external canister filter. There is an internal filter in the transmission that resides in the pan and serves at the pick-up for the fluid. That internal filter does not get identified in the Owner's Manual as ever needing to be changed. I attached an illustration of it. It is very likely a coarse filter and so it will not remove all the material that is removed by the external canister filter. The filter that is changed at specified mileage intervals in the Owner's Manual is the external canister filter. That is a fine filter. It was speculation (and I identified it as such) that the dislodged debris would end up back in the fluid pan, then be drawn though the internal filter, make its way to the external canister filter where it is filtered out before it gets to the lines and valve bodies. And I agree completely with @RKCRLR posts 27 and 28. That what matters whether GM accepts it - and that there may be other explanations for exactly how the hydraulic system flush clear the debris and moves it to the external canister filter.

The following, by the way, is the exact description of the purpose of the hydraulic system flush taken from the hydraulic system flush procedure posted by in another thread a while back:

"DescriptionThis procedure is used to flush the transmission fluid through the filtration system before a repair is performed that may have been caused by debris in the fluid."

Interesting that it says "through the filtration system". The steps to replace the external canister filter occur after the hydraulic system flush is complete, and are not actually part of the hydraulic system flush procedure. And, the procedure is not only called out as a step in the DCT external canister filter replacement, it is also called out by other procedures. Attached is an interesting service bulletin that also invokes the hydraulic system flush procedure as step 9.

 

I’ll let y’all keep discussing the flush procedure and filter change. I didnt realize who was posting. This subject has been discussed for over a year in other forums and post.
Have the dealer do the flush and filter change if “anyone” is so worried about keeping warranty.

I can only put if in layman terms. Canister filter or filter system doesn’t matter. And this will be the last post.

the flush is simply a procedure to capture particles in the DCT canister filter that they will be replacing.

.

 

I agree with you now and I agreed with you in post 25 - that is the purpose of the external canister filter replacement when used as part of the DCT external canister filter replacement, and the car can probably be driven between the hydraulic system flush and the external canister filter replacement like you said. And I also cannot think of any reason why it could not, same as you. And I agree with @RKCRLR that we do not know if GM would view it the same way from a warranty perspective. I am sorry that what I wrote about how it might work and why that is consistent with what you said has offended you. It was not my intent.

 

All that follow is hypothetical/speculation.

So, I gave this some further thought - and to do so would require extra valves to open and close a separate fluid return path from the pressure lubricated components to a point that is between the internal filter and the fluid pump, would it not? Attached figure 1 is my simple minded illustration of how the fluid flow my normally work. I have been called out before other threads for assuming how this flow works - so I want to be clear is hypothetical. In the first figure fluid is drawn up through the internal filter by the fluid pump, then sent to the external canister filter, and after that it flows to pressure lubricated components and the valve bodies, after which that fluid is returned to the pan. The first figure is based on an assumption that all pressure lubricated components receive fluid that has passed through the external canister filter, and that all that fluid is returned to the pan after it passes through the valves and lines. It also shows the there are some splash lubricated components (gears, clutches, etc) that ride in oil from the pan. The second picture allows for the possibility that there are other pressure lubricated components that do not receive fluid that has been through the external canister filter. As if it were a bypass filter rather than a full flow filter. I do not know if there are any such components. The third figure illustrates what might happen during the hydraulic system flush. The fluid/debris could still be returned to the pan, then makes it way back to the filter though the normal flow path. Or, it could be returned directly to the pump. To do would require an extra path, and some extra valving. It looks a little complex, which is the reason I first suggested it just returned via the normal path. I do not believe it could be returned to the filter at a point after the pump - the pressure differential would not allow that flow path.

 

Without having a hydraulic schematic we don't know the system operates. But it may be possible to have a circuit that returns fluid to the inlet side of the cannister filter by operating the valves in a sequence or putting them in a position that isn't seen during normal driving. And it uncommon for systems to have valve(s) that are strictly for servicing. The reason I question it going through the main filter is that is a lot of fluid to filter in a relatively short time and it would take many passes to capture a majority of the particles because of the dilution that occurs.

 

I think I see what you mean. Normally, the highest pressure in the system is going to be between the pump and filter, so it might be difficult to return something from downstream, on the other side of a valve because the pressure will be lower there. But I suppose there could be a way - I'm still having trouble picturing it. A valve that re-directs the pump pressure maybe? Anyway, this is just a discussion of how the contaminated oil gets back to the filter - but we know it does get that there - and that recently posted updated procedure drives that home by making the point that the filter must not already be overloaded when the hydraulic system flush is initiated.

I

 

If there is an electronically controlled pressure regulation circuit between the pump and the canister filter it wouldn't be that hard to incorporate a circuit that returns fluid to the inlet side of the cannister.

 

We need a TREMEC insider..... I do recall seeing Conti's sump filter on his channel when he replaced it at 30K miles. It looked new (no discoloration like you see on the external DCT filter). Makes me think that at least the clutch debris is getting to the filter before the fluid ends up in the sump. If the discoloration we've seen on the DCT filter is actually clutch debris. I was going to change the sump filter at 2500 miles along with the external filter but decided not to replace the sump filter based on what I saw with Conti's. I'll be doing a fluid replacement and DCT filter change in the next two weeks and report my observations (fluid and filter conditions).