how often should the transmission oil be changed?

[bigi]

Manual no way, if you go by what the manual says you will never do any maintenance on it, 20,000 is more than enough for any fluid the cleaner the better overheating and fluid breakdown is the biggest problems in transmissions and having the catalyst converter inches from the transmission don't help matters any.

Based on this logic, weekly changes or better yet daily oil changes are advised.

 

[Pacfanweb]

50k is a good number. All manufacturers are going to stretch the limits in their recommendations to make it look like their product doesn't require as much maintenance.
Always remember, it's not in their best interest for your car to last forever. They don't make money off servicing it, they make money off sales.

The dealer makes money servicing, but the manufacturer only cares about sales. So 50k is a good rule of thumb for tranny fluid. Maybe stretch it to 75k for synthetic, but I'd not go further than that.

 

[mindless1]

1) Consult the owner's manual to see what is recommended.

2) Research your specific vehicle model and tranny to see if it is known to have problems and what the owners' evidence is of fault and resolution. Sometimes they might have sludged ATF and sometimes they might have changed it religiously and it made no difference.

3) Consider your climate and driving habits/hills/load. If you're likely running the tranny hot, it should be changed a lot more often.

4) Check the fluid. If it looks or smells quite burnt it is time to replace it.

5) If you have very high miles between changes (depends on specific transmission and as above the other stress factors, but generally around 100K mi then shorter intervals for each subsequent change), it may be too late to replace it and putting fresh in may just dislodge gunk that causes clogs.

6) If it starts shifting funny and is intolerable (replacement imminent if all else fails) you might try changing the fluid since you're running out of options.

7) You should always use synthetic fluid. Anything old enough that it didn't come from the factory with synthetic, is likely too old to benefit from a fluid change (see #5 above). There may be rare exceptions if there is no alternative to a special OEM formulation of ATF.

It might have helped if you mentioned the specific vehicle and engine/tranny in it, and the driving conditions. If there was only one magic number, nobody would have to ask.

 

[BonzaiDuck]

I replaced my auto-tranny in 2004, in a car that was 7 years old in 2002 when I bought it. Thus, the replacement tranny has been through ~ 63,150 miles and an additional 17 years. The Tranny was flushed after 36,110 miles, or ~27,000 miles ago.

The factory shop manual -- a $100 tome the size of an encyclopedia volume -- shows the block diagram of a maintenance schedule measured in thousands of miles, and only mentions "checking" the transmission fluid over increments spanning 100,000 miles. It does not mention "changing" the fluid (Dexron III) or "flushing" the transmission.

The tranny cooling lines connect to the radiator, and these radiators are made of aluminum with plastic. To flush the tranny, you have to disconnect the cooling lines at the flange nuts near the radiator, and connect them to a machine. The connections to the radiator can develop a leak. The leak is harmless enough, provided that it only leaks a few drops here and there and you can keep the tranny fluid topped up with Dexron. I am still making observations, but it appears that something like the Valvoline "stop-leak" formulation which is otherwise Dexron III with an additive like brake fluid -- can attenuate such a leak.

So I'm beginning to think that I should never have let my mechanic flush the transmission. And I'm looking at the mileage milestones, to assess what I should do this year. There's nothing wrong with the way the tranny works -- it shifts smoothly. There's nothing wrong with the tranny fluid -- still looks like cherry juice or pink drops on the drop pan. Should I have the filter screen cleaned? Should I have the fluid replaced?

I think I should just let it go for the next five years. Five years at the current rate is merely another 12,500 miles added to the odometer.

 

[fritzfield]

I think that once every 18 YEARS is about right.

 

[BonzaiDuck]

I think that once every 18 YEARS is about right.

Unless you frequently tow 2,000 lbs, go 4-wheelin' in wildlife refuges, and any number of things that would put a strain on a tranny . . . .