Ford 5R55N/S/W's Overdrive Planetary Failure

It’s not unusual to see a 5R55N, S or W type transmission come in to the shop with the overdrive planetary destroyed due to a lack of lubrication. After overhaul the transmission exhibits erratic shift behaviors. The shift complaints vary from having flared shift, slide bump shift and harsh shifts. 

One cause for the lubrication problem can be a malfunctioning cooler bypass/thermal valve line-up in the valve body (figure 1 - 5R55N shown). This affects Coast Clutch drum’s (figure 2) lubrication oil coming from the front pump assembly (figure 3) and center lube for the OD planetary (figure 4). Rear lube remains unaffected for the main gear box as it is supplied by the Pressure Regulator Valve (figures 5 and 6).

FIGURES 4-6

The erratic shift feel behavior after overhaul can be attributed to an inconsistent solenoid regulating valve (figures 7 and 8) and/or the flow control valve (figures 9 and 10) in the pump, especially after an overheat failure and/or over torque. The inconsistent regulation of the flow control valve in the pump compromises solenoid feed oil to Pressure Control Solenoids A, B & C while the solenoid regulating valve compromises solenoid feed oil to the TCC and shift solenoids. The inconsistent supply of oil to these solenoids causes harsh shifts, soft flared shifts, slide bumps and the list goes on. One shift cycle all the shifts work well and the next shift cycle you never know what you are going to get.

FIGURES 7-9

Restoring the proper operation of the cooler bypass/thermal valve, the flow control valve and the solenoid regulating valve is imperative to resolve these issues. There are several aftermarket kits available that provide a new flow control valve for the pump. But there is one kit available (figure 11) from Superior Transmission in conjunction with Dean Mason/TransLab that provides the flow control valve, an updated solenoid regulating valve for the 5R55N and one for the 5R55W/S as well as a completely redesigned cooler bypass/thermal valve line-up called the Sure-Cool® Bypass/Flow Control valve. Tests have revealed that when Ford’s thermal valve is working according to factory design, the unit runs far too long before starting cooler flow. With this updated line-up, along with other included modifications, you near triple the flow of lubrication oil to the overdrive planets eliminating repeat failures.

FIGURES 10-11

The 5R55N/S/W kit by Translab is available from Superior Transmission Parts Inc. through your local supplier:

5R55 Kit Number — K5R55/W/N/S

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