I just got my new Stingray and believe it or not, the 8 speed automatic went out at 81 miles. GM flew a new one in and replaced it, so all is well but I'm seeing something that I don't understand so I would like to get some feedback from other 8 speed auto owners. When I do an acceleration from a dead stop, using the sport mode and sport mode tach on the heads up, I first see green dots, then orange dots, then blue dots flash, then engine sounds like I hit the rev limiter, it sounds like it backfires almost, it then shifts and does exactly the same thing on the next gear. It does this through all gears. I did a PDR recording of this and the transmission shows that it shifts at 6500 RPM consistently and doesn't appear to be hitting the rev limiter. First, what do the blue dots mean? Second, does the engine hit the rev limiter at 6500 RPM? Anyone else experience this, or it is normal?

Thanks
TeaMan

 

My 2015 C7 A8 shifts the same way -- at 6500 rpm and sounds something like a back fire.

I have an aftermarket Trifecta tune installed, which re-calibrates both the ECM and TCM and their interaction.

 

That is normal, you are not hitting the rev limiter (I think that is at 6600 or 6700 rpm) what you are experiencing is a 100ms fuel cut off which allows the transmission to shift fast, this results in fuel igniting in the cats, again this is normal with both the A6 & A8 autos and has been discussed many times on this forum.

 

Thanks. I searched the C7 forum for Automatic, but didn't see anything. I get the green lights, and orange lights, but don't get the blue ones. Any idea what blue means?

Thanks.

 

The lights are the shift warning. These are useful in paddle mode or with the manual transmission since you can see them easier in your peripheral vision rather than having to watch the tach. You will see similar lights on F1 steering wheels.

The flashing blue (I thought they were white) are the last stage.

 

They could definitely be white, on my HUD they look Cyan. Could simply be an optical illusion.

Thanks for the help.

TeaMan

 

This topic brings up an interesting story. I mentored a COOP student about a year ago that was working on a sprint car that the University raced in competitions. He was working on the shifter, and used my shop to finish it. He built the circuit that did almost the same thing here. It cut the spark, then shifted so the transmission wasn't under load. He was able to get it to shift in less than 20 milliseconds. When he went to the competition the big auto engineers were extremely interested in how he was able to get this down to less than 20 milliseconds. He actually got a job offer on the spot from a company in Texas that did embedded controls in the auto industry. He ended up taking a job in his home state for a very small company doing embedded controls with transmissions that go into motorcycles. He was a very interesting young gentleman. He was so smart it was scary. Some of the things he was able to do for us in the short time he was there were some of the best work I've ever seen from a young engineer.

I didn't put two and two together, that GM was doing the same thing to shift their auto's. It would be nice if they put a line in their manual that explained this so when it happens to a $70,000+ car, you wouldn't think you broke something. It scared the bajesus out of me the first time it happened. It really cracked.

TeaMan

 

I think in our Vettes it is a combination of fuel & ignition cutoff and you are right. GM should at least mention it in the manual, they always have those annotation boxes in the manual and this could be one. This question has been brought up and talked about in the A6 & A8 and did not happen in the C6 because the C7 A6 was changed (Smaller torque converter) to handle quicker shifts than the one in the C6.

 

Mine are blue as well.

 

Off topic, but what exactly happened to your A8? What symptoms were you experiencing before it failed?

 

I didn't think there were any symptoms at all, but thinking back there were a couple things that didn't seem normal. One was, in park you could feel it pull like it wanted to move. If I rev'd it, you could really feel the pull. The other thing is when I was driving slow, when it shifted from first to second it lurched. I just thought it was because of the high performance and wasn't used to Auto's. Not sure if either of those things were actually abnormal, but could have been. It did shift pretty smooth, almost so smooth that you had to pay attention to it to be able to tell. Only very slow did it lurch.

I brought it to a dead stop, stepped it down fairly hard and let it go through the gears, I didn't notice anything until it reached about 60 mph, then heard the backfire sound or fuel in the cats, and it dropped into neutral. I though that was way too strange. I brought it to a stop, decided to try again to see if it repeated, and when I gave it throttle, the revs went up and car went no where. It had two codes when I got to the dealer, they were stuck solenoids in the valve body. The mechanic that tested it, put it in reverse to move it back and it pulled forward, no reverse. I explained the incident in detail to the mechanics there and they all agreed that this car should have eaten that performance up. No reason it did what it did. They thought it could have been a particle in the valve body after machining.

I did find out what it's doing as a follow up: My service manager called GM engineering. The new multi port fuel injection in the C7 is required by emissions standards for GM to cut the fuel when the valves start to overlap. Some fuel gets pushed into the cats, and it ignites when fuel comes back on. He said GM can't program it out at this time. He said it didn't have to do with making it shift faster and seemed disappointed that they had to program it this way. I mentioned to the service manager that the valves are mechanically tied to the cam and any valve overlap would be through the range of the engine. The only thing I could think of is that possibly the lifters pumped down at high RPM and caused a little overlap. Guess that's the scoop from GM.

Later
TeaMan

 

I think it sounds fantastic when it crackles between shifts! And I love the way sounds as it downshifts! I am impressed with the automatic in the C7!

 

I have a 2015 C7 with 8 speed (non Z51).

I routinely exit my subdivision onto a highway and need to get to 55 mph (I never go over the speed limit ) very quickly, and of course it is way fun. Have had the car for 7 months and never noticed a backfire till recently and it happens only in sport mode when I go from 0-55 in a few seconds. Tested Touring and Track modes and it does not happen. Many comments say this is normal but it seems unusual that it never happened for 6 months and then started. If anyone has seen this and has information on what was done to correct it, please let me know.

Especially curious if there is any way to change a setting etc. Since curing a rough shifting problem with service bulletin and specific driving routine, expecting this car to be able to fix itself... either way this is my favorite car in the world!!

 

I am having a transmission problem in the Sport mode. Sometimes when I punch it to change lanes or get on the freeway in the sport mode, it downshifts, but does not shift back to a higher gear it just over revs until I shift to neutral then back to drive. Has anyone else experienced this?

I have a 2015 A8 base (non-51)

 

Wow I went to Lebanon Valley 2 weeks ago and had this same "backfire" issue at 6500 rpm. Just went to my dealer today to discuss this issue. This thread came just in time for me. One less thing to worry about. Now i just have to get my whell spin under control

 

This is normal for spirited or aggressive throttle applications. It's also been discussed in many threads.

If you look at the center display, it will say Performance Shift Active. This will hold lower gears and higher revs since the car thinks you're on a track or driving a twisty road or spiritedly. And this is what you'd want in these cases: instant power available without needing to downshift, better engine braking, quicker shift algorithms, and downshifting under braking to hold revs higher.

Once this mode is engaged, I find it will ease its way back to normal in about 10 seconds of steady non-spirited driving. Or you can temporarily manual shift by clicking the paddle to up shift yourself (while still in D) to the gear you want. It will return to auto on its own. Or switch to Manual mode and back to D; I think this may work but haven't tried in a while.

Bottom line this is what Corvette engineers intended for Sport and Track modes to engage Performance Shift more easily. Not sure if it does this in Touring. I like it, and I wish it was easier to engage sometimes as I think the standard shift logic up shifts too quickly in all modes and the shifting is too smooth. Sport and Track should be more aggressive.

 

In this case it over-revs way beyond useful power. It prevents the car from speeding up because it does not upshift. It will not shift up regardless until I shift to neutral or shift it manually. I think it may be a software glitch, or at least I hope that is all it is.

 

Got it. Then I do agree this sounds like an issue. So it will just bounce off the rev limiter while in D and not up shift?

 

Yes, it just revs out to the max and I have to either manually shift it or shift to Neutral and then back to drive. It shifts fine in every other mode so I hope it is just a software glitch. I am on my way home from the Museum but next week she goes back to the dealer..

 

From my experience when the trans shifts below 4000 RPM's the TCM commands the shifting as they did in the previous A 4 trans----It feels like more of a delay than anything but still a crisp shift However when you get above 4000 RPM's the algorithms changes for the shifts----It sounds more like a sputter than anything----GM has decided that momentarily shutting off the fuel can create faster shifts than ever before and NOT be so evasive on trans life---At 1st I was concerned about this sputter until I learned it was by design----The new A6 and A8 is the fastest shifting auto trans than ever produced before by GM and now even quicker than the auto Trans's in foreign super car autos with a dual clutch system ( GM uses a priority clutch to clutch system) Plus it can handle huge amounts of TQ and HP--