Transmission slipping in fifth gear: An elusive cause of a common problem - Transmission Digest

Transmission slipping in fifth gear: An elusive cause of a common problem

Sometimes, finding the cause of a complaint isn’t as straightforward as one would expect; sometimes, these causes can be very elusive.

Sometimes, finding the cause of a complaint isn’t as straightforward as one would expect; sometimes, these causes can be very elusive. Take, for example, a 2014 Ford Expedition 5.4L engine using a Ford 6R80 transmission. It was a call that came into ATSG’s help line taken by Rino Partipilo. It was from a shop where he had worked around four years ago called Continental Transmissions. The complaint was that the transmission slipped in fifth and generated a fifth gear ratio code. When the unit was removed, only the 4-5-6 (Overdrive – E) clutch plates had a slight burn to them. The drum was heated and checked for cracks, but nothing was found.

During the rebuilding of the transmission, the stator bushing was replaced as a standard process. Since the vehicle had high mileage and no real damage was found, the valve body assembly was also changed. The vehicle road tested well and was then delivered to the customer. Not long after, the customer returned; this time, the transmission was shifting in and out of fifth gear. A scan tool showed the 4-5, 5-4, 4-5 up and downshift. This began a different diagnostic route looking for the reason such a command would occur. Engine loads, speed sensors, misfire possibilities—all external possibilities were investigated, revealing nothing.

Read more columns from our Shift Pointers series here.

The transmission had been out and back in again throughout this time to check the 3-5-R (Direct Clutch B) drum assembly. Meanwhile, the problem with the transmission was escalating. It was with great hesitation that the unit was pulled for a third time. But, this time, the problem revealed itself: The 4-5-6 piston was cracked as seen in Figure 1. This crack did not reveal itself at first; it had worsened over time since the original rebuild.

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Typically, this type of failure causes a problem with a shift into fourth rather than into fifth gear adding to the elusiveness of this problem. In this case however, the problem reared its ugly head in fifth gear. The next problem encountered was to locate another piston for the 6R80 transmission. This was not as easy as it was first thought to be. But then, Rino remembered there was a ZF6HP24 in the shop when he left, and it was still there. He recommended that they pull the 4-5-6 (Overdrive – E) clutch drum and see if the piston was the same. It was. So now, they have their customer on the road and a cannibalized ZF6HP24 transmission in the shop.

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