It felt like driving across rumble strips that weren't there. Holding steady at 45 miles an hour on the Belt Highway, light on the gas, the whole truck would vibrate for a second or two, like a low drumroll through the seat. Speed up or let off, and it stopped. Hold it at 45 again, and there it was.
I first noticed it in May with the Silverado around 146,000 miles. At first I thought it was the road. Then I felt it on three different roads, including the new pavement on US-36. Then I thought it was a tire. But a tire vibration gets worse the faster you go, and this didn't. It only happened in that narrow band of speed, under light throttle, once the truck was warmed up.
What the shop thought it was
I described it to our shop over the phone and the service writer said "torque converter shudder" before I finished the sentence. He said they see it a lot on GM trucks of this age with the six-speed automatic.
Here's the short version as I understand it. The torque converter sits between the engine and the transmission. At steady cruising speeds it locks up, so the engine and transmission turn together instead of slipping through fluid, which saves gas. On these trucks that lockup is controlled to slip just a little. When the transmission fluid gets old, it loses some of its friction properties and the lockup starts to grab and release very quickly. That's the shudder.
The good news is that old fluid is often the cause, and fresh fluid of the right type can fix it. The bad news is that sometimes the converter itself is worn, and fresh fluid only helps for a while, or not at all. A new torque converter means pulling the transmission, and the shop said that would run somewhere in the $2,000 to $3,000 range.
Why fluid first
The shop's approach was to try the cheap fix first and see. I appreciated that. They could have pitched a converter on the spot. Instead the service writer said, "Let's do a fluid exchange with the updated fluid and drive it a couple weeks. If it's still there, we talk."
A fluid exchange is not the same as a drain and fill. A drain and fill only gets out a portion of the old fluid, because much of it stays in the converter and lines. An exchange uses a machine to push new fluid through as the old comes out, until nearly all of it has been replaced. It takes more fluid and costs more, but you aren't mixing new fluid with mostly old.
The truck went in on the first Monday of June. The bill was $289: $148 for about 14 quarts of fluid, one hour of labor at $132, and $9 for disposal and supplies.
I asked whether I should have been changing this fluid all along. The answer was yes, probably. The maintenance schedule on these trucks treats transmission fluid as a long-interval item for normal driving, and plenty of owners never touch it. My notebook says nobody has changed it since I bought the truck at 74,000 miles, and I can't find any record from before that. That's on me. I change the engine oil like clockwork and never thought about the fluid on the other side of the bell housing.
What we'll watch for
Three days in, the shudder is gone. I've driven the Belt at 45 more times this week than I've ever done on purpose, and it's smooth. The shop told me it can take a few hundred miles for the fluid to fully settle in, so I'm keeping notes.
Here's what I'm watching, and I've written it in the notebook so I don't fool myself later:
- Any return of the shudder at 40 to 50 mph under light throttle.
- Slipping, like the engine revs climb without the truck speeding up.
- Harsh or late shifts, especially when it's cold.
- Any burnt smell or red fluid under the truck.
If the shudder comes back in a few months, the converter is the likely next step, and that's a much bigger conversation about how much more money this truck gets. At $289, though, the fluid was the right first try. If it holds, I'll have a shop do it again around 196,000 miles, or sooner if it starts acting up.
Truck work at $289 has a way of feeling cheap after the deer and the 4WD actuator. That's a dangerous way to think, and I know it. It still went in the spreadsheet in red.