Rebuilding your own automatic transmission can save hundreds, if not thousands, of dollars over paying someone else to do it or buying a new one. Mechanics routinely charge hundreds of dollars for work that any properly trained minimum-wage employee can do in less than an hour.
Though the idea of rebuilding a transmission seems intimidating, fear not. The secret: get the right directions for your transmission, and follow them to the letter. Haynes is an excellent resource for transmission rebuild manuals, especially if you have an older domestic vehicle.
Most automatic transmissions can be disassembled and put back together with nothing more than basic hand tools (sockets, extensions and screwdrivers). But there are some specialty tools that will make things go smoother.
A universal spring compressor runs about $120 and will make it a lot easier to remove and replace the retaining springs that keep the low/reverse piston and return spring in place. This can be done using flat-head screwdrivers, but it's easy to damage the parts that way.
If you rebuild your fluid pump (kits typically include the parts), you'll notice in short order how difficult it can be to realign the gears inside a central shaft. Get a pump-gear alignment tool. It costs about $35 and could save you hours of aggravation.
Don't snatch up the cheapest rebuild kit available. Buying a kit from an established distributor like AutoZone, Advance or Pep Boys will ensure that your parts are at least up to factory specs. Kits come in four basic flavors:
Complete: Include gaskets and seals, clutches, bands, new metallic pressure plates, replacement springs for the valve body and a new filter. These kits are generally only available directly from manufacturers, since they are preferred by professional shops.
Refresher kits: Include all but the metallic pressure plates and replacement springs. This is what you'll find at auto parts stores---it's the basic kit for rebuilding a transmission.
Performance: Include everything found in either of the kits above, depending on what is required. Most include heavier-duty clutches, additional clutch plates for stronger clamping, Kevlar overdrive bands and a shift kit.
Shift kits: Though not technically a "rebuild," shift kits let you alter the shift firmness and timing of the transmission. Because it allows less slippage, a shift kit alone can strengthen the transmission's torque capacity; but this should never be used as a bandage to cover up worn clutches.
You may want to consider doing other preventive maintenance while you're in there rebuilding the transmission.
Oil pump: The pump's internal gerotor gears can wear down, making transmission fluid pressure inconsistent or weak and throwing metal shavings into the oil pan. Go ahead and replace it.
Torque converter: Stock torque converters are generally reliable, but the welds can weaken---and metal shavings from the old internal bearings may have killed your tranny in the first place. You can gain performance by moving to a higher stall torque converter.
