What you're hearing is a common misconception and it's going to affect your experience in the same negative way that happens to most people. First of all, that stuff you're referring to that's circulating is not there. It's the friction material flaking off the plates that is believed to be circulating but that had better be stopped by the filter. If a chunk of that material made its way past the filter, it would chew up the lip seals in the clutch packs or it would cause a valve in the valve body to stick. You'll find that material on the bottom of the inside of the pan but not on the other side of the filter.
What typically happens is a shifting problem develops, then owners run to the repair shop thinking a flush will solve it. In fact, a flush will not magically fix a mechanical problem.
To approach this a different way, if there is no problem now, a flush won't change anything but it won't get the debris out of the pan either. The fluid is moved in and out of the transmission by the transmission's pump, and that is supplied with fluid that has just gone through the filter. What I suspect can happen is most flushing procedures include running a can of strong detergent through the the transmission first for a few minutes. That chemical might attack the fiber plates and tear the friction material off. I don't know that for a fact, but I am confident that if a normal flush on a transmission with no developing problem were to CAUSE a problem, it would be common enough that we would know about and stop doing them.
The late shifting you described is likely the result of a sticking governor valve or throttle valve. If your transmission uses a vacuum modulator valve, also check the vacuum hose to it for leaks. When road speed overcomes throttle position, a shift valve initiates the up-shift. Varnish buildup can cause one of those valves to stick in its bore. They fit in there with a very tight clearance.
I had the same issue with my '88 Grand Caravan. I've used it to drag a tandem-axle enclosed trailer to the nation's second largest old car show swap meet for the last 15 years. The trailer is bigger and heavier than the van, but I've never needed the trailer brakes. 65 mph is a dream yet to come true, even when going downhill, because of the horrendous wind resistance. Given all that abuse, the transmission fluid and filter have only been replaced once, at 90,000 miles, because the $3.50 side cover rusted out and I already had most of the fluid drained for the repair. Today the transmission has 269,000 miles and has never been touched. (It has also been 13 years of daily driving, and about 100,000 miles since the oil was changed)! This is obvious abuse, not just neglect, but I'm doing it for a reason. I'm not suggesting anyone else do this.
A few years ago I was experimenting with the cruise control, and a poke in the wrong place with the test light activated one of the solenoids that pulled the throttle wide-open for a few seconds. I hadn't hit wide-open-throttle for over a decade. Immediately after that, I had the same delayed up-shift you're describing, and once in third gear, it wouldn't downshift at stop signs by itself. I had to manually shift to "L" to start out.
There is nothing that can be damaged by hitting wide-open-throttle, but that varnish that builds up will not get worn away where the valve never travels. The more I drove the van, the better the shifting got as the valves began to slide freely again. It took about six months for the last hint of the problem to be gone.
What you might consider is using an additive that's meant to dissolve that varnish. What I would do is use that to try to solve the problem, but then get it out soon afterward, especially if that is what's specified on the label. A simple fluid and filter change might be sufficient. That gets about half of the fluid replaced with fresh additives in it. The flush would get all the old fluid out.
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Thursday, April 30th, 2015 AT 7:25 PM
