The first thing we need to understand is that transmission fluid is full of great cleaning properties. Years ago our Grandfathers, Fathers, Uncles, and some Technicians would pour it into the crankcase and fuel tanks to help keep engines clean of sludge that gummed up piston rings and everything else in the engine.
The real question is, when is the right time for a pan service or a transmission flush? It is best to perform a flush only if you know the transmission has had regular fluid maintenance performed or if it has under 70,000 miles. There are two occasions when it is not wise to flush a transmission.
(1) if the transmission is having mechanical shifting issues (flushing is not recommended.) it may cause the transmission to fail quickly due to its cleaning capabilities.
(2) If the transmission has over 70,000 miles or you are not able to verify transmission fluid maintenance services. (flushing is not recommended.) It may cause needless premature transmission failure due to its cleaning capabilities. Remember, new transmission fluid is a detergent, it can dislodge sludge, and debris quickly causing valves to stick, plug solenoid screens and fluid passages.
I have seen the customer return to a Kwik lube facility and say, “at least the transmission was working before you flushed it.”
If the transmission has a Torque Converter shudder or you cannot confirm that regular fluid maintenance has been performed prior to 70,000 miles, it is best to perform a pan service with a good transmission fluid conditioner like Lube Gard or BG. This way the 4-6 quarts of new fluid and conditioner mixing with the old fluid will slowly and harmlessly clean the transmission. Then drive the vehicle 2,000 miles and perform the transmission fluid flush, and every 30,000 miles thereafter to protect and prolong the life of your transmission.