2020 chevy trax hill start assist issue.

So as the title says, Im working on a 2020 chevy trax with a hill start assist issue, or so it seems. The symptom is when on a hill and the assist engages and holds brake pressure to keep the vehicle still it doesn't always release the pressure after the pedal is released. As this is happening if I pull up manually on the brake pedal it will release. There's not a lot of slop in the pedal and I don't physically pull up very much to get it to release. but when i do that it acts normal and drives fine otherwise.

The kicker to me is as this going on the second I release the brakes when this symptom is happening while I'm looking at live data it goes no comm with the scanner. I went on a half an hr drive watching live data and it never goes no comm until hill start assist activates and I try to drive away normally. But it boots me every time.
One thing I haven't done is check freeze frame data yet. I will do that as soon as i get back to the car later today.

 I do have some dtcs C0161 and B1527 are the ones that that pique my interest. I've seen some posts that say a BPP sensor, a master cylinder replacement due to swollen seals keeping the system slightly pressurized, and an abs unit have all been known fixes to this issue. Looking to get some insight so I can make a concrete diag on this and want to see what yalls next steps would be as I'm no diag master and am still learning. I've relearned the bpp in both the bcm and ecu. no change. Its shows active and not active when on the brake and off it respectively in live data. The Bpp sensor shows a change in voltage when applying break pressure and is around .49 volts showing with no pressure on the break. Shop key says up to 1v is normal when brake not depressed. 

Geddy, I think I'd follow the C0161 if this were in my bay. From the code set criteria for C0161:

• The electronic brake control module detects the brake pedal is not applied when the brake pressure is greater 15 bar (218 PSI) for 3 seconds.
• The electronic brake control module does not detect that the stop lamp switch signal changed after driving the vehicle at a speed greater than 40 km/h (25 MPH) and performing three brake stops

I'd compare the BPP signal against the brake system pressure after stomping on the pedal a few times. I'm guessing there's gonna be some residual brake pressure there. That'll set C0161 AND cause the brakes to drag. 

Having to pull up on the brake pedal would suggest something like a binding pedal or brake booster. Has anyone replaced the brake booster or master cylinder recently? 

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Sorry for taking so long to reply. So there's no data on the scan tool for brake system pressure and i don't have any gauges to hook up to the caliper to check it manually but im guessing that's my best bet. I guess ill have to get some. and as far as i know nobody has replaced anything on this car. my guess is master cylinder but i want to be sure. and with all the other stuff it could be i don't know how to pinpoint this one down exactly. im thrown off by it going no communication when its happening too. what do you think about that Tyler?

Geddy wrote:

Sorry for taking so long to reply. So there's no data on the scan tool for brake system pressure and i don't have any gauges to hook up to the caliper to check it manually but im guessing that's my best bet. I guess ill have to get some. 

No offense meant, but you're certain there's no brake pressure sensor PID in the EBCM? Service info says there is, but that wouldn't be the first time SI lied.

 

Anyway, you don't necessarily need that PID, it's just more convenient.

and as far as i know nobody has replaced anything on this car. my guess is master cylinder but i want to be sure. and with all the other stuff it could be i don't know how to pinpoint this one down exactly.

You could confirm the presence of residual brake pressure by lifting the vehicle (or even one wheel) and trying to spin the wheel(s) after pumping the brake pedal a few times.

If you can't spin the wheels by hand, take the master cylinder nuts loose and recheck. If the wheels now spin by hand, there's something keeping the master cylinder applied.

Digging around in SI, I found this step in the brake booster removal procedure:

lemon-manuals.la/Chevrolet/2020/Trax%20L...Removal%20Procedure/

"Compress the tabs on the top and bottom of the brake booster pushrod retainer (1) and lift the brake pedal (2) rearward to disconnect the brake pedal from the power brake booster pushrod."

 

In other words, the brake pedal doesn't have a physical stop, it's determined by the brake booster. Something to keep in mind.

im thrown off by it going no communication when its happening too. what do you think about that Tyler?

Yeah I got nothing on that one.   May be part of the problem? May be incidental. I think I'd still go after the brake pressure first.

Only similar condition I can think of is Honda/Acura products losing communication with the ABS module above a specific speed. Nothing wrong, just normal operation. 

None taken my friend. I meant there’s no actual system pressure pid that displays system pressure in psi. My thoughts were even if the pedals released it may have residual system pressure even if it’s not displaying voltage. I did reference that in the original post saying I’m showing .49v with pedal released and it does go up with pedal application and back down to .49 when released. Spec from SI is 0-1 normal release and 1-5 when applied.

So I had time to do some testing. I back probed the pedal apply wire and am getting 1.19v with brake released. More when applied And when I pull up on pedal or drops to 1v or slightly below. When Hsa is engaged it holds the brakes at the 1.19v and releases when pedal is pulled up to get it down to 1v. releases right when it hits 1v.
I pumped the pedal up and check all the wheels and the front two spin freely with seemingly no drag and the back wheels have slight drag.(nothing major but noticeable audibly and doesn’t spin as freely as fronts It is awd so I figure that may have something to do with it)

Loosened the booster thinking it was the master (it was dark and didn’t notice the master nuts I thought the booster was held on with studs and the master was secured on those) with the same results.
BUT with booster loosened  the Hsa problem disappears. It functions as normal. Tighten the booster back to 14 ft lbs and problem is back. Loosened again to confirm and normal hsa function 

Thanks for the help so far man greatly appreciated this whole things has baffled me a little. The cars been to two other shops, the dealer and an independent. I’m friends with the Independent shops owner and they’re really good. I think the issue was the owner didn’t have a good explanation to what the symptoms were because she couldn’t always replicate it. She said the brakes got hard and it made a weird noise. She didn’t realize it was a hsa problem. I just so happened to be facing down hill at my place and she said it happens in reverse more. And I popped it in reverse and viola brakes locked and made the “chattering noise” she was hearing.

The owner is one of my best friends wives so that’s how I got a shot at it. Her plan was to trade the car in if she couldn’t get this figured out.

And also I don’t always have the best approach to some of this stuff as I’m no expert.
I just do side work pretty much on problems that have been to other places before and the people are ready to give up. So I dig in as much as I can with my limited knowledge because I’m infatuated with diag and am trying to strengthen my knowledge any chance I get. I don’t get a lot of experience because this is not what I do for work nowadays. I worked at a shop in my younger years that fired the parts cannon at stuff and never liked that. So again thanks for the help I’m sure my lack of experience shows but I don’t care I just wanna fix stuff lol