Thanks Hillbilly, I'll contact the dealer tomorrow. For now I cleaned the battery terminals and drove around the neighborhood several times and all was good except the check engine annunciator still on.

 

Well if the light is on after you pulled 12V, it's real upset about something. There are a limit on freeze frames. New codes will over-write and push out old. But the data on what was going on when the code set is valuable. Especially when trying to re-create the event.

 

Sorry, didn't make that clear, I used a chemical battery terminal cleaner, leaving the terminals still connected. I wasn't too sure about pulling the positive cable without some sort of memory backup device installed. I didn't know what the repercussions of completely resetting the 12v system would be.

 

Removing 12V deletes codes and memory. Sometimes that is a last resort in clearing a code that won't let you go.

 

Thanks for the help hillbilly

 

Summary of Discrepancy.
Loss of brake pressure, pedal went almost to floor.
Brake warning annunciator
Stabilitrak caution annunciator
Service brakes soon message ON DIC
Stabilitrak failure message on DIC
Occurred during drive home after work, vehicle had sat for 10 hours. Radio on low volume, air-conditioning on high on a warm day, about 10 minutes into the drive.

Duplicated the discrepancy several times after that, always with the air conditioning on high. Three days in a row. The fault would go away if the air conditioner was turned off and the vehicle left to run for 5-10 minutes.
Also duplicated it one time after a trip to auto parts store to have the 12 vdc battery tested. This of course would have placed a high load on the 12 vdc battery and the battery would have needed to be charged back up. Again after 5 minutes of running the vehicle, the faults went away and the brake system worked normally. The sales guys at the auto parts store told me my battery was fine.

Of note in all of this, when this fault happens, there must be a electric pump for a brake accumulator pressure, I don't hear this pump running. However, when the vehicle runs for 5-10 minutes and the faults go away, it is because I can hear this brake accumulator pump come back on line and charge the brake pressure back up.

Took to dealer and they found the following codes; C12B1, C0110, C0277, C12E0. They verified TSB 08-05-007 software update is current. They were advised by tech assist to replace the master cylinder. I didn't do that at this time.

My main thought has been that it seems like a electric power issue, possibly related to the 12 vdc battery. I understand the air conditioner runs from the 300 vdc battery, but possibly when it is on high, the power management system cant keep the 12 vdc battery charged up properly.

I replaced the 12 vdc battery with a new one and have driven the vehicle for the last two days with no further issue (so far).

Question, does anybody know what voltage this brake accumulator pump runs off ? I'm suspecting the 12 vdc battery, but haven't verified this yet.

 

Brake controller and pump run off the 12V system. You have a solid state alternator or Accessory Power Module APM. That APM can produce about 175 amps in the 12-to-14 volt range when loaded. Combined 300V loads like APM input and A/C compressor including High Front and Rear Blowers are maybe a around 6 kW depending on outside temperature and humidity..

6000W/300V = 20 amps from the HV system.

Now to your brake hydraulics. There is an almost softball sized round high pressure accumulator mounted to the main brake pump/valve/pressure sensor assembly. It is under floor on the driver's side. Like vacuum brakes, power-steering pump assist Hydro Boost, and now your accumulator equipped vehicle, you should have one (brake pedal) pump action of stored assist energy at almost any time after starting and running.

Even if someone could cut the 12V cable while you are driving!

A long lecture here on the three systems. I'm ok giving one but not today. I have been turning wrenches for a living 42 years now and every brake pedal that I have seen go to or almost to the floor was a hydraulic problem. I can't guarantee that here but that's where I would start.

A loss of brake assist should feel like a very hard pedal but not lots of travel. In all 3 systems mentioned above. When the rubber cups inside a master cylinder start to go there can be intermittent (excessive) pedal travel incidents. When these cups begin to go, a very slow pedal apply will have the most internal leaks. So a very slow, light, and steady pedal apply can have a very long and almost to the floor pedal travel without stopping power. Then release the pedal and a FAST jump on the pedal will result in a high and firm pedal with plenty of brake force. Two very different behaviors of the same hardware.

Hope you get this fixed.

 

Thanks again Hillbilly, you're a wealth of information I quickly discovered! I understand electrical systems, especially complex electrical and hydraulic systems as I've been a corporate aircraft technician for about 33 years now. But, obviously I don't know the GM hybrid design, nor does there seen to be a lot of technical information that I can get my hands on. So again, thank you for your help in this.

I'll continue to keep you updated with any additional troubleshooting information as it comes up.

 

No problem. You will catch on fast. The 2-Mode was a huge jump in automotive technology. Some systems almost by-wire. However your brakes still have a real master cylinder to push fluid to the calipers. Also, you still have a mechanical connection to the steering.

One can still "dead-stick" land one.

 

Hello I'm interested in what was the outcome? I'm thinking of buying a 08 looking at all issues before I buy ..mine will be out of warranty so I'm being cautious