Transmission Power Flush

First off, I know this subject has been discussed in here before. But I can't get a good feel if there's any kind of majority opinion one way or another by automotive pros.

I have a 1999 Honda Accord V6 with 48,000 miles that came up with a PO740 code ("Lock-up clutch does not engage". This car has been operated under
"normal" conditions as defined by Honda. "Normal" service recommendations call for tranny fluid change at 90,000 miles. Bad idea. "Severe" service recommendation is 30,000 so I figure around 50,000 is probably about right.
Before I let them do it, I checked with a respected independent transmission specialist in my community. He advised against the procedure. He said he was aware of catastrophic transmission failures on vehicles immediately after having this done. He recommended having a drain and fill, a couple days of driving, and another drain and fill. At 40 bucks a pop for the drain and fill, this was more cost effective than the power flush.

If the power flush is more effective than the multiple drain and fill method, and non-destructive, I would gladly pay for it. But if the drain and fill IS as effective, I obviously shouldn't take the risk the power flush may represent. So I guess the opinions I'd appreciate from you guys are:

1. Are there risks involved in having a power flush performed?
2. If so, is there a difference in risk between a vehicle with 48,000 miles and one with significantly higher mileage that had never been serviced?
3. Is doing multiple drain and fills as effective as a power flush at purging the old fluid and contaminents?

Thanks for any help you can give me,

"shiden_kai" wrote

Should just clear up something. This machine is not an essential dealer tool, but it is GM "approved". The machine that I was thinking about as an essential tool is a new transmission "cooler flush machine" that we recently got.

"Pete" wrote

Pete, it really depends on what type of machine is being used.
I've only had experience with one machine and it happens to be a Wynn's machine that is labeled as a GM dealer essential tool. It does not "flush" the fluid....or perform any kind of
"backflush". It simply hooks up in series with one of the trans cooler lines and pumps out the old fluid while pumping in new fluid. It can sense whether you hooked up to the pressure cooler line, or the return cooler line and it adjusts as needed.
So there is no fear of inadvertantly hooking the lines up wrong.
It also maintains line pressure as it exchanges the fluid.
I think that there is no doubt that this type of machine does a far better job of "exchanging" all the trans fluid then a simple drain and refill. But, the drain and refill has it's advantages.
You get to change the filter and clean the pan. And have a look at what debris is being deposited in the pan. Often though, debris in the oil pan can scare the living daylights out of the untrained person.
Which only works to the advantage of unscrupulous transmission shops.

My personal preference on my own vehicles is to drop the trans pan and replace the filter. Top up the trans fluid and
"then" do a complete fluid exchange. Obviously it's less expensive for me as all I have to do is pay for fluid. You can run more fluid thru the transmission then it actually has as a capacity and be guaranteed that you have almost completely changed all the fluid.

The other machines that I've heard about....(anecdotal evidence) appear to do things that you wouldn't really want done to a transmission.

I will note that in our dealership, we use these transmission
"flushing" machines daily and have yet to see a trans failure that could be attributed to using them. Believe me, if there were failures like that, we would stop using them in a hurry.
We don't need that hassle.

A lot of people will say yes....I will say no...if the right machine is used.

If a person religiously changed their trans fluid and filter every
2 years/40k kilometers.....they probably would be just fine. I do know that a top notch transmission rebuilder on these forums recommends servicing transmissions even more often. He's the expert and see's what happens when people refuse to perform good, solid, preventative maintenance.