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There seems to be a lot of debate regarding when and how to service an automatic transmission, and what, exactly, a standard service consists of. Even though I'm not in the industry anymore, I was a certified diagnostician and tech in the automatic transmission repair industry and a service manager at a transmission shop. Here's my professional OPINION regarding automatic transmission service intervals and what's required.
Regarding service intervals… I’ve seen the most durable transmissions fail as early as 80,000 miles for no other reason than old, nasty fluid and a dirty filter. I’ve seen the most fragile of transmissions last 300,000 miles because they were serviced regularly! I recommend a normal pan/filter service (NO FLUSH) at 15k – 20k intervals for any vehicle that drives in mountainous terrain, tows a trailer, goes four-wheeling, or gets a lot of stop-n-go use. Vehicles that see mostly highway use in the mid-west can probably go as far as 30k/mi between services, but that’s the MAX!
It needs to be understood that ATF, whether synthetic or petroleum based, has several additives that make it perform is job correctly. There are FRICTION MODIFIERS that help the clutches grab (and release) when they're supposed to. There are VISCOSITY ADDITIVES that help maintain the "weight" of the "oil" for proper lubrication and hydraulic properties. There are DETERGENT ADDITIVES that help control deposits by suspending particles in the fluid until they can be filtered out. All of these additives need to be replenished from time to time, as they are "burned away" with normal use. The BASE oil in ATF doesn't really break-down the way engine oil does, so it doesn't really need to be replaced in its entirety, just refreshed. Five to Six quarts of ATF contain enough of these additives to ‘refresh’ the five to six quarts that remain in the torque converter and cooler lines when the pan is dropped. Therefore, it is not necessary to flush a transmission at normal service intervals.
A normal service STARTS with dropping the pan, which releases about 1/3 to 1/2 of the total volume of ATF. Only by dropping the pan can you replace the filter, so there's no way around this if you want to do a correct service! Once the pan is off, remove and CUT OPEN the filter! By examining the contents of the filter and the sediment on the bottom of the pan, an educated tech can accurately assess the condition of the transmission as a whole. It's ok to have SOME material in the pan and filter, but if you see any chunks (1/16" or bigger) of friction material or metal shavings, take the filter (contents intact) to an expert for evaluation, because you have problems. The little ½” circle of grey past in the middle of your pan is normal wear… Don’t panic, just clean it and continue with the service…
Modern automatics have a lock-up style torque converter that contains one pressure plate and one ring of clutch material. This clutch is very susceptible to overheating and slipping (anyone heard of the dreaded P1870 GM code?) and will start to crumble and shed material (that will end up in the filter and pan) if it starts to carbonize from heat. One preventative solution is to keep fresh friction modifiers (less slippage) and viscosity additives (better hydraulic pressure) in the fluid via regular maintenance.
Shops that suggest a flush and fill service instead of dropping the pan and changing the filter should be avoided. Shops that try to sell you a flush along with a pan/filter service at normal intervals are just trying to pay for their new flush machine a little sooner. A reputable shop typically charges about $70 to $80 for a service including bulk ATF, pan gasket, filter and labor. Chevy S-10 4x4’s usually warrant an extra ½ hour labor because the exhaust x-over pipe is in the way and needs to be moved, so be prepared to shell-out another $35 - $40 for that.