Fixing an output speed sensor fault on Fords and Mazdas

Jan. 1, 2020
Sometimes after rebuilding a 4F27E/FN4A-EL transmission, an intermittent P0721 Output Speed Sensor Fault suddenly appears with no drive-ability complaints.

Ford & Mazda Vehicles with the 4F27E/FN4A-EL 4 Speed FWD Transmission Intermittent P0721 Output Speed Sensor Fault

Sometimes after rebuilding a 4F27E/FN4A-EL transmission, an intermittent P0721 Output Speed Sensor Fault suddenly appears with no drive-ability complaints.  Since this was not a problem when the vehicle arrived the wiring from the sensor to the computer becomes suspect. At times a wiring harness can get hung up on the transmission as it is being removed. Depending on when this is noticed determines if the wires were pulled hard enough to compromise a connection. But after a thorough investigation of the wiring no issues are found.

If the AC signal is viewed on a scope, one or more systematic drop outs will be observed. This is indicative of a problem with what is exciting the sensor such as a Reluctor wheel with a lug or lugs missing.  

In this case, there are two different design Reluctor wheels used in a 4F27E/FN4A-EL transmission (4F27E for a Ford and FN4A-EL in a Mazda, same transmission). The first design Reluctor wheel which is mounted on the differential uses bent over tabs to excite the speed sensor rather than lugs. These tabs can be easily bent during the cleaning and assembling process causing an inconsistent air gap resulting in a P0721 trouble code.  If the tabs are not too badly distorted it is possible to remove the speed sensor to gain access to the tabs. Rotate the differential until the bent tab or tabs are located. Carefully bend the tab back into proper position using a suitable tool.

The second design Reluctor wheel is made of a soft material, which has exciter lugs that can be easily marred during the cleaning and assembling process. This too will affect the sensor to lug air gap causing code P0721 to set.

If the lug is marred, it may be too difficult to correct it with the transmission in the car as with the first design tab type. The transmission will need to be removed and disassembled. Only then can it be determined if it’s repairable or in need of replacement.

Being aware of the sensitivity in this area when handling the differential assembly, care can be taken to avoid damaging the Reluctor. Especially so when loading parts into a clean machine as this is typically where the problem occurs.

About the Author

Wayne Colonna | POWERTRAIN PRO Publisher

Wayne Colonna is president of the Automatic Transmission Service Group (ATSG) in Cutler Bay, Fla., and a frequent speaker/instructor for transmission training around the globe.

Sponsored Recommendations

ADAS Applications: What They Are & What They Do

Learn how ADAS utilizes sensors such as radar, sonar, lidar and cameras to perceive the world around the vehicle, and either provide critical information to the driver or take...

Banking on Bigger Profits with a Heavy-Duty Truck Paint Booth

The addition of a heavy-duty paint booth for oversized trucks & vehicles can open the door to new or expanded service opportunities.

The Autel IA700: Advanced Modular ADAS is Here

The Autel IA700 is a state-of-the-art and versatile wheel alignment pre-check and ADAS calibration system engineered for both in-shop and mobile applications...

Boosting Your Shop's Bottom Line with an Extended Height Paint Booths

Discover how the investment in an extended-height paint booth is a game-changer for most collision shops with this Free Guide.