Beware of Code P0720: Part 2 - Transmission Digest

Beware of Code P0720: Part 2

When the computer observed an increase in rear prop speed as a result of the axle popping out, it determined the front vehicle speed sensor as being faulty due to it decreasing in rpm. Of course the loss of rpm was due to the loss of power. The computer either didn’t or couldn’t distinguish the difference. Nonetheless, the front vehicle speed sensor was being blamed when nothing was wrong with it or the transmission. Considering the strain this had on the transfer clutch with the axle popping in and out, it explains why they were burnt originally.

Beware of Code P0720: Part 2

Shift Pointers

Author: Wayne Colona, Technical Editor
Subject Matter: Subaru TG5D9-5EAT unit
Issue: Front vehicle speed sensor

Shift Pointers

  • Author: Wayne Colona, Technical Editor
  • Subject Matter: Subaru TG5D9-5EAT unit
  • Issue: Front vehicle speed sensor

Slow-speed tight turn makes the vehicle suddenly slow down and not move

The October 2017 Shift Pointers column, “Beware of Code P0720,” addressed Subaru’s TG5D9-5EAT transmission (Figure 1) producing a front vehicle-speed sensor code P0720. This sensor is oftentimes confused with the turbine speed sensor (Figure 2) as it is the most forward externally mounted speed sensor.

The front vehicle speed sensor is mounted internally in the rear of the transmission (Figure 3). For obvious reasons one would think the turbine speed sensor would be the front vehicle speed sensor.

Kevin Quinlan reminded us of another reason to beware of P0720. When he brought it to my attention, I remembered we mentioned this problem in our 2014 seminar. If you attended the seminar you will see this on page 134 in the Blue Manual. Kevin lives in Vermont and sees his fare share of Subarus compared to the days when he lived in Mahopac, N.Y.

A wholesale shop brought to Kevin a transmission they pulled from a vehicle they said would intermittently experience a very bad slip and then produce code P0720. When he disassembled the unit, the transfer clutches were burnt. The rest of the transmission looked good. After going through the unit and repairing the transfer clutch damage, he also replaced the front vehicle speed sensor as an extra precaution.

When he returned the transmission to his wholesale account, he went over his work with them explaining that he didn’t see anything that would explain a severe slip. They installed the unit and reported back to him that it worked perfectly. However, one week later the customer brought the vehicle back complaining that it was doing exactly the same thing before the work. This time, the shop brought the vehicle to Kevin. He drove it for several days and did not experience any problem. The customer getting impatient (that rarely happens, right?), took the car back. This was a concern as nothing was done to remedy a problem no one else other that the customer was experiencing. As expected, one month later it came back with one very upset customer threatening to reverse his credit card charges (that rarely happens, right?).

When you read the diagnostic process of P0720, one would conclude that an internal problem is the cause. But Kevin knew it couldn’t be as he never felt or observed anything wrong.

He took the vehicle out for a more thorough road test. After driving the vehicle under various conditions, he suddenly experienced the problem. Only when he would make a slow-speed tight turn would the vehicle suddenly slow down and not move. His scan-tool data would show his rear prop shaft rpm suddenly rise while the front speed sensor rpm decreased, as the vehicle was slowing down to a no-move condition. While at a stop he could hear the transmission shifting, yet he was not moving. He turned the ignition off, and immediately after, he felt a pop. He started the car and it would drive away normally. He could duplicate the problem every time he made a slow tight turn. Having the ability to duplicate the problem at will, he timed it as he brought it back to the shop. Without turning the vehicle off, they pushed it in and onto the lift. Once in the air, they saw that the right rear axle had popped out of the differential (figures 4 and 5). Making normal turns would allow the axle to pop out and back in. Only on slow tight turns would it pop out and stay out until the vehicle was turned off.

When the computer observed an increase in rear prop speed as a result of the axle popping out, it determined the front vehicle speed sensor as being faulty due to it decreasing in rpm. Of course the loss of rpm was due to the loss of power. The computer either didn’t or couldn’t distinguish the difference. Nonetheless, the front vehicle speed sensor was being blamed when nothing was wrong with it or the transmission. Considering the strain this had on the transfer clutch with the axle popping in and out, it explains why they were burnt originally.

In conclusion, beware of code P0720.

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