FNR5/4F27E Servo Concerns - Transmission Digest

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FNR5/4F27E Servo Concerns

A 2-3 shift flare before or after overhaul is a common complaint with the FNR5/4F27E transmission. Even a slight flare can be accompanied by codes for gear-ratio error and solenoid function. The cause for all of this is usually a worn servo-pin bore where it goes through the case.

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By Jeff Parlee
Jeff Parlee is the Sonnax remanufactured valve body product support manager and member of the Sonnax TASC Force (Technical Automotive Specialties Committee), a group of recognized industry technical specialists, transmission rebuilders and Sonnax Industries Inc. technicians.
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FNR5/4F27E Servo Concerns Body of Evidence Subject: Shift problems caused by wear in servo-pin bore Unit: FNR5/4F27E Essential Reading: Rebuilder, R & R Author: Jeff Parlee

Body of Evidence

  • Subject: Shift problems caused by wear in servo-pin bore
  • Unit: FNR5/4F27E
  • Essential Reading: Rebuilder, R & R
  • Author: Jeff Parlee

A 2-3 shift flare before or after overhaul is a common complaint with the FNR5/4F27E transmission. Even a slight flare can be accompanied by codes for gear-ratio error and solenoid function. The cause for all of this is usually a worn servo-pin bore where it goes through the case.

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The servo on these transmissions not only applies the 2-4 band but also releases the band in third gear. The apply pressure is not released in third, but rather the spring side of the servo is filled with third gear oil and along with the return spring pushes the servo piston back toward the servo cover to release the band. When the computer shifts the transmission to fourth, the 3-4 shift valve stokes to cut off forward-clutch oil and cut off the third oil to the release side of the servo, allowing the band to reapply for fourth. A leak at the servo-piston pin lowers the direct-clutch apply pressure, causing the 2-3 shift flare and the codes.

Testing can be performed in the car or on the bench during disassembly. With the valve body removed, blow air into the servo-release port with a rubber-tipped air nozzle (figures 1 and 2). The release side of the assembled servo should air-check just like a good working clutch. Any air leakage past the servo pin at room temperature will only get worse as the transmission gets to operating temperature. If oil leaks out of the servo-apply hole during the release-side air check, replace the servo piston.

If you get into the habit of air-checking the release side of this servo during disassembly you will know right from the start whether you need to repair the servo-pin bore by reaming and installing a sleeve into the servo.

Jeff Parlee is director of product support at ValveBody Xpress.

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