2012 Silverado lunges forward while braking to a stop

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When I brake, it seems to really lunge forward. I can only describe it as aggressive downshifting? I have been advised by a fellow Silverado owner to turn off the Active Fuel Management with Flashpaq? I would love to hear what some of you guys have to say on this. It's a 1/2 ton crew cab 5.3L 4 speed automatic. 2012. Thanks
 
Other forum discussions I have found describe it as "lurching" forward. Happens as transmission downshifts.
 
Just my random experience: last year my passenger car experienced a lounge forward per se when I was almost to a full stop. It only happened once, but I checked it out the next day and discovered that my brake fluid had gotten really dirty, and probably contaminated due to the extremely humid summer and stop-and-go tunnel traffic I have to go through. I'm thinking the hydraulic pressure didn't force the brake pads to catch and hold, so I did a brake flush and haven't had that issue again. Given the fact that we're going through another humid summer, I'll probably take a precaution and flush the lines again during my next tire rotation.
 
By aggressive downshifting if you mean earlier at higher speed that would cause higher revs and would be the opposite of lurching forward. If it were older I'd guess maybe the Idle Air Valve? Not sure what the AFM would have to do with it.
 
Originally Posted By: 901Memphis
Nearly impossible to diagnose across web with the limited info we have


Nonsense! We're all psychic!
smile.gif
 
You're not doing something silly like driving around with tow/haul mode activated are you? (button at the end of the column shifter)

Some people like to drive around all the time with it on because of the firmer and later shifts, but doing so will cause more aggressive engine braking and downshifting while slowing down.
 
I am not in tow haul. Definitely high revs. From 2 to 1 is the worst. I have a feeling it's normal. But I read where a guy turned off DOD/AFM and never experienced it again. Stopping gradually it doesn't do it. Only in harder more sudden braking. I maybe wrong in describing lunging or lurching, but it definitely feels like it's under more power than what I am used to. I appreciate the replies, I will check back again. Maybe it's the computer, and it's normal, like an engine braking program? I'm no expert. I service fluids and change filters and gaskets but after that, I am not a knowledgeable mechanic.
 
I have been in several 2010-2012 Silverados with the 6 speed auto. They ALL clunked from 2-1 downshift and have a "surge" feeling when almost stopped. Even worse with the 4x4 models. I have a buddy with a 2014 that claims the same thing. His words not mine, "the forums say the all clunk and downshift rough". Nature of the best?
 
Originally Posted By: SquirrelMan
I am not in tow haul.


So what does it do when you are in tow/haul? Get worse, stay the same, etc?

I also believe that transmission has adaptive shifting, perhaps try disconnecting the battery for a bit and letting it relearn? Worth a try, don't you think?

Just noticed you said it was a 4 speed? I didn't think that was still available in 2012 behind a 5.3? If that's really the case then there isn't adaptive shifting but a PCM reset still might do it some good.
 
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Originally Posted By: SquirrelMan
When I brake, it seems to really lunge forward. I can only describe it as aggressive downshifting? I have been advised by a fellow Silverado owner to turn off the Active Fuel Management with Flashpaq? I would love to hear what some of you guys have to say on this. It's a 1/2 ton crew cab 5.3L 4 speed automatic. 2012. Thanks



This is a "feature" of those trucks. It is caused (in part) by the AFM. What I would suggest doing to test this out is to move your selector to the manual position-and then move the gear button to the "5' position. This is great for around town. If the transmission is prevented from going in to 6th-the AFM will not come on.

A tuner is not needed to turn off the AFM. There is a company that makes a black bot that plugs in to the OBD port. You just unplug it when you take it in to the dealer. It DOES NOT OVERWRITE the ECM.

https://rangetechnology.com/

I have the same year as yours in a GMC Sierra with the 5.3. This is my second truck-the first one was a 2011. So I'm pretty familiar with the issues with these trucks.

The issue with a tuner-is that it over writes the ECM (and-yes it can be discovered by the dealer) and will void any powertrain warranty you have.
 
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Just noticed you said it was a 4 speed? I didn't think that was still available in 2012 behind a 5.3? If that's really the case then there isn't adaptive shifting but a PCM reset still might do it some good. [/quote]

I just noticed he said 4 speed as well... If it's indeed a 4 speed then he has a 4.8 V8 or a 4.3 V6 which neither of those have AFM in those years.

I have driven, rode in, and owned countless 4L60e's for 2 decades and never experienced this 2-1 downshift and surge. So if it's a 4 speed then my experience tells me that something is wrong with that unit.
 
Thanks for your time guys, I have never engaged tow haul. Only had truck since December. I could swear when I bought filter and gasket for tranny I had to specify 4 speed or 6, deep or shallow pan. I also don't remember manual mode showing 6 speeds, I will check. Also, I don't feel the transmission moving through 6 different gears as I build up to 60mph on highway. As a matter of fact, it happens when I pull into McDonald's and it's very rough bump pulling in. I will double check my transmission specs.
 
My cousin actually recommended the Range unit. If I want to turn off AFM. He couldn't stand AFM but he is very particular
 
One thing for certain is the AFM is not responsible for this issue. AFM only engages in sixth gear.

I have a hard time believing this truck has a 4 speed auto as well.

OP, your truck is out of the AFM bad history. All resolved now and still suffering from internet amplification. Due to the bad experiences of a small percentage of purchasers the AFM gets blamed for everything! This could be as simple as a throttle body sticking due to deposits.
 
Sorry, I have the 6 speed. Man it sure is smooth. I only notice 4 gears. When I get to 60mph on a flat it will settle into a relaxed drive. The flowmaster rumbles low when it does it. I thought that was the engine kicking into AFM. Maybe it's just 5th or 6th gear. I seem to remember having to specify 4 or 6 speed while looking at gaskets online. I ended up going with local parts store maybe I gave him the VIN code because the filter was identical and the gasket matches up it even had little rubber Dowels for location. I could have run across the different speed specifications when I was trying to figure out what type of rear differential I had as well I probably just don't remember correctly. You guys had me second-guessing myself I looked under the hood to see if it was a 5.3 which I was quite certain of, then I started it up and put it in manual and I'm able to go to 5. Did not go further as number 5 told me it was not a 4-speed. I think when I used manual before I didn't go beyond 3 and that's what had me thinking 4-speed. I always learn something on here I appreciate your knowledge.
 
No. AFM engages at lower speeds as well.Even when your slowing down to stop. You Post is incorrect. However-it will only engage at all speeds when selector is in D or manual 6th position. If manual selector is in 5th or lower it will not engage.
 
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Originally Posted By: CKN
No. AFM engages at lower speeds as well.Even when your slowing down to stop. You Post is incorrect. However-it will only engage at all speeds when selector is in D or manual 6th position. If manual selector is in 5th or lower it will not engage.



Thanks for that. My experience is only with 3500 6.0/6L90E service vans and they are programmed completely different than even 3500 6.0 Silverados. I would still suggest OP's issues are unrelated to AFM...
 
I am not too excited about turning off any settings if I don't have to. If it's normal I can accept it. It doesn't clunk. Just revs high when downshifting. You can feel it. It's quite noticable.

I appreciate everyone's input. I check back in case anyone else has something to add. Sorry I didn't break up the last post for easier reading. Also, sorry for the name, got tired of choosing names already taken so came up with something off the wall.
 
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