GM’s 6L40/50/80 series rear wheel drive transmissions referred to as GM6 may come in to the shop experiencing a complaint of a delayed reverse accompanied with a slipping reverse as well as a flare or slide 2-3 shift. The problem may be severe enough to produce solenoid performance codes placing the vehicle into a failsafe condition.
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One cause may be due to a crack developed in the 1-2-3-4 / 3-5-Reverse Clutch Drum (figure 1). Five or more years Chris Spick at Lawrence Transmissions sent in to us some pictures of this problem showing how difficult it is to see this crack. It takes place either in the weld or by the bearing seat where the drum sits against the pump (figure 2). The size of the crack will obviously determine the severity of the condition but it will also determine how easy or hard it will be to find the crack. If the complaint is not temperature related the crack is typically severe enough to locate with a wet air check. But if the problem occurs when hot, the crack is usually hard to see. To verify and locate a crack with a drum that checks well cold, load the drum and place it into a plastic bag and run it through a clean machine to heat it up. Then perform a wet air check when the drum is hot (figure 3).
A more recent dilemma is related to a cracked 1-2-3-4 piston (figure 4). This too can be difficult to see (figures 5 and 6). Lorenzo Ortiz from Phillips Transmissions sent us the piston which caused him to have a rough day. He shared with me his experience with one that he tangled with. It was with a 2007 GMC Yukon Denali AWD 6.2L vehicle using the 6L80 transmission. The customer first noticed a problem when he was on an entrance ramp to a highway and he needed to punch the accelerator. The transmission went to neutral initiating failsafe. After an ignition cycle all was well again. From time to time when he needed to be heavy into the throttle it felt like it would slip. It was decided to have the transmission removed and rebuilt at Phillips Transmissions. After it was rebuilt, the customer had it for two weeks before he noticed the problem remained. This is when Lorenzo took it out for a long road test and full throttle it until it slipped badly going forward.
When they took it apart the 1-2-3-4 clutches were toast. He loaded it up and air checked the drum looking for the leak. With just a normal supply of air, the drum air tested nicely. But when he blasted it with full pressure (100-120psi), it blew air out past the piston. When he took it apart, it took some looking but he did locate the crack which is seen in figures 5 and 6 between the black markers. But once he discovered the problem, his day didn’t get any easier. When he ordered a new piston (part # 24224146), it listed for $419.00 and wholesaled to the shop for $293.34. Some pretty tall dollars for one aluminum piston. To make matters worse, if the drum was cracked and it too needed to be replaced, this empty drum assembly lists for $269.63 and wholesales to the shop for $188.74 (part # 24237185). Talk about crack, snap, split or fracture, it will break your day and your bank. But there is an interesting alternative. A different style OE piston can be purchased which is smooth compared to the rib type (figures 7 and 8). The smoother piston is a fuller, sturdier type piston. It appears that aluminum material was removed with the ribbed type to make it lighter in weight. Apparently it also makes it weaker in strength. The good news is, this non-ribbed type piston (part # 24238700) sells for $25 to $30 dollars. It’s a great alternative in every aspect.
If the drum was cracked, it too is quite expensive. The empty housing lists for $269.63 and wholesales to the shop for $188.74 (part # 24237185). Can you imagine if both the piston and the drum need to be replaced?
There is a non-ribbed type piston (part # 24238700) which sells for $25 to $30 dollars.