Transmission oil change

when changing the transmission oil is it better to remove the pan and replace the filter or hook up a device that changes the transmission oil without having to remove the pan?

That depends on the torque converter. Some have a drain plug , some don’t. When you drop the pan and can’t drain the converter, you’re just changing half of the total oil. This may be ok depending on mileage and the condition of the old oil.
Sometimes we do both to change all the oil. Drop the pan to change the filter but use the flush machine to get all the oil.

I would always (always, always) drop the pan and change the filter before hooking up a machine. And on a routine basis will only do the pan & filter w/out full fluid exchange.

I’m in the camp of only needing to drop the pan, replace the filter, and refill.
Do that every 30k-50k miles and you shouldn’t have any need for a flush.

I believe flushing is OK after a long set of “ifs” is satisfied. That list includes:

  • The pan has already been dropped and cleaned
  • The filter has already been replaced
  • The flushing machine operator is skilled (not a 17 year old new-hire).
  • The flushing machine uses the exact fluid required for your vehicle, and not some generic fluid with additives thrown in (which is often done).

With the above long list of “ifs”, and with the fact that millions of transmissions over decades have done fine with simple pan drop/filter changes, I don’t see a big need for flushes.

Many shops have transmission flush machines primarily as a profit-maker. Be careful.

I would suggest that even with the FLUSH you don’t get all the oil changed. It depends on just how much fresh fluid they want to run through there.

A flush does not shove out the old and replace it with fresh, it really just mixes with the old diluting it.

Just to clarify, you are talking about an automatic transmission, right? I ask because for ATs, it is usually referred to as transmission fluid, but for MTs, it is usually referred to as transmission oil.

For an AT, the only device you need for changing the fluid without dropping the pan is a drain plug. In my opinion, transmission flushes are unnecessary if you change the fluid on schedule.

Joe! You are a riot! Have ever operated or even seen a transmission exchange machine? They DO replace ALL the transmission fluid!


But they STILL leave all the trash in the pan AND the filter. Its unfortunate though that something that could be a good thing is taken for granted and is just being used as a quick money maker. Ninety percent of the shops that use flush/exchange machines tell you that the filter does not need to be replaced and I have even heard of several shops telling these poor uninformed customers that the machine will CLEAN THE FILTER. You can hardly blame them though because I had a salesman come into my shop one day several months ago trying to sell me a flush machine. I listened to his pitch and he told me how much money I could make flushing transmissions. When I asked him about the filters and pans he just smirked and said “You dont need to do that, it takes up too much time” What a crock… This guy is making his money selling machines to auto shops filling them full of BS about them making hundreds of dollars a day by selling flushes and these shops are selling the flushes and performing them improperly. Its all about the $$, not the customer. I LOVE handing a bill for a transmission overhaul to a fast lube shop, corner repair shop or tune up shop that has screwed up someones transmission and are now paying for their errors. Oh yea, about the salesman, I told him that I am only a rebuilding shop, I dont deal with R&R’s so I have no need for a flush/exchange machine. I did show him the transmissions on my ready shelf and on my pending shelf AND on the bench which were there because of improper servicing. I still believe that only transmission shops should have access to such equipment.


No, I have never operated the machine. I was told that they just used a flush type exchange. Over a long time here I have found that you are usually right, so I will take this as something new I have learned and will accept your comments as valid unless I find a reason to question them.


In MTs, we’ve always called it ‘Gear Lube’ or ‘Dope’. I never have referred to it as ‘Transmission Oil’. Every time I buy some, it is labeled ‘Gear Lube’.

Also, most transmissions today have a true filter, not a ‘bug screen’. Even with a drain plug, you still need to drop the pan and replace the filter.

Joe makes a good point. Allow me to expand on it.

If you could drain out all the fluid first, flush it second, completely drain it again, and then fill it, in four separate steps, you would basically have all new fluid. However, since only about 2/3 of the fluid comes out when you drain it, some of the flushing fluid must come into contact with the old fluid. When that happens, they mix, and you would have to flush it for a long time to have full replacement of the fluid.

To demonstrate this principle, conduct an experiment. Fill a bucket with a colored liquid. You can use food coloring, milk, or any fluid, but make sure it is as opaque as possible. Then place a hose in the bucket and turn on the water. How long does the hose have to run before the liquid flowing out of the bucket is clear? Is the water ever totally pure of the liquid you had in it when you started?

If the guys running the transmission fluid exchange machine run it until the fluid coming out is perfectly clean, then Joe and I stand corrected. However, something tells me these profit-makers would cut into their profits if they ran the machines that long, and pushed that much clean transmission fluid in.

Tester, have you ever operated a transmission fluid exchange machine? Do you run it for a set period of time, or do you monitor the quality of the fluid coming out? How are you able to tell when all of the old fluid is out? Please help educate us poor sots.

A transmission fluid exchange machine operates on the same principle as bleeding the brake fluid system. Both are hydraulic systems. The brake fluid system is a closed system. The transmission fluid system is a closed loop system.

When you bleed the brakes, you remove the dirty fluid from the reservior and fill it with fresh fluid. Then as the brakes are bled that fresh fluid forces the rest of the dirty fluid out of the brake system from hydraulic pressure. This prevents any of old fluid from mixing with the fresh fluid. Because high pressure always moves to low pressure. And the low pressure is at the bleeder valves when they’re opened.

A transmission hydraulic system is closed loop system. This means the transmission fluid is pumped from a high pressure side to a low pressure side. Between this high and low pressure side is the transmission cooler in the radiator. It’s here where the transmission fluid exchange machine is connected.

The machine I use works like this. Remove one of the transmission cooler lines from the radiator. Connect the exchange machines two hoses to the cooler on the radiator and the cooler line. The machine is now connected into the closed loop system. Fill the clear cylinder with fresh fluid. At the bottom of this cylinder is piston that seals one end of the cylinder from the other. Start the engine and in some cases the transmission has to be put into neutral. The pump in the transmission forces the old fluid into the bottom of the clear cylinder, pushing the piston up the cylinder to force the fresh fluid back into the transmission. This is repeated until the fluid entering the bottom of the clear cylinder coming out of the transmission matches the color of the fresh fluid going back into the transmission.