Fixing an output speed sensor fault on Fords and Mazdas
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.
