I have heard that in higher mileage vehicles (My 2000 4WD Blazer LT has 132,000), that changing the transmission fluid can actually be detrimental to the internals of the transmission, and you are better off just leaving it alone. I haven't had the fluid changed in awhile, and just wanted to be certain before attempting it...

 

This is chicken or egg stuff. Bought mine w/130K and changed it, had never been done. Positve it shifts smoother. 156 now no apparent problems. You will get diffrent answers on this. I just sleep better with clean fluids.

 

That's what is wrong with getting partial wisdom. Not necessarily. It DOES apply to vehicles you buy and have no idea of the service history.
Since you had it changed yourself and I assume within about 50K miles, why not.

I'd like to point out that it was the 'Transmission Flush' syndrome that started concern about all this.

Probably invented by AAMCO, I know they pushed it a lot. Yeah.. they're REALLY concerned that your tranny stays in top shape!

 

Jiffy Lube flushed the transmission in my 1995 Silhouette (4T60-E) at 158,000 miles. A week after I changed the fluid and filter - mind you it was shifting fine and the fluid was clean, they told my wife it was dirty and charged $150. At 165,000 miles the trans was toasted.

 

It CAN cause rapid failure. Its like this...as the trans wears all sorts of clutch material,ultra-fine metal and contaminants are released from parts. If the oil is allowed to get dirty and the detergents wear out this stuff settles in all the nooks and crannies in the unit. For the most part it isn't causing any issues as it isn't circulating through the system.

When you add new fluid (ATF is VERY detergent strong) over the next few hundred miles this stuff is loosened up and starts to circulate through the unit(like moving around the gravel in a seemingly clean fish tank) and can clog solenoid screens, affect seals...and most of all cause valves to hang.

Most automatic transmissions dont get to the point of needing to be rebuilt because a clutch or band "just wore out" evenly over 100k miles. The actual friction material is about .010 thick and you can pull clutches out of a 100K mile unit and it will be the same thickness as a brand new one if it hasnt "burnt down" and failed. 90% of the time the clutches burn up because something else failed (hard or leaking seals, leaking rings, low oil, a cracked piston, failed pressure control system etc) and the clutch was allowed to slip and burnt up in a few hundred mile span. This is why transmission repairs have such a high comeback ratio...too many people just replace the clutches or bands and fail to find or fix the real issue and the failure repeats at increasingly more frequent intervals.

You can smoke brand new clutches in a mile if they are slipping....So, 1 stuck valve and you can kiss a trans that has made it 150k miles goodbye. I have worked in transmission shops for 23 years and any HONEST shop will drive your vehicle before filter service and make sure it has no problems. I actually have my guys ALWAYS drain the fluid into a clean container and if it looks burnt to a crisp or the pan is full of "mud" I will stick a new pan gasket on, put the old oil back in and send the customer on their way for free if they choose not to repair a impending issue. I'm not saying this is going to be the case on every vehicle but if your trans has over 100k on it and is working fine IMO just keep it full. A new filter and oil never fixed anything except for RARELY a TH400 filter will clog. This is why manufacturers don't worry about installing inaccessable filters on modern cars, If something is clogging the filter it is likely pieces of internal transmission parts and the thing needs to be opened up anyway. Todd

 

^^^^^^^^^^^ This!



There's other factors like never getting your trans fluid above 200 deg. Checking your pan like above and not messing if there's a lot of hard sludge in corners, metal on the magnet, etc.
But I dumped Lucas in a tran that was slipping in hot weather {but no burnt smell}, stuck an aftermarket cooler on it and drove it another 150K .. to 290,000 mi. Damned if I was ever gonna THINK of dropping that pan.

Other 'oopsies' like the guy on Ford Forums who changed his filter, looked up in the bore for the filter neck saw some gunk and thought nothing of it, until two days later when alla sudden it wouldnt shift. THEN he realized...

 

Thank you Todd!! That was the most informative and we'll written forum post about transmissions I've ever read. If there were more people like you, shops would be bored. Thank you for taking your time to share your expertise, it is appreciated.

 

I changed the filter and fluid in my 04 at about 180k, ONLY because I had a shift solenoid fail and I lost 2 gears... and I had to drop the pan and dig in to change it... I replaced every solenoid on the valve body and just went ahead and did the filter and fluid change while I was at it. No ill-effects and that was about 3 years and about 30k miles ago. But the fluid was surprisingly clean that came out (obviously well used, but not burnt and not particles in it or in the pan)