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DIY Transmission Flush

I want to replace all the old transmission fluid in our '04 Jeep Grand Cherokee, but I understand that pulling the transmission pan would only drain half the fluid (even though I could replace the filter when I pull the pan). A mechanic would be able to force all the old fluid out, flush the transmission, then put fresh fluid back in.

I really enjoy maintaining our cars, and would like to do this myself. Is there a way for me (a decent shade tree kinda mechanic and Navy engineman from the Reagan administration) to flush my own transmission? It looks like it would cost me ~ $125 to get this done at a shop, but that’s big money right now, and I love doing my own work.



Berea, KY

The “Flush” machines connect to the cooling lines and “trade” fluid…Nothing gets “flushed” other than your wallet. They don’t necessarily change the fluid trapped in the torque converter either, and they ignore the filter…

So drop the pan, change the filter, and refill the transmission, changing as much fluid as you can…I have started engines with the transmission pan off and let the tranny pump itself out somewhat, 30 seconds or less, no more time than that, others will claim this can damage the transmission, but I think 30 seconds at idle speed is safe enough…

This can be a very messy job, so have a BIG drain pan…

You can do it, but it’s a bit of a challenge. Disconnect the cooling lines. Put a funnel on the inlet line, and put the outlet into a drain pan. Start the car and pour fresh fluid in at the same rate the old fluid is being pumped out. Once fresh fluid starts coming back out, stop. After you reconnect the lines, you will have to correct the fluid level in the transmission. I’ve never tried this, but I have heard from others who were able to make this work.

There is no need to flush the transmission. The proper procedure is to pull the pan, clean it out, replace the filter and refill with transmission fluid. That’s the way the transmission was designed and that’s the correct procedure.

Why wouldn’t the torque converter fluid be changed during a flush?
As far as I know, the flush machine I’m familiar with uses the pump in the transmission on a running engine and works on a diaphragm principle of sorts.
The transmission pump should be circulating the entire mess out, from pan through the converter.

And some converters have drain plugs anway. (My Lincoln does.)

As I understand the fluid exchange, you want to get the new fluid into the torque converter. What I would do is drop the pan; inspect it; clean it; replace the filter; install a new gasket; bolt the pan back on; and refill the transmission. Now disconnect the transmission outlet cooler line and run it to a bucket. If you accidentally put the return line in the bucket, fluid will exit from the radiator cooler fitting so switch lines. Now run the engine in Park or Neutral until 1 or 2 quarts of fluid is in the bucket; refill the transmission with the same amount of new fluid; and repeat until nice clean fluid starts coming out. You might run the transmission through the gears while the fluid is coming out to flush out any remaining in the valve body; clutches; and servos. That should get you an almost complete fluid exchange.

As others have stated, if you drop the pan, change the filter, and refill every 25K miles, you probably are wasting your time and AT fluid by doing a complete transmission fluid exchange each time.

You keep saying that, and in many cases (including this one) that may be true. However, there are in-fact transmissions that are designed to be serviced only (or mainly) by fluid exchange machine. The ones in my two cars are examples of such. They are sealed, have no dip-sticks, and the factory service manual calls for the use of a fluid exchange machine to change out the fluid. The factory service interval is 150K miles, so it is very important that all of the fluid, not just 30% of it, be changed.

To get 75% of the fluid replaced, do a drain/filter/fill, drive for a few dozen miles, then do a drain/fill. Unless the old fluid is burnt (which means you have bigger problems to deal with) this should get you roughly 75% fresh fluid. Or just do it once, replacing 50% of the fluid every 30k miles or so, you should be fine.

I would just drain and refill a couple times in close succession. Doing that will replace enough of the fluid to make me happy.

This seems to fly in the face of conventional wisdom since a lot of manufacturers went to a sealed transmission because so many “quick change” shops added transmission flushing to their list of “services”. I can accept “fluid transfers” if properly done but transmission flushes do more harm than good in my opinion.

From the service manual:

SECTION 307-01: Automatic Transmission ? 5R55S

2006 Lincoln LS Workshop Manual

Procedure revision date: 06/24/2005
Transmission Fluid Drain and Refill ? Automated Equipment Printable View (16 KB)

Special Tool(s) Automatic Transmission Flush and Fill Machine

Automatic Transmission Flush and Fill Machine
199-00010 or equivalent

Material Item Specification
MERCON? V Automatic Transmission Fluid


CAUTION: Use only clean automatic transmission fluid specified for this transmission. Do not use supplemental transmission fluid additives, treatments or clean agents. The use of these materials can affect transmission operation and result in damage to internal transmission components.

CAUTION: Always refer to the instructions supplied with the flush and fill machine.

  1. With the vehicle in PARK, position it on a hoist. For additional information, refer to Section 100-02 .

  2. Use a suitable flush and fill machine to change the fluid.

  3. When connecting the flush and fill machine, connect the machine to the fluid cooler tube after the fluid cooler on the cooler return line. This will help remove any foreign material trapped in the fluid coolers.


  1. Use only clean automatic transmission fluid.

  2. Once the fluid exchange has been completed, disconnect the flush and fill machine. Reconnect any disconnected fluid cooler tubes.

  3. With the engine running, check and adjust the transmission fluid level, and check for any leaks. If fluid is needed, add fluid in increments of 0.24 liter (0.5 pint) until the correct level is achieved. For additional information, refer to Transmission Fluid Level Check in this section.

CAUTION: Always refer to the instructions supplied with the flush and fill machine

  1. When connecting the flush and fill machine, connect the machine to the fluid cooler tube after the fluid cooler on the cooler return line. This will help remove any foreign material trapped in the fluid coolers.

These must be the steps that gets bypassed during a usual flush at a lot of places that do transmission flushes. I still don’t like them but I guess I will have to live with them on newer transmission. Thanks tardis. I will just add a “caveat” to my future advice.

A lot of people on this board tell vehicle owners to avoid the FLUSHING of the transmission fluid. And these people have never used or even seen a transmission fluid exchange machine. This is not a flush. It’s a transmission fluid EXCHANGE.

Caddyman suggests disconnecting the cooler line and dumping the fluid coming out of the transmission into a pan. Well, doing this can burn the pump up inside the transmission. Very risky.

Why not instead of dumping the fluid into a catch pan, just connect the cooler line to a clear cylinder that has a piston inside of it. With the piston at the bottom of the cylinder, the cylinder is then filled with clean transmission fluid. The engine is then started and the pump in the transmission forces the old fluid into the bottom of the cylinder pushing up on the piston causing the new fluid to be forced back into the transmission. This continues until the fluid coming out of the transmission is the same color as the fluid going into the transmission. Doesn’t that sound easier, and cleaner? And it replaces ALL the transmission fluid.

That’s how my transmission fluid exchange machine works.