Back on the Bench - August - 2021

What a Drip! 6L80 Drain Back, That Is

As time goes by, we see more and more of these 6L80s showing up at the door. It reminds me of how often we used to see 4L60Es. Although, the problems with the 6L80 are usually consistent. Almost every 6L80 that shows up is either a converter failure, plugged filter, burnt 4-5-6 clutch, or a damaged stator and bell.

As people across the country began buying these parts, supply could not keep up with demand. This drove up prices and made it hard to sell the job and make money. So, we began machining the old components to reuse them and saved quite a bit of money in doing so. It helped increase the profit margin and lower the end price, making it slightly easier to sell.

At the shop I work at, we have built so many of these, sometimes seven or more a week. We have repeated the same procedures over and over with excellent success, to the point that they have become habits. This could lead someone to getting into that mode of just doing the procedures and not paying critical attention to all of the details. Sleeping at the bench is what I call it, and I’m sometimes guilty of it. That’s usually when something sneaks up on you and bites you in the rear, which is what happened to us recently.

We had rebuilt a 6L80 for one of our customers. Everything seemed to work ok, so we delivered it. The customer drove it around for that day, and everything was good until he parked it and let it sit overnight.

On initial start-up, it had a delayed engagement, but after that, it worked perfectly all day long. The customer stated he couldn’t be happier with the way it performed throughout the day and that it never worked better. But he was concerned about this morning delay and that it might lead to something worse. The vehicle was left with us so we could let it sit overnight and check it out. We installed a pressure gauge on it as well.

The next morning, we found the fluid level was high on the stick. We fired it up, and the transmission had no pressure. We suspected a worn pump rotor, so we removed the transmission.

We grabbed a couple of new rotors on the shelf to measure thickness while another technician removed the transmission. The new rotors measured .708″ thickness (figure 1). It would only take a couple of thousandths to allow fluid to leak by. The one in the vehicle was .699″ (figure 2). The top and bottom surfaces looked smooth, like they were designed that way.

So, here is a new addition to the normal procedure, measure rotor thickness even if it looks good; calipers don’t lie. We have found bad rotors before, but they were scarred up with deep grooves on the surface from riding against the bellhousing or stator support body.

After installing the rotor and letting it sit overnight, we rechecked the fluid level in the morning and checked for the delayed engagement. The fluid level was good, and there was no longer a delayed engagement, which made our morning.

It was time to call the customer to get his vehicle back home. A couple of weeks later, he stopped by for the ten-day check and stated the transmission had never worked better. He thanked us for the diligence we applied to getting it right and would leave us a five-star review, which made the boss happy with us as well.

Even though I enjoy rebuilding transmissions, there are days where it becomes a job. When I get those common transmissions like the 4L60E and 6L80 land on my bench, I tend to take a nap while building or play games like “let’s see if I can build three of these today.” Rushing or sleeping through a rebuild is never good; it’s usually times like that when the little things like .009″ of thickness get by you and eats your lunch. So, you might want to add measuring those rotors to your playbook if you don’t already, and let’s keep those transmissions from landing back on the bench.