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Ford’s 4R75 and 5R55 overheating transmission

Tuesday, February 3, 2015 - 09:00
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A general list of reasons that cause transmissions to overheat was presented in a previous article called “Transmission Overheat.”  In it the comment was made that with variations between manufacturers along with transmissions having their own unique designs, it prevents an article from getting into the finer points of things. This short newsletter will hone in on a couple of overheat issues related to Ford transmissions.

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A late model Ford Crown Victoria (usually a police vehicle) comes into the shop with the transmission slipping. Upon disassembly it is obvious that the transmission was overheated.

Once the transmission is installed, and the vehicle is road tested, the technician notices that the transmission temperature is rising above a normal operating level. The vehicle is brought back into the shop and cooler flow is checked only to find out that the flow rate is quite excessive. Generally speaking, a return cooling line disconnected at the transmission may pump out a quart in 20 seconds and would be considered normal. The best way to check cooler flow is with Sonnax’s SonnaFlow® meter connected to a lab scope or digital graphing multi-meter. In this instance the line was pulled and one gallon in 15 seconds was observed.

Daryl Keels at Econo Transmissions in Wilmington, N.C., quickly realized that there was a bypass valve somewhere in the system stuck in a position where it was directing the fluid back to the transmission without it going through the cooler. This resulted in transmission fluid reaching operating temperatures of about 270 degrees.

These as well as some other Ford vehicles use a transmission cooler that is integrated with the A/C condenser (figure 1). Located at the lower driver side corner of this assembly where the cooler lines enter (figure 2) is where this thermal bypass valve is located (figures 3 through 5).  

To correct this condition will require replacing the A/C condenser and transmission cooler unit. This can be a costly repair, for any owner including municipalities.

As a result, some have rendered the bypass valve inoperative which will allow fluid to circulate through the cooler at all times. This is not a good option in colder climates due to the possibility of fluid jelling. This will prevent no cooler flow and if driven, would cause premature failure of the drivetrain due to lack of lubrication.

If one chooses to go to perform the bypass valve modification seen in Figure 6, remove the bypass valve assembly from the cooler. Using round aluminum stock .865” in length. Machine a locating dowel pin in one end .382: in diameter and about .135” in length. Next machine a .337” length of the aluminum stock to .739” in diameter. Next, machine a .428” length of the remaining aluminum stock to .700” in diameter.

Insert the machined aluminum plug with the .700” end into the bypass valve housing (figure 7), then install the inner heavy spring next followed by the inner plug and retaining snap ring.

After Daryl made these modifications, the transmission 

operating temperature was at 175 degrees regardless of run time and accessory usage.

Other Ford vehicles have the bypass valve built into the transmission rather than in the external cooler system. The 5R55N/S/W transmissions are an example of that. Figure 8 shows the thermal bypass valve line up in the 5R55S and W model. Repeated overdrive planetary failures can many times be traced back to this bypass valve malfunctioning. The good news is with this configuration the A/C condenser and transmission cooler unit will not need to be replaced. Superior Transmissions completely redesigned the cooler bypass/thermal valve line-up called the Sure-Cool® Bypass/Flow Control valve. With this updated line-up along with other modifications, the flow of lubrication to the overdrive planets near quadruples eliminating repeat failures.

These are just two examples of getting into the finer points of the cause to overheat problems. In some future newsletters, we’ll get into other transmissions as well.

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