Changing Automatic Transmission Fluid - Completely

 

Changing Automatic Transmission Fluid - Completely

 

 

You probably know you should change you Chevelle's automatic transmission fluid every 50,000 miles or less.  Less meaning, if you do a lot of stop and go driving, towing, or subject it to other severe duty, as in "spirited driving".  Did you know if you just drop the trans pan and replace that fluid, you're replacing less than half the total amount?  The remainder is in the converter, trans passages, cooling lines, and cooler (factory coolers are inside the radiator).  BTW when doing this you should change the filter too (inside the pan).  Filter and gasket kids are available at most auto parts stores.  

So how do you get the remaining fluid out?  Well you can to it the easy but expensive way by taking it to a dealer or shop that has a specialized machine that flushes the entire system.  This will cost you approximately $100-$150 before any additional work is done that they recommend (and you know they will).  Plus you don't know for sure if they got ALL the old fluid out.  If you're a do-it-yourselfer like me, you won't let anyone else touch your "baby", you can do it yourself without any fancy machine.

You do this by basically intercepting the "out" trans cooler line at the radiator and letting the trans pump out the remaining old fluid as you replace it with new.  Here's how:  On most older Chevys the lower trans cooler line at the radiator is the one that's the "out" line from the trans.  After pulling and draining the pan, replacing the filter, gasket, reinstalling the pan, and replacing the same amount of fluid you drained (approx 2-4 qts depending on trans), disconnect the lower cooler line at the radiator.  A tiny bit of fluid will leak out of the radiator.  Take approx. 2 or 3 ft of 3/8" fuel line hose and a worm clamp and force onto the flared end of the line about 1 inch.  Tighten the clamp behind the flare.  Place the other end of the hose in a drain pan.  You might want to tape the hose to the rim of the pan so it doesn't come out.  Start the engine briefly and shut if off to make sure the lower line is the trans "out" line (a helper comes in handy).  If all is well, start and run the engine til about one quart of old fluid is in the drain pan.  Stop the engine and add a quart of new fluid to the trans fill tube.  Start the engine again and run it til another quart comes into the drain pan.  Stop the engine and add another quart.  Keep doing this until the dark older fluid stops coming out and new red fluid is.  It will probably take another 5 to 7 quarts.  If you get air bubbles at anytime after initial startup, stop the engine immediately and add 2 quarts to the trans because you've pumped the trans pan almost dry.  After adding the 2 quarts you can continue.  When finished, remove the fuel line hose from the cooler line and reconnect to the cooler line to the radiator.  Then fill the trans to 1/4 below the 'ADD" mark on the dipstick.  Do not add more until the engine and trans are fully warmed up after driving a short distance.  The fluid expands when warm and you don't want to overfill it.  Then after fully warmed add more if necessary to between "ADD" and "FULL" marks.

Factory spec fluid for Chevys is Dexron-Mercon.  It will take around 8 quarts for a Powerglide and TH-350, and around 10-11 quarts for a TH-400.  It can be a messy job, so protect the floor with news papers or cardboard and have plenty of paper towels or rags handy.  But just think of all the money you'll save and the satisfaction of knowing you did the job right, yourself.