I checked all of the plugs today in front of the fuse block and did parasitic drain test and every thing checked out fine. The problem is that right now every thing on the car is fine, it starts, the battery is at 12.5 volts and holding and the alternator is charging between 14 and 15 volts. Whatever the issue is it has disappeared again as often happens. I will drive tomorrow to see if I can get it to show up again. Still looking need to do some of the other suggestions.

 

mine did the same thing ,as u say yours is , it would come and go and drive great for days then go crazy ,mine was a 08 and the brain for it was 08 only we tried to reflash other years and it would not take but as soon as i found the same year and part number reflashed it my dash lights went off and its never done the crazy thing since , and again this was sorted out by a 40 year GM tech i also just had a hhr with a check engine light i could not reset and a trac control light on i changed the tcm, and bam the light reset and the trac reset and no flash with that , so u never know some times u have to change parts , and yes its nice to have 30 plus hhrs for parts don't hate only trying like u to help ,,,

 

Thanks for the input. I am trying to avoid taking it to the dealer, but I may need to do that eventually. Since it is not acting up right now I am not sure I will find anything right now. More research needed for me. I wish I had 30 parts cars but I am trying to downsize. :)

 

I thought you were going to do the CANbus diagnoning. You don't think it would be helpful to know which module is complaining?

 

Diagnosing CANbus
Yes, I completed the CANbus diagnosis. Now the condition presently with the car is that it is running fine, starting right up, the gauges are NOT fluctuating and there is No check engine lights displayed. So I really didn’t expect to find anything and this is what I got.

On the DLC’OBD2 Connector
Going from pin 4 (chassis ground) to pin 16 (battery): 12.43 volts
Going from pin 5 (signal ground) to pin 16 (battery): 12.43 volts
Going from pin 6 (HS- Can high) to ground : 2.58 volts
Going from pin 14 (HS- Can low) to ground : 2.37 volts. This should total 5 volts so it is pretty close.

The Ohm integrity measuring between pins 6 and 14 was 61.3 ohms so both resistors in the computers are working.
So all of the measurements and readings described in the videos (post 16) are within the range. This is honestly what I expected since the check engine light is off and the car is running fine.

The frustrating part is that the problem is not constant and that is why I have been checking fuse locations, grounds and all of the electrical connections. The car has spent its entire life in Charlotte NC, one owner, 181,000 miles, no rust or accidents and well maintained.

In the back of my mind I still think is has something to do with low voltage from the battery but this is the second one I have put in from AutoZone and the load test came back fine, as well as the charging system and starter, although I am still a little suspect. The alternator (I replaced it August 2019) is working fine right now, but does anyone think that as you drive the car that is might go from charging to not charging, thus lowering the battery voltage to below acceptable conditions and causing it to throw the codes and go into limp mode.

I am frustrated right now but we will figure this out. Fortunately, I have my Mini Cooper that I can drive so I can take my time before going to the dealer. I don’t think they will find anything either right now until it starts acting up again. If one of the computers is acting up sometimes, but fine when I take it in, will they see anything other than the stored codes?

Sorry to go on for so long, but thanks to everyone for their input.

 

You probably fixed it by plugging/unplugging things. That is the usual problem.
Your battery is low at about 70%..
The BCM controls the alternator, the output is based on need so it varies constantly. NOT like a 70's on/of regulator.

 

donbrew

In post #2, I already said that after my wife waited 30 minutes and started it up, everything was fine except that the check engine light was on because of the codes. When she got home and it sat for another 30 minutes when I started it the CEL was off. That is when I pulled the codes, but it was running fine at that point. It is an intermittent problem, I didn’t fix anything by checking all of the connections, I was just looking for possible poor connections and grounds and I have not found any yet.

Yes I too think the battery is low for a brand new battery but it checks out as good. That is my concern but this is the second battery AutoZone has given to me, I traded in the first after 2 days and they gave me another one. I need to drive it so when the CEL comes back on, and it will, I can check everything while it is in limp mode.

 

Have them load test the battery and check the charging system.

 

Here's a thought, and probably is not the problem. But it can be tested fairly easily, so I'm going to throw it in here for discussion.

The large wire that connects the starter to the alternator, has what may be a fuseible link in it. It's not identified as such on the wiring diagrams, but it kinda looks like one. Anyway, that wire could be removed and tested for continuity, wiggling and flexing it. If it is a link, maybe it's conducting intermittently.

 

Well you didn't check wires very well if you didn't jiggle them around a bit. I.E. bad pins in connectors, broken wires etc.

​​​​​​​ At this point I don't know what you have or have not done. You seem to be all over the place. None of the codes you have cited point to the EBCM without further testing when they are still current., not pending or permanent/historical. They all do point to a loose/broken wire or connection. If you do the CANbus diagnosing when C0550 is current you can narrow down the particular module that has the bad wire.

At this point all you can do is jiggle wires and connectors until you duplicate the symptoms or accept that the problem is gone until it comes back.