@razmigb
that's not true. Some flushes use the pressure of the transmission's pump to move the fluid. The pump can generate 80PSI or more.

@mountainmanjoe
In the case of his transmission, definitely not! Look at the mileage! And almost all modern transmissions are very sensitive to fluid flushes because of how sensitive the internals are to pressure. For example, any ZF transmission CANNOT be flushed because it will damage internals, as stated by ZF directly. Performing any flush on any transmission is risky. I'm a strong believer of just draining whatever fluid comes out, refilling it, and changing the filter. It's safer than flushing. And if you do this every 50,000-60,000 miles or so, you don't even need to think about flushing since you're cycling the the fluid (some old fluid comes out some new fluid goes in) at those intervals. I know people who had their transmissions flushed, especially at higher mileage, and started to have problems.

@razmigb
I don't think you are comprehending. I'm talking about using the transmission's own pump to move fluid. If that pressure could destroy a tranny, then the tranny would just self destruct under normal driving, ok? The transmission internals have to be able to withstand pump pressure. Anyway, when you hook up a flushing machine correctly (and this probably depends on the particular vehicle) then it's just gravity moving it through the transmission body because the fluid gets picked up by sump action. I think the flush is mostly to clear out the cooler lines and a bit of the torque converter. The dealers DO use flushing machines.

I agree that most people do not need to do a full flush. If the fluid is full of metal then it's probably time for a rebuild. But there could be situations where a flush might be a suitable course of action. I have done flushes at 150k without issue.

@mountainmanjoe

I understand the pump pressure and all that, but it's not just about the flush procedure itself. It's whether or not the transmission will operate well with slick new fluid. As the clutch-packs get older, they lose friction due to the frictional compound wearing out. The same reason why the clutch on manuals have to be replaced every so often, because they wear out. But in an automatic, that frictional compound wears off the clutch-packs and gets suspended into the fluid. So now the fluid plays a vital role in providing the necessary friction for the worn out clutch-packs to operate properly (to not slip). When you flush ALL that old fluid out, you're removing the friction with it. So when you add fresh new slippery fluid, the transmission may or may not start to slip depending on how well the clutch-packs are inside. This is why MOST mechanics do not recommend flushing at a very high mileage like this, especially if fluid work has never been done on the transmission before. That's the main argument behind flushing older transmissions...not so much the flushing pressure. At only 60,000 miles, a fluid flush won't run the risk of slipping because the clutch-packs have a ton of life left. The risk at 60,000 miles is whether the flush may or may not damage internals.

@razmigb
> The risk at 60,000 miles is whether the flush may or may not damage internals.
which internals exactly? How do they get damaged?

@mountainmanjoe
The seals and valve body if there are any abrasive metallic materials in the fluid or even caught somewhere in the lines. Or simply by using higher pressure to flush. Some mechanics are idiots these days and will use really high pressure to flush the system and damage everything. When you do a flush you don't remove the pan, correct? So you don't see the condition of the fluid inside (if no dipstick), the pieces of metal the magnet has caught (which suggests the health of the transmission and whether some of that metal got stuck somewhere in the lines), and you don't even replace the filter. You wouldn't know what you're going to be pumping through the delicate system, and you can then start to clog lines. Or if you do end up unclogging lines, those debris will be free inside the transmission and damage other lines and valves. You can argue "but the transmission was pumping bad debris anyway while driving". But in this case, you're forcing to pump out all the fluid and all the debris with it. Anyway, I'm done answering your questions...you're questioning my knowledge which I don't appreciate. Flushing or not flushing is an opinion, and my opinion is to not flush. Avoid it in the first place...

@razmigb
If there are pieces of metal large enough to block a fluid line then that transmission is finished. If that's happening at 60k then it has serious problems and needs to be opened up.

If you can't tolerate different opinions, or having your knowledge questioned then you're going to have a really hard time in online forums.

@mountainmanjoe
I'm tolerating differing opinions just fine. I stated my opinion at the beginning (the second comment on this answer). I think you're not tolerating it well since you started all this back and forth arguing saying I'm not comprehending your comments and unnecessarily questioning me on things you clearly should know if your responding on this post. You're calling me out for my opinions, and that I don't appreciate. Don't take my opinion personally, I was just giving an elaborate explanation on the downsides of flushing, and allow the original poster of this question to make their decision based on our combined opinions. I do apologize if I sounded rude in my comments. We have different experiences with this topic, and I believe it's great to share both sides.