Originally Posted By: Jim Allen
Originally Posted By: onion
But I see a [censored] of a lot of unwarranted certainty on the matter from people around here- backed up by nothing in the way of evidence. It's common knowledge that return oil from the cooler IS the lube oil for parts of LOTS of transmissions. How can ya'll be so certain that the forward clutch bushings (some of which ARE turning with the trans in park) are not among these parts that are lubed by cooler return oil?
No unwanted certainty on the cars/trucks I own, I can say because I have looked in the manual.
C6 Ford. '86 F250HD: Cooler flow lubes planetary gears. Forward clutch is lubed off the main pressure circuit.
BAXA Honda, '00 Accord V6: Cooler is at the very end of flow and dumps right into the sump. It appears everything gets oil prior to the cooler, so there should be no issue with a cooler line exchange at all.
4R75E Ford, '05 F150HD: I'm the least sure on this trans because the manual doesn't have a hydraulic flow chart perse, but indications are that cooler return goes to the rear of the trans. I'm less sure about forward clutch lubrication but on every Ford trans diagram I have seen, and GM as well, it's lubed off the main hyd circuit.
Last time I got into this argument- a couple years ago- I looked up the hydraulic schematic in the factory service manual for my 4T60E. The schematic wasn't 100% clear, but like the example you cite, it appeared that the return oil from the cooler lubed only the planetaries. So in the case of that transmission and similar transmissions, I'm reasonably sure that a 'bucket flush' is a safe procedure.
But there are dozens of different makes and models of automatic transmissions out there. And without specific info, I would be hesitant to categorically state that a 'bucket flush' is 100% safe in all cases. Anybody can make that CLAIM... but backing it up is another thing entirely. We KNOW that cooler return oil is often used for lube. Is that lube oil required only for parts that turn while the vehicle is moving? We can be pretty sure of that in several common applications... but for all applications? That would be conjecture at best.
And as far as I'm aware, all professional transmission flush systems pump new ATF back into the cooler return line. If a bucket-flush was 100% safe and effective for all transmissions... why isn't it a common practice in the repair industry? Why are shops paying big $$ for completely unnecessary equipment?