FNR5/4F27E Servo Concerns - Transmission Digest

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.

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.

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.

You May Also Like

Sometimes, a diagnostic code is all you need

With ATSG having the opportunity to help shops solve problems, sometimes we get faced with some real doozies. A shop will call and give us a laundry list of DTCs, leaving us to think someone must have a bulkhead connector unplugged. We then go through the arduous task of deciding which codes prompted other codes

With ATSG having the opportunity to help shops solve problems, sometimes we get faced with some real doozies. A shop will call and give us a laundry list of DTCs, leaving us to think someone must have a bulkhead connector unplugged. We then go through the arduous task of deciding which codes prompted other codes to set—we’re actually diagnosing diagnostic codes themselves at that point. So, when an issue comes up on our help line with codes that actually tell the story, it makes for a nice change, as well as a quick pathway to a repaired vehicle.

10L80 and 10R80 pump gear differences

You may have seen an article in the August 2023 issue of Transmission Digest called “GM 10L80: A new kind of pump noise,” which goes over how the front cover housing in the 10L80 is fitted with a converter drive gear and idler gear. The idler gear drives the pump’s driven gear, and is press

Shift Pointers: What to do when the 62TE TRS tab breaks

How frustrating it is when on a hot summer day, as you go to open a nice cold can of your drink of choice, and the tab breaks off? You are outside, away from any tools to remedy the problem quickly. It now requires a MacGyver mentality looking around at the resources available to get

Going the extra mile: Proving your transmission repair suspicions

A 2003 Honda Pilot with a five-speed three-shaft transmission came into our shop with a customer concern that the vehicle had no power, and the “D” light was flashing. I first did a scan for codes to see what it came up with, and the scan tool returned four DTCs: P1298 (ELD voltage high), P0135 (H02S

RRfeature-1400
Diagnosing Ford 10R60, 10R80 and 10R140 series speed sensor issues

Ford 10-speed 10R series transmissions utilize four two-wire, Hall-effect sensors — TSS, ISSA2, ISSAB and OSS — for providing speed signals to PCM or TCM. They are supplied nine volts by a PCM or TCM and assist in the control of clutch apply/release timing that is used in determining shift quality, including TCC. Related Articles

Other Posts

Shift Pointers: Failures caused by incorrect tire sizes

For years ATSG has produced a wide range of issues related to improper tire sizes on vehicles. Even under-inflated tires have been known to cause issues. Problems such as premature failure with an active 4WD transfer case will occur with incorrect tire sizes. Related Articles – Valve body and component suppliers: A comprehensive list –

Shift-Pointers-Jan-Figure-1-1400
Understanding lube flow control valves in Toyota/Lexus UA/UB80 transmissions

The Toyota/Lexus UA80 and UB80 transmissions first came out in 2017 in Highlanders and Siennas. The UA80 is used in V6 applications, and the UB80 is paired with four-cylinder versions. They have been called Toyota New Global Architecture type transmissions, and alternately referred to as the “Direct Shift 8AT” eight-speed automatic transmission. This transmission was

Tasc-Tip-December-Figure-1---LFC-Valve-OE-Partial-Circuit-Diagram-1400
How reading through service bulletins can turn a technician into the customer’s hero

Over the last 28 years of being a technician, I have developed the habit of checking for and reading technical service bulletins at the forefront of the diagnostic process, especially when an unfamiliar vehicle exhibiting blatant or straightforward concerns comes into the shop. I have found many valuable nuggets of information while reading over these

Sometimes, you should sweat the small stuff

It’s a common phrase: There may have been a time when you worried about something, and someone who knew what you are going through said, “Hey, don’t sweat the small stuff.” Sometimes, this may be good advice. But other times, it may be wise to handle the small stuff before it becomes bigger “stuff.”  Related