I suspect BCM... thoughts?

[Joseph Garcia]

Actually, Joseph, new BCM’s for the NNBS are closer to $200. Search GM 25835966, the correct one for the OP’s truck. I noticed this because I’ve replaced mine in my 08 Silverado and remember it wasn’t cheap. I ended up buying a used one.

You are absolutely right, Mark. I stand corrected.

I just re-looked at my BCM replacement invoice, and it was $172.

 

[intheburbs]

What I don't understand is that when the problem(s) occur, a quick disconnect and reconnect of the battery and everything is set right again. That's what was making me think computer related - like it was being reset.

The loose connection causes a voltage drop, throwing the electronics into disarray. You're right, doing a disconnect/reconnect reboots everything and gets it back to normal.

But the problem is wiring, not the computer(s).

 

[Doubeleive]

I find it highly unlikely that any intermittent problem is computer-related. They either work, or they don't.

This is completely and utterly a bad electrical connection issue. I'd even replace both battery cables. Check the primary feeds to the alternator, to the mega fuse holder, and to the load center. You checked grounds? Check them again.

And yes, those wires in the footwell were for a brake controller.

you are partially correct, however heat can cause problems with electronics. A electronic/computer can sometimes work perfectly fine until it gets hot, a component expands and looses connection. not saying this is the OP's issue but it can be a problem.

OP what you can try also is get a cheapy code reader if you don't want to spend the bucks for a tech2
when the no start issue happens, instead of disconnecting the battery try clearing the bcm codes even if it doesn't show any, doing this reset's the bcm
if it works and starts that may be a clue doesn't mean it's the bcm as it could be anything attached to it, but it's a clue that may lead to the root cause with a deeper diagnostic.

 

[Just Fishing]

New ground for battery to passenger head and to frame coming today. Other main ground was braided cable from rear drivers head to firewall, and also checked one under driver's footwell at the frame. My truck is pretty corrosion free and the connections are tight.
While I'm in there I'll take them off and sand/wirebrush the connection points as a double-check.

What I don't understand is that when the problem(s) occur, a quick disconnect and reconnect of the battery and everything is set right again. That's what was making me think computer related - like it was being reset.

Look down under the driver side.
there are several ground locations on the subframe.
one right under the driver seat, and another under the second row.
IIRC there is a third in there as well.

The one that was really bad was under the driver seat (If I Remember Correctly).

it was a large spade connector with a few wires going into the location.
I had to cut the shrink tubing off to see how nasty it was.
otherwise, it looked perfect.

If you don't want to cut and redo the connector,
a little rosin or flux with a little heat will flush the connector out.
then flow in some solder.

I noticed I didn't have very much wire length, so I decided to just clean and flow solder into the current connectors.


To re-seal,
I got creative.


The easy way would be to just use some liquid electrical tape.

but since I didn't have any of that on hand, I used some marine grade shrink tubing (built in glue)...
but since the connector is already installed, I used a light coating of black rtv (since I always have it on hand).
then took some of that shrink tubing that fit the wire nicely, cut it into nice tight spirals, spun it on the wire and then did it once more to ensure the loops crossed with slightly looser spirals.
Applied some heat to seal it up, after the rtv had a chance to thicken up, and finished with another coating of RTV.

 

[TrybalRage]

OP what you can try also is get a cheapy code reader if you don't want to spend the bucks for a tech2
when the no start issue happens, instead of disconnecting the battery try clearing the bcm codes even if it doesn't show any, doing this reset's the bcm
if it works and starts that may be a clue doesn't mean it's the bcm as it could be anything attached to it, but it's a clue that may lead to the root cause with a deeper diagnostic.

I have a super cheap bluetooth dongle that I use with an app - not the type that can read BCM codes, just basic engine codes and clear them, view ready status, etc.

When the truck starts acting up, it cannot connect so won't give me a reading of anything.

I'm afraid if I had one that could read BCM codes it might have the same problem.

 

[TrybalRage]

I got the new cable today but didn't get it installed yet. I also ran out and got another generic negative cable to add an additional ground (one of my searches here led to a video that showed that, and you can never over-ground, I suppose).

I also found this document: https://www.autocodes.com/uploads/gmc/08-07-30-021E.pdf So after I get the new grounds installed, and clean up the other existing connections, I'll take a look at the transmission connector as well to see if anything looks awry.

 

[petethepug]

If you need a code reader / clearing tool that stays the in the truck 24/7 and invisible there’s this option for under $2h.



There’s more that accompanies this but it allows you access to every code generated on the fly.

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www.btdieselworks.com

 

[Doubeleive]

I have a super cheap bluetooth dongle that I use with an app - not the type that can read BCM codes, just basic engine codes and clear them, view ready status, etc.

When the truck starts acting up, it cannot connect so won't give me a reading of anything.

I'm afraid if I had one that could read BCM codes it might have the same problem.

that's pretty odd, well whenever you get it resolved be sure to post up what it was
sometimes replacing the ground cable seems to fix things for a short period then the problem comes back so be sure to give it a few days

 

[chevylade]

I find it highly unlikely that any intermittent problem is computer-related. They either work, or they don't.

A bad BCM can cause very random and intermittent problems though my experience with a bad BCM didn’t cause any of the symptoms that the OP described. I’ve used Highway 71 for a programmed BCM and was satisfied with what they sent me. I agree that it seems like a connection problem.

 

[TrybalRage]

Well added the new negative cable today, sanded and wire brushed the connections at the lower front passenger frame, firewall on the driver's side, and ground under the driver's footwell. Added a new 4GA cable from the driver's head to the same point as the braided line on the firewall.
Reinstalled the BCM, heat shrunk the ends of the auxiliary trailer brake wires and zip tied them out of the way.
Truck fired and runs fine, so I guess now we wait. I parked it outside in the sun to simulate the conditions of last week, so we'll see what happens.