Transmission service intervals and procedures are a highly controversial topic. There are those who believe a complete fluid flush is ideal, those who believe a transmission flush is ill advised, and then there are certain transmissions that the manufacturer claims are "filled for life" and do not require service. There are basically two options with regard to transmission service.
Power flush - Take the vehicle to a reputable service shop that has access to a power flush machine. Have them remove the transmission fluid pan, replace the transmission filter, and then flush new ATF into the transmission while simultaneously removing the old fluid.
Basic transmission service - Drain the transmission fluid, drain the torque converter (if applicable), replace the transmission filter (requires removal of the transmission pan), then top the transmission off with new ATF.
The benefits of a power flush are obvious, as all traces of the used ATF are theoretically removed. A basic transmission service does not remove ATF from the cooler lines and passages inside the transmission that hold fluid. However, the power flush is widely controversial. Keeping in mind that there are obviously strong opinions on both sides, some argue that a power flush 1) may dislodge particles and deposits that otherwise pose no threat, 2) often use detergents and/or cleaning agents that may precipitate complications, and 3) force fluid to flow in unnatural directions, placing unwanted strain on seals. Additionally, some service shops may sell you a "flush" but will neglect to drop the transmission pan and replace the filter; alliterate, time is money.
In the end, it's hard to argue with the traditional service procedure, which is less controversial and has been a time proven method of prolonging transmission life and maintaining shift quality. As an added bonus, it's a relatively simple procedure for the DIY mechanic who has access to basic hand tools. Every drop of ATF may not be replaced, but done periodically the transmission will, for all intents and purposes, face the same inherent benefits. To preserve shift quality and promote longevity, 25,000 to 30,000 mile intervals are an industry standard; more frequent service may be advised if you tow frequently. You should always refer to your owners manual for the OEM recommended service intervals.
With regard to the "filled for life" precept, we simply don't see the merit in any claim that attempts to justify not changing a fluid that endures such harsh conditions. The following procedures are based on Ford's 4R70W, but the basic approach is largely universal for many automatic transmissions. This may not apply, however, to transmissions that are non-serviceable or "filled for life". Some modern transmissions may also feature an auxiliary transmission filter that must be replaced; refer to your owners manual.
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• Identify whether or not you're torque converter can be drained. Not all transmissions have serviceable torque converters. Remove the plastic/rubber inspection grommet from the bottom of the transmission bellhousing so that the torque converter is visible. If no such access port exists on your transmission, you will not be able to drain the torque converter.
• Rotate the engine clockwise using the center bolt on the crankshaft pulley until the torque converter drain bolt is visible through the access port. Alternatively, bump the starter until the torque converter drain bolt is visible. If there is no drain plug on the torque converter, it is not serviceable and you may skip this step.
• Place a suitable drain pan under the transmission bellhousing, then remove the torque converter drain bolt (11 mm socket for the 4R70W) and allow the torque converter to drain. Once the torque converter has drained completely, replace the drain plug and access port grommet.
• Place an oil drain pan beneath the transmission fluid pan. Generously loosen the (2) x front bolts of the transmission pan until they are roughly halfway removed (10 mm socket). Loosen the remaining (12) bolts slightly until fluid begins to drain from the front of the transmission pan. You may need to lightly pry the front lip of the pan open so that fluid begins to drain.
• Support the transmission pan with one hand while completely removing all (14) transmission pan bolts, then lower the pan and empty the remaining fluid into the drain pan.
Note - some transmissions may have a drain plug on the transmission pan, in which case the transmission/pan should be drained completely before removing.
• Thoroughly clean the transmission pan with a mild solvent and a clean, lint free rag. Remove all particulates stuck to the magnet in the bottom of the transmission pan.
• The magnet captures metallic clutch material as the transmission wears. An accumulation of fine particles on the magnet is completely normal. However, larger pieces of metal or debris are not normal and signify that the transmission has broken parts.
• For many Ford transmissions, like the 4R70W/4R75W, the pan gaskets are reusable so long as they are not cracked or deformed. If in doubt, play it safe and purchase a new one. If reusing, clean thoroughly, but do not use a solvent.
• If you find a plastic plug in the bottom of your transmission pan, don't be alarmed - when the dipstick tubes are installed from the factory, they are plugged. The plug is later dislodged once the transmission dipstick is installed and ends up in the bottom of the pan.
• To remove the old transmission filter, work it side-to-side while pulling downwards from the point that it attaches into the underside of the transmission. Before installing the new filter, make sure that the rubber seal has been removed with the old filter and that no part of it is stuck in the mount.
• Install the new filter after coating the rubber seal/gasket with clean ATF. The red arrow in the image at left (click to view fullsize) is where the filter inserts on a 4R70W or 4R75W. Be sure the filter is fully and properly inserted before reinstalling the transmission pan.
• Reinstall the transmission pan and gasket, hand tightening the pan bolts. Coat the transmission pan gasket with fresh ATF to ensure a positive seal.
• Torque the transmission pan bolts to 108 - 132 in-lbs (inch pounds!) in a criss cross pattern for the 4R70W and 4R75W.
• Refill the transmission with new ATF through the transmission dipstick located on the passenger side of the engine bay. Fill until fluid level reads full on the dipstick indicator.
• Start the engine and move the shift lever through all positions (P, R, N, D, 1, 2), then return to park (P) and recheck the transmission fluid level. Add fluid as necessary.
• Drive the truck, allowing the engine and transmission to come to full operating temperature. Check transmission fluid level, add as necessary, and repeat this process once more.
• The fluid level needs to read normal (within the crosshatched area of the dipstick) while the transmission is hot - the fluid level of a cold transmission only serves as a baseline for this process. Do not overfill the transmission.
• Always mind the OEM ATF specification. The 4R70W and many other Ford transmissions require MERCON V automatic transmission fluid. Refer to the owners manual for your vehicle's OEM fluid spec. Do NOT use universal fluids that "meet" the OEM spec. Your transmission fluid only needs to meet one specification, and the OEM fluid will meet it best. We highly recommend only using Genuine Ford/Motorcraft automatic transmission fluid.