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Transmission fluid flow


Active Member
June 29, 2009
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City, State
atlanta ga
Year, Model & Trim Level
1996 explorer
I have a 5r55e Transmission and want to know which way the trans fluid flows through the radiator, i.e does the fluid flow form the top of the radiator down or the bottom of the radiator up. I nned toknow this as I am installing a in line tran filter. \


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While I can't asnwers your question directly, I can say that the A4LD, 4R44E, 4R55E, 5R55E all return trans fluid on the UPPER line ON THE TRANSMISSION.

So, just see which line on the radiator goes to the upper on the transmission so you know which way it's flowing.


Which port - does it matter?

I don't know about the other transmissions but this is what I learned about my 2000 5R55E:

Neither port has any suction. The pressure of both ports is greater than atmospheric. The cooling loop valve in the transmission is not supposed to open until the ATF in the torque converter reaches 150 deg. F. However, as shown in the photo below there was flow from each port even when the ATF was cold.

I installed a temperature sender at my remote filter.

I installed an indicator in my A pillar pod. I learned that it takes about 20 minutes of normal driving after a cold start for the ATF to reach 150 deg. F. This was also confirmed datalogging the ATF Temp pid with my X3 PowerFlash via the OBD-II port.

Does it really matter where in the external loop the filter is installed? It could be before or after the radiator internal ATF cooler. If you have an auxiliary ATF cooler it could be before or after it. As long as all of the coolers and the filter are in series the effectiveness will be the same. Ease of installation and filter replacement as well as protection of the filter from damage should be considered.


Thanks, with 3 filters you have the cleanest trans fluid of any of us

engine oil filters

I only have one full flow ATF filter. The other filters are engine oil filters: one is a full flow and the other is an Amsoil bypass flow filter. I also installed a thermostatically controlled engine oil cooler.

Everything fits behind the bumper and I only have to remove the air deflector below the radiator to replace any of the filters. The core on the driver side is a boost pressure controlled forced air heat exchanger for the intercooler.

just to clarify

The instructions in bold for the inline transmsion filter say to go with the flow (no pun intended) . It is only supposed to work one way I assume. The way I have it now is that i have the filter coming out of the bottom of the radiator. i.e the filter is flowing from top of the radiator to the bottom.
Any insight? thanks . Just want to make sure I am doing the right thing

Thanks Again

direction of flow

Sorry, I was thinking more about where to place the filter instead of the direction of flow which I agree is important. My Mobil 1 full flow filter has a check valve to prevent the filter from draining when there is no oil pressure. That reduces the duration of "dry starts". I'll have to look thru my notes to confirm which transmission port is the external loop output.

I really appreciate all your help.

There are two lines which are connected to the transmission on the passenger's side. The lower one is the "out" line which goes to the cooler. The return line on the cooler goes to the upper fitting on the transmission. Check the links in # 52 in my list of useful threads.

Transmission Fluid Cooler Flow Test

I did a lot of searching before making my ATF cooling modifications. I found numerous conflicting descriptions of the port purpose. Initially I used the table in the link below but did not have any flow thru my external filter. After that I did the jug test shown in my previous post. After the transmission was fully warmed up the upper port on the transmission produced twice the output of the lower port. So I rerouted my hoses to connect my filter inlet to the top port and have good flow thru the filter.

The table in the following Cooler Return Line Chart agrees with Maniak's post.

I found the following in my 2000 workshop manual:

Transmission Fluid Cooler Flow Test

1. Remove fluid level indicator from fluid filler tube.
2. Place funnel in fluid filler tube.
3. Raise the vehicle on a hoist, refer to Section 100-02 and place suitable safety stands under the vehicle.
4. Remove the cooler return line (rear fitting) from the fitting on the transmission case.
5. Connect one end of a hose to the cooler return line and route other end of the hose up to a point where it can be inserted into the funnel at the fluid filler tube.
6. Remove the safety stands and lower the vehicle. Insert end of hose into the funnel.
7. Start the engine and run it at idle with the transmission in NEUTRAL position.
8. Once a steady flow of fluid (without air bubbles) is observed, remove the hose from the funnel and
place the hose in a measuring container for 15 seconds. After 15 seconds place the hose back into
the funnel and turn the engine off. Measure the amount of fluid in the container. If adequate flow was observed, approximately 237 ml (8 oz) will be in the measuring container; the test is now complete.

I was thinking the two fittings were vertically aligned but found a drawing of the heat shield in the Workshop manual that shows the external cooler ports. The lower one is perpendicular to the case and the upper one is at a 45 degree angle to the case pointing forward.

Therefore the lower one (rear fitting) is the return port and the upper one (front fitting) is the supply port. I don't know why the cooler flow test uses the return port to route ATF into the funnel.

My advice is to try one way and see if the filter gets warm from the ATF flowing thru it. Mine initially stayed cold indicating no flow so I rerouted hoses.

On my '98 SOHC, the upper port on the radiator is flow coming from the trans, the lower port is the return to the trans.

Guys thanks so much! I now feel confident that I did the right thing. Hope my 96 gets many more miles.