Originally Posted By: wafrederick1
A flush with the machine will hurt the transmission.Problem with the machine is that it backflushes the dirt,crud,metal shaving and clutch material back in the valve body resulting in sticking valves.It also uses too much fluid.I know a transmission shop in my area doing lots of valve body cleanings,rebuilds due to flushes with the machine.I even had to replace one transmission in a 1999 Monte carlo once,acted up after a flush with the machine.Read some of the factory shop manuals,GM does not recomend flushing their transmissions.Honda does not also fearing transmission damage.A transmission flush with the machine is huge no and waste of money.
Sorry to correct your reply but you are wrong about how a transmission flush machine works. None do backflushing as you mention. The flush machine merely takes old ATF pushed out through the cooler hose and dumps it into a used ATF bucket and pumps new ATF into the pan so the transmission pump can pick it up (as it normally does) and pump it through the transmission. The flush machine has absolutely NOTHING to do with pumping or backflushing ATF through the transmission. It does what anyone could do by draining ATF out one cooler line and keeping the pan filled with ATF.
Now many shops will add a flush agent to the transmission prior to the flush (or kero) and run the vehicle for 20 minutes then do the flush to loosen up crud.
There is nothing wrong with the concept of a flush. Where things can go wrong is when you have 150K miles on original fluid (looks like driveway sealer) and someone does a flush without first dropping and cleaning the pan and replacing the filter, since at that mileage it has to be covered with crud.
New ATF will loosen up crud in the transmission no matter how the new AFT was introduced (flush or a drain and refill of the pan).
The best way to service a transmission is to drain & refill the pan then do a cooler line flush. Few shops bother to drain and refill the pan, they just blast through a few extra QTs of ATF to overcome the mixing that will happen in the pan. If the current ATF was really neglected, then the pan should be dropped and cleaned (filter replaced if a felt filter) and then do the flush.
Adding a Magnefine filter during (or at end of) the flush will prevent any loosened crud not flushed out from being recirculated.