- Subject: Pattern failures resulting in loss of reverse, third and fifth gears
- Unit: GM 6T70, Ford 6F50
- Essential Reading: Rebuilder, Diagnostician
- Author: Pete Luban, ATSG
The definition of a pattern failure as far as transmissions go is seeing the same wear issue or broken part in every transmission of that type. This is such a case.
We have similar transmissions in two different car makes, the 6T70 from GM and the 6F50 from Ford. These transmissions are almost identical as far as the internal parts and pieces go, but they are controlled differently. The parts that fail in the 6T70 are different from those that fail in the 6F50, say the manufacturers.
However, both failures will result in the same complaint: no reverse and no third or fifth gear. Are we confused yet? Good, me too.
First, let’s take the GM 6T70 failure, a broken 3-5-reverse waved plate allowing the clutch piston to over-travel, which cuts the lip seal and results in no application of the 3-5-reverse clutch.
As a result of this failure, pressure-control-solenoid performance codes P0776, P0777, P2723, P2714 and P2715 may be stored.
Because of the hard-part damage resulting from the broken waved plate, metal debris carried through the transmission could become attached to the input-speed sensor, resulting in codes P0716 and P0717 being stored.
If you’re working on a transmission that has not yet experienced this failure, I would recommend installing a new waved plate. The part number for a new waved plate from GM is 24254103.
When the waved plate breaks, the ensuing damage also includes the 3-5-reverse and 4-5-6 clutch housings as well as the 2-6 clutch-piston return spring and the end cover (figures 1, 2 and 3).
Now comes the unusual part of this scenario. To replace these parts from your local GM dealer would be more expensive than buying these same parts from your local Ford dealer. That’s right; these parts are the same ones found in Ford’s 6F50 transmission, and this is assuming the parts are available from the GM parts dealer. They are available from Ford. Figure 4 lists the parts and the Ford part numbers.
OK, now let’s get to the Ford 6F50 failure. This also is a loss of third, fifth and reverse. Gear-ratio codes P0733 and P0735 may be stored along with Shift Solenoid B performance code P0756.
The failure for the 6F50 is the front sun shell. The sun gear breaks off the shell, resulting in the previously mentioned condition (Figure 5).
The Ford part number for the front shell which includes the #3 thrust bearing is FT4Z-7A019-A.
So, let’s recap: Ford says it does not have issues with the 3-5-reverse waved plate; it has sun-shell issues. GM says it does not have sun-shell issues; it has issues with the 3-5-reverse waved plate. So what happens if you get a 6T70 with a broken sun shell or a 6F50 with a broken waved plate? Follow a pattern and get the appropriate parts from Ford, using the Ford part numbers included in this article.
Well, that’s enough confusion for now.
Many thanks to the crew at Greenville Transmission Clinic in Greenville, S.C., for the heads-up on this problem.