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My 2010 Silverado 1500 (500,000 km) has a problem with the transmission. It maks s clunking noise when shifting from second to third gear. I've been driving around it by letting off the gas when it shifts from second to third. The clunking isn't nearly as bad when I do that.

Question:

The noise has been getting slightly worse over the year that I’ve owned the truck. It will inevitably get much worse over time. What will happen when the transmission eventually fails? For example, would it give me lots of warning (loud noise) when accelerating at low speeds? Or is there a high-risk safety issue, when driving at highway speeds?

Notes:

  1. My mechanic noted that the truck struggles to get from 2nd to 3rd gear (revs up to 3000 RPMs), which indicates that it's a problem with the transmission, not a U-joint. He says the U-joints look fine.
  2. He doesn't recommend servicing the transmission (changing the transmission fluid) since that could do more harm than good at this mileage (500,000 km).
  3. The transmission gets warm when idling: When should engine fans turn on to cool transmission?
  4. Replacing the transmission would cost $3,000-$4,000 CAD, so my mechanic recommends just driving it as-is/taking it easy. That's what I do anyway—I try let off the throttle when it shifts from 2nd to 3rd gear, reducing the clunking.
  5. I know an obvious solution would be to replace the truck. But that’s not feasible right now due to budget constraints.

2 Answers 2

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More than likely, when the transmissions goes, it will just quit working. It may exhibit some noise when it gives up the ghost, but that isn't a given. The transmission in your truck is the 6L80E (six-speed ... if that's wrong, correct me). The major weak part of these transmissions is said to be the torque converter. These are known to expire and cause issues. Considering the amount of mileage on your truck, you've done pretty good with it. The "usual" time with the TC expires is before 150k miles (~250,000KM), if the TC is actually your issue. It is my understanding, unfortunately, that when the TC is going (or has gone out), it does so in spectacular form, which means a complete rebuild or replacement of the transmission.

Is it a safety issue? Usually not. About the only time where it might be is if it finally goes while heading uphill with zero momentum. There'd about be no way to get the truck to the side of the road without letting it roll backwards.

Replacement is probably your only option. That will need to happen at some point. A cheaper alternative may be to purchase a used transmission. The only problem with it is how long will it last. Getting one would be far cheaper than a rebuilt/new one. Either way with a new, rebuilt, or used transmission, you'll want to ensure you get a new torque converter to install with it.

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  • "heading uphill with zero momentum" sounds like a situation that by definition has little risk of getting rear-ended immediately, because you're at a standstill to begin with, possibly also with brakes already applied. Nothing like if it seized up at high speed.
    – TooTea
    Commented Apr 10 at 11:48
  • @TooTea - I don't think I've ever heard of an automatic seizing. They pretty much just quit working. I threw that statement in there as sort of a "this might happen" scenario. Like most things automotive related, Murphy plays a big hand and makes sure it ALWAYS happens when you can least afford it to (whether time wise or money wise). Very rarely does a vehicle quit working in the driveway or when you are pulling into the shop's parking lot. Commented Apr 10 at 15:00
  • @paulster2 by "quit working" you mean they quit transferring any torque and don't resist any rotation?
    – Andy
    Commented Apr 10 at 19:54
  • @Andy - Yup, that's pretty much what I mean. Commented Apr 10 at 20:05
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There isn't a single answer to this, there are many failure modes for a transmission. If the torque converter goes then it's pretty catastrophic event, all your transmission fluid will spew out and it will be a smoking heap. More likely in your case a gear will stop working, or the whole thing will just grind on you. It's not likely to give you any audible warning.

It's not a particularly high risk safety issue, for instance a brake failure. It's not going to seize up on the highway, the worst case is you will suddenly have no power to the wheels and you'll coast to a stop, which is unfortunate but not usually that dangerous a situation.

While I understand what your mechanic is saying, it all depends on how long you plan to keep the truck. Although it could be your transmission is on its way out, clunking into gear is a classic symptom of old, broken down transmission fluid, in which case a service could fix it. What your mechanic is saying is that once you open it up it could be a can of worms. The theory is that disturbing the fluid could dislodge gunk that could later block valves and cause a catastrophic failure. There is some truth to this but it can be overblown sometimes. If you're a mechanic it's an easy answer to a thorny issue, but it's not always the right one for your customer.

The problem you have is going to get worse, with more clunking and slipping, and eventually a complete failure. Remember that transmission fluid is not only a hydraulic fluid but also acts as a lubricant, once it breaks down too far it loses its protective properties and your transmission starts to wear badly. If you are only going to have this truck short term you'll probably be fine. If you need to keep this truck longer term you've got to deal with this issue, and you have a few servicing options:

  1. A transmission fluid flush: when you drop the pan out of a transmission only about half the fluid comes out, the rest is all trapped in the hydraulics. A flush gets all the old fluid out. I would not do this with an old transmission such as yours because this is where the biggest chance of dislodging crud lies
  2. Fluid change service: a fluid change service drains the available fluid and replaces the fluid filter, it's relatively non-invasive process, but it can in some cases cause issues
  3. Replace fluid without dropping the pan: this is something you can do yourself. You'd stick a syringe with a long tube on the end down the filler tube where dipstick goes, suck the fluid out of the transmission (do not try to do this while it's running! You also want the fluid to be warm, not hot) and replace it with fresh fluid of the same quantity. This replaces the fluid with fresh without disturbing anything. The benefits of this approach are it's super-cheap, and it won't make anything worse. You may have to repeat the process a few times, running the engine to circulate the fluid, but eventually you'll have mostly fresh fluid in there

I would suggest you try option 3, as it's a low risk and cost effective way to try and fix the problem. It may not solve it but there's no harm in trying.

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  • I'd expect an automatic to fail the same way a handshifted transmission fails: whichever disks got cracked up no longer function. I suspect it's more inconvenient/dangerous with an automatic though. Last time my manual failed, I could keep driving a small distance by keeping it in a gear that worked. That got me out of a dangerous spot. An automatic won't know which gear failed so you immediately have to cease driving.
    – Mast
    Commented Apr 10 at 13:25
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    Automatics are far more complex than manuals, they can sometimes lose a gear and keep working but that's rare.
    – GdD
    Commented Apr 10 at 13:55
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    @GdD - I think you are understating the complexity differences between a manual and auto ... :o) Commented Apr 10 at 15:02

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