Drain/refill vs diy flush?

[DoubleWasp]

Originally Posted by caprice_2nv
Agreed. We flushed lots of high mileage transmissions at the dealerships I worked at and I can't recall any "coming back" shortly after with issues. I'm sure it's happened, but more likely when someone already had shifting or slipping issues and tried the fluid change to see if it would fix it.


I actually had this happen to a transmission. Had nothing to do with the fluid though. Had a clogged vacuum line causing terminal issues.

Mechanics have been spreading this misinformation for a long time for one kind of practical reason: Too many people want a fluid change because they are already having terminal issues, and they don't want to be the guy who hears, "It was fine until you touched it."

People are stupid. I once pointed out a water pump that was so bad that the pulley was cocked at a 15 degree angle and the belt was half way off of the water pump. Customer refused to have the pump replaced. Guess what they called up to tell me when their belt flew off the pump that night?

"It was fine for weeks before you touched my car!!!!!"

Yes, customers can be THAT stupid.

So I don't necessarily blame mechanics for spreading that fertilizer, but it doesn't change the fact that it is misinformation.

 

[caprice_2nv]

As far as the flush vs drain argument, I guess it depends on the vehicle. I have 2 auxiliary trans coolers mounted in front of the rad on my Oldsmobile. Easily accessed rubber hose and clamps. I can flush 12qts through it in less than half an hour if I don't have to change the filter. Pan drop and filter is a big pain. I have to do it this time because I skipped it the past 2 and I think it's been in there for possibly more than 60-70k miles now.

 

[4WD]

Right, and I'm not going to touch the cooler lines … both of mine have factory trailer packages and I personally prefer to leave that alone. (In the past I had TruCool add on and Gates hose)

A key difference in this discussion is some of us start a drain/fill program as the original owner … it's a more difficult situation to look at a vehicle purchased with some miles.

 

[T-Stick]

After reading through all of this ... Its still seems clear to me that all the evidence suggests that new ATF in a neglected transmission will tend to clean and circulate more particulates and risk failure. This is why in a neglected transmission doing one drain and refill with time between is the safer approach. You typically need about 3 cycles to replace most of the fluid. Each cycle of refill gives a higher content of new fluid which creates more active cleaning and each drain removes some of those dislodged particulates. This surely has less risk than introducing all new fluid with aggressive cleaning and then removing none of the particulates for the next 30-50k miles. I wouldn't leave a neglected transmission neglected--I would do the drain refills over a period of time.

 

[OnTheRocks]

Originally Posted by caprice_2nv
Agreed. We flushed lots of high mileage transmissions at the dealerships I worked at and I can't recall any "coming back" shortly after with issues. I'm sure it's happened, but more likely when someone already had shifting or slipping issues and tried the fluid change to see if it would fix it.


Depending on when you worked at the dealership, technology could have been different. Old machines actually flushed with pressure from the machine. The machine flush pressure could have caused damage. Modern machines only exchange fluid through the transmission lines. Fluid is pushed out by the transmission's pump.

A lot of cars already have problems. Then the owner blames it on the last guy to lift the hood. "It didn't do that, or make that noise, before I brought it here."

 

[DoubleNickels]

Drain & fill is good. Flush is good. I prefer complete fluid exchange. Only doing a few quarts is better than nothing. With the car lifted, and I already have the tools out, I always drain and fill 3 times. No reason not to.