Po571 Chevy 2500 6.6 2008

HI guys I have a po571 on this truck I have power on pink wire of brake pedal switch there are four wires the other wire I tested was the blue whtstrp it had power with pedal depressed both tests with key on *brake lights work I am leaning toward a bcu ecu com problem (not a parts changer)there is logic at light on dash (logic screws me up)*the service brakes soon light keeps coming on the light on dash comes back at same exact time interval and other tech in shop said it will kick off the cruise control I am Fairley new at this and I have access to a modus ultra in our shop with two channel scope *pretty confident settin it up after a few years of watching scanner danner in action either I am not checking the right part of this circuit as it pertains to the cruise control and why its getting shut off and service brake soon light on dash keeps coming on I looked at code setting criteria but this one is a bit much for this beginner thank you ! PS I am the new tires guy in this shop

Hey chief! You didn't say exactly which Chevy truck you've got, but I figured it's a K2500 or the like. Here's the diagram with the brake switch:





Based on your test at the light blue/white wire, we can say that half of the switch is operating correctly (at the time you tested it, anyway). This is your basic pull up switch input - the light blue/white won't have power on it until the switch is closed. The other half of the switch works the same way. With the switches closed, pin D purple and pin A light blue/white should show 12V or close to it.

Before you go too far in your pinpoint testing, you might have a glance at scan data. Both of these switches have corresponding PIDs which may be beneficial. Here's a shot off a known good '08 GMC 3500 that uses the same switch:

Thank you Tyler that is very helpful I will check for a pid on that brake switch I will also study that known good and scope the switch

Hi as far as I know the brake switch is in question I am not sure what wave forms are good and which ones are bad and also I do not know how any of those would effect the shutting off of cruise control and put service brakes soon light I think I am missing the strategy being used and exactly what parts of circuit to test I do not understand why the 1v ignition sign or any of those waveforms I have no idea how they would pertain to this code and why its kicking off cruise control and putting brake message on dash I do see the brake switch gets signal from bcm I would test at that point I am not thinking logically or do not know what to test on that switch I watched some videos where the same truck received a new brake switch and it cleared the code but I am not gonna just change the switch unless I know exactly why I am doing it thank you

chief eaglebear wrote: Hi as far as I know the brake switch is in question I am not sure what wave forms are good and which ones are bad and also I do not know how any of those would effect the shutting off of cruise control and put service brakes soon light I think I am missing the strategy being used and exactly what parts of circuit to test I do not understand why the 1v ignition sign or any of those waveforms I have no idea how they would pertain to this code and why its kicking off cruise control and putting brake message on dash I do see the brake switch gets signal from bcm I would test at that point I am not thinking logically or do not know what to test on that switch I watched some videos where the same truck received a new brake switch and it cleared the code but I am not gonna just change the switch unless I know exactly why I am doing it thank you

Nah, you're thinking logically - you don't wanna throw parts at it!

I don't have Mitchell in front of me, so I don't know the exact strategy at work, but I can take a guess. The brake switch has two switches inside it, electrically independent of each other. One reports to the BCM, and one reports to the TBCM/ECM/TCM/PTO. These are both pull up switches if you want to refer to Engine Performance Diagnostics.

Both switches will change state when you step on the pedal. My guess is that the BCM and ECM are watching BOTH switches at the same time, and this code is setting when one switch or the other didn't change state when it should have. That's where the scan data comes in - if you're stepping on the pedal and you don't see 'BPP SIGNAL' or 'TCC/CC BRAKE PDL SW' change, then you're witnessing the problem.

Again, just a guess, but I'm betting the issue is with the 'TCC/CC BRAKE PDL SW', or the part of the switch that reports to the ECM/TCM. You get all those messages on the dash because the ECM/TCM no longer trusts it's brake pedal input, and is forced to turn the cruise off as a safety precaution.

I dunno how intermittent this problem is, but I'd suggest watching scan data first to see what's going on with those sensors when the code sets. You could also use a DMM connected to the purple or light blue/white wires and cycle the brake pedal repeatedly or wiggling the connector. If you catch either switch not changing state, you've found the problem.

Thank you after a few years of studying with scanner danner I thought this would be a little easier lol but this will be the first time I have had a scope in my hands I do have a multimeter it seems like I need to go to the omputers in question and see if they are getting a good switch signal I have virtually no guidance with sc ope in my shop Thier good teks but a little old school thanks for the awesome strategy explanation I will revue my pull up pull down circuit designs thanks

If I monitor ign 1v signal would I see it disappear when cc gets shut off by ecm

OK I am seeing a little better I am gonna just do basic volt checks from bcu to switch on power side and I was thinking about unplugging daytime running lamps to see what happens also I see the trailer circuit tied into ecu side of switch maybe just check power and grounds for modules in question thank you

I think the hard part will be getting to bcu I might need to do some piercing

chief eaglebear wrote: I think the hard part will be getting to bcu I might need to do some piercing

You can totally go to the ECM and BCM if you want. But, I'd honestly suggest keeping it simple and starting at the switch itself. Knowing circuit design, it'd be very easy to verify module and circuit integrity from the switch. If you end up finding a circuit issue, then you can totally dig whichever module out that you need at that point. Don't worry about the IGN 1 voltage for now.

For example, if you end up finding a failed brake switch, you can do bypass testing. Watch those two PIDs on the scanner, and short across pins C and D, or pins A and B. One PID or the other should change state as you apply the short, as this is exactly what the switch does.

Don't let the diagram make this look more complicated than it is. Observe the scan data and the switch behavior when the fault isn't there, so you know what to look for when the fault happens. I'd suggest leaving the Modis hooked up to the scan data, and watch the switch with a DMM.

Thanks for all your experience and advise I am not sure if I will get back to this truck I only work in this shop wed Thursday Fri as a tire guy but I will go in on my time off Monday today and let the tech who put me on the truck know which test and what direction we should go on this po571 I appreciate the advise as I refuse to be a parts changer you don't learn nothing from that and there is no excellence in guessing

Hi little update I was not paying attention when Senior Tech gave me the problem and told me at that point the truck had received a new brake switch I will attempt to measure voltage at computer bcu and ecm or would measuring current be better I will attempt to inspect wires visually

I dunno that current will help you, especially on the BCM switch. Given that the switch has been replaced, I'd start looking for aftermarket wiring under the dash. People love to find those wires and ScotchLock into them for aftermarket BS.

Very interesting that I got an '08 GMC 2500 with the exact same complaint. :lol: It actually does have a bad brake switch. I can post scan data when I get home tonight. Are you sure you're not in Kansas? :silly:

Hi thanks for the insights turns out they told me the fix was topping off brake fluid I am closing this one out

Really? :huh: Wow, that's surprising. Not arguing with your coworkers, just wasn't aware that a level problem could cause this code.

Thanks for keeping up updated!

I did not argue but kinda felt the same thing I could not put it together I did ask a couple times.?? I also asked if they took it for a test drive to see if it was still kicking off cc they did not ??

chief eaglebear wrote: I did not argue but kinda felt the same thing I could not put it together I did ask a couple times.?? I also asked if they took it for a test drive to see if it was still kicking off cc they did not ??

Eh, sounds like it was out of your hands! :silly: Maybe you'll have another shot at it if it comes back again?

On the '08 2500 I saw last week, I found the right data list to watch:



I finally caught it acting up when I'd tap the brake pedal the right way. CC would kick off, wouldn't engage again, and the code would go to 'Current DTC'.



You can see the 'BPP CIRCUIT SIGNAL' shows applied up to frame 300, while the other brake pedal PIDs show released. :silly:

Ah OK I see those waves and those circuits I wonder I would go directly x3 13 and x5 b1check voltage?

chief eaglebear wrote: Ah OK I see those waves and those circuits I wonder I would go directly x3 13 and x5 b1check voltage?

Definitely. I dunno how tough the BCM is to dig out of this truck... underneath the steering column, IIRC?