Some automobiles and pickup trucks have factory or aftermarket transmission oil coolers, either attached in front of the radiator or built into the radiator itself. The cooler is a simple device: transmission oil is pumped by the transmission oil pump through lines to the front of the vehicle where it is channeled through several finned pipes. If a transmission oil cooler develops a leak, the average backyard mechanic can remove, repair and replace an aftermarket transmission cooler in about an hour.
Determine the source of the leak. This may sound easy, but oil can be deceptive and drip from a location other than where the leak is. The rubber lines could have a wear hole or a puncture that may drip to the lower steel lines, making it appear to come from where the two meet, not the actual location of the leak. Wipe all of the oil with a towel, and make sure the line is as dry as possible. Start the vehicle and watch for the leak to appear.
Uncouple the leaking line from the cooling system and replace it. Start by placing the drain pan below the lines to catch any spills. If it is a rubber line, turn the hose clamps with a screwdriver counterclockwise and pull the hose from the nipples on either side. Replace the line with one of equal length and diameter by placing hose clamps on the ends, then pressing the ends onto the nipples. Secure the hose clamps by turning the screw on each clamp clockwise. Some clamps could be a "pinch" style clamp, which require that the tabs be pulled together with a pair of pliers and the clamps moved.
If the line leaking is a steel line (either from a bad gasket or puncture), remove it from the transmission by turning the line nut counterclockwise or by simply pulling the line free. Steel lines must be sized to fit the vehicle. Attach the steel line (with a new gasket) into the transmission by pressing it into the opening and turning the line nut clockwise, if applicable. The other end routes to the rubber line, which uses a hose clamp.
Start the engine and check the transmission for the proper fluid level, filling as necessary. Usually a transmission cooler will require an extra quart to accommodate the lines and condenser.
