C7 AFM / Transmission issues.

[Stingray]

If you don't have a problem you can't really expect GM to do work under warranty. As they have probably told you a requested fluid change will cost around £500.

There's loads of information about the A8 on Corvetteforum and elsewhere if you google for it. My understanding is that if you have the problem you will know it, typically described as being like driving on a bumpy road and people say you may also see the rpm needle fluctuating a little. It's also my understanding that changing the fluid is a genuine "fix" for any issues which may arise. Only if people keep on driving around when there's an obvious problem will the lock-up clutch suffer accelerated wear.

Apparently the lock-up clutch is released and re-engaged as the cars switch between V8 and V4 modes in order to smooth the transition. The "problem", which is believed to relate to hygroscopic properties of the early transmission fluid, arises when the transmission gets confused about whether it's locked up or not due to slipping of the clutch and gets into a cycle where it keeps on releasing and engaging the clutch, causing torque at the rear wheels to surge up and down which gives a feel like driving on a bumpy road. That's why they say you'll know if you have the problem.

As they say, if it ain't broke don't fix it!

 

[Chevrolet]

Ian Allen did say that. I explained that I recently acquired a UK car still under warranty and wanted to know if it had been flushed, or would not be affected as it may have had the newer transmission fluid in it. They said that due to GDPR - they could not tell me if it had had a flush (under previous ownership) which sounded rather ridiculous to me. They could not tell me anything about the car or if it had the new fluid, out of the factory (September 2018 registration, I do not know when built but it is a 2019 model).

Was the car originally supplied and always serviced by Ian Allan (their stamps in the book)? If so, odd that they cant/wont tell you if it has been flushed? Only red '68 plate UK C7 I have seen for sale recently, was the one being sold by Parts USA who also serviced it. Think Stingray is correct above, when he says the latest non hydroscopic trans fluid came out circa March 2019?

 

[Stingray]

I'm not a wizard on these things but take a certain interest as my car is an imported 2015 with A8 transmission. Yes, I've had a precautionary fluid change done by Ian Allan as it's an early car. My understanding is there have been at least two changes of fluid specification since A8 was introduced. Presumably later cars (such as 2019) will have left the factory with whatever fluid was OEM specification at that time.

TeamZR1 has posted up links to GM service bulletins about the A8. If I have understood correctly, only North American market has the concept of "flushing", where a special machine pumps new fluid through the transmission and torque converter. The European and RoW solution seems to be conventional fluid change by dropping the pan.

 

[Oneball]

Unless there’s a drain plug on the torque dropping the pan from the box will only drain 50% or less of the fluid.

 

[Stingray]

What's needed is a mix of fluids which enables the lock-up clutch to grip. For owners who have had problems the Euro bulletin specifies two fluid changes one after the other which, compared with a single change, will increase the proportion of new fluid in the mix. If an owner has not had problems and simply wants a precautionary fluid change a single fill may well provide sufficient comfort.

 

[Chevrolet]

Latest Mobil 1 trans fluid from July 2019 according to this. No mention of the AFM V4/V8 operation being part of the problem:

TSB Seems To Fix 8-Speed Automatic GM Transmission Problems

A transmission flush using a new fluid may finally fix the shudders and harsh shifts of GM's 8-speed gearboxes.

gmauthority.com

 

[Stingray]

My understanding is the TCC (Torque Converter Clutch) is used for gear shifts and also to smooth driveline disruptions in steady speed running, such as when the AFM (Active Fuel Management) transitions between V4 and V8 running. Hence if a driver feels shudder it's best to switch to a different drive mode where V4 is locked out, reducing the use of TCC.

Amongst the comments on Corvetteforum somebody posted, "In the case of my car and the descriptions I've heard from others, the shudder occurs while in V4 when light acceleration is applied. Apply too much and it goes back to V8 and the shuddering stops, as it does when decelerating. When transitioning there is often a minor bump, especially noticeable going from V4 > V8 going up a grade. This is normal and doesn't bother me in the least, but the TC shuddering is quite a different animal."

GM-techlink summarises, "A torque converter shudder vibration may be induced by the torque converter clutch in normal driving mode (with an expected TCC slip amount) and is most often due to TCC friction material/ATF degradation). Other sources of a vibration include insufficient damping or isolation of the engine or driveline, an irregular combustion event, and Active Fuel Management (AFM) disturbances."

I suspect the "clever technology" in C7 transmission is what's led to complications. For instance, "The torque converter is 258 mm in diameter and features a lock-up clutch, although clutch and converter specifications vary depending on the application. The 8L90 uses electronic controlled capacity clutch (ECCC) technology, which employs a small, regulated amount of slip to dampen out engine pulses for a smoother running drivetrain, especially during shifting." My guess is it's the monitoring of that regulated slip that goes adrift if the original fluid gets water in it.

Fortunately it appears that if owners notice any change in the behaviour of their driveline a simple fluid change will head off any issues.