The myths of the Deadly Trans Flush:
This is one subject that leaves a lot of people in mistrust, doubt, wrongful information and a bitter feeling when mentioned. First of all let me clarify what a transmission flush REALLY does since it is very commonly misunderstood. A transmission fluid flush machine does nothing more than hook up in line with your transmission cooler. One line hooks up to the supply coming from the trans and the other line hooks back up to the return side, which is in series with the cooler. This IN NO WAY flushes the inside of the transmission! All it does is take the old fluid from the transmission side out and replace what goes back in with fresh new fluid. There is no magic wand that crawls inside the transmission and pressure washes anything to disturb particles and gunk buildup. The transmission fluid flush machine has a bladder inside that transfers existing pressure that comes out of the transmission, roughly 10-40 psi, and passes it along to the opposite side of the bladder containing new fluid to properly displace the same amount that came out. NOW with that said here are some problems. I have seen many times where a shop did a flush and NEVER checked the trans fluid before OR after the flush! I have found cars up to 4 quarts low on fluid because of this neglect and leaving the customer stranded with a burned up transmission. It is more of a user or operater problem than the machines fault, its just putting back in what came out.... or is supposed to. During transmission fluid flushes there is also an additive that commonly is added before the flush to help clean the inside of the trans and can be harsh if the transmission has been very neglected over its life. After a transmission ages, and the fluid ages respectively, things chemically change inside the trans as far as friction materials and the adhesives on them. If you never change your trans fluid and it has very high mileage on it or is severly burnt then NEVER expect a flush to magically cure problems or expect new fresh fluid with strong detergents not to cause problems. It is a shock to the transmission to remove all of the old well past due fluid with all new fluid and the new fluid with its fresh strong detergents can break down the friction materials and adhesives that bond them to the clutch plates. On a vehicle with higher mileage and an unknown past it is always much safer to drop the pan and change the fluid and filter, that way you are giving the fluid a chance to slowly adapt, new with old and not be such a chemical shock to everything. Again checking the fluid level is a MUST! Neglect is the biggest problem with transmissions and making sure the fluid level is full and periodically serviced is the best you can do for it. Another problem is that new transmission fluid has friction modifiers in it and tend to keep shifts smooth. A lot of times if you change trans fluid that is old and burnt with new fluid it will feel like it is shifting softer, that is because the fluid is doing its job again. Fortunately on computer controlled transmission this can be troublesome since the pcm controls shift timing and now has to play catchup to firm up shifts because it has been softening them up over time. Not a common occurance but again neglect will only lead to worst case scenarios. I have nothing wrong with flush machines IF used properly and not used on a trans that is just plain worn out, again it is not a solution fix all, it is maintenance pure and simple and if neglected too long then only time will tell longevity of the trans. I hope this clears up some of the confusion and mislead information that has been floating around for years. I have NEVER seen a flush machine cause a failure, I HAVE seen an operater of a flush machine cause a failure.
Originally Posted by SnowDrift
