If the batteries are truly in parallel, they will try to equalize their voltage,
so the 9.5v battery was dragging down the 12.5v battery to the point there was not enough voltage to start your truck.
I don't have dual batteries in my truck, but I do in my boat and have had that happen before.

 

Is it dual battery system or auxiliary battery? Does it have the relay for Aux battery in the junction box under the hood?

TP2 should be aux battery which is not tied to the starting system, and only connects to charging system once vehicle is running via the relay.
Dual setup is different and are tied at all times.

 

I am going off of what I see based simply by initial looks under the hood and what the RPO codes are. If you are able to provide clearer understanding of how its wired I can look deeper.

I will look for said relay too.

 

Sorry, I didn’t me to over complicate it. If you have the auxiliary battery setup (relay in junction box) then you likely just have a dead battery and that’s why it started when you swapped them. It will only ever start off of the main battery.

In other words, your relay separates the battery from the starting system, and only connects the 2 together once the vehicle is already running. When it is running, you should see charging voltage at both batteries.

 

Oh you didn't over complicate at all and cleared up a lot of what my thinking was. My truck before was a 2500 HD Duramax so I thought it worked in the same sense but then found out it does not and really confused me.

I plan to figure out this system and make sure its working properly before putting another battery back in.

From what I have read and got notes on the TP2 code is the following:

• Secondary battery
• Secondary battery is only used when the truck is running and not during start
• Primary battery (driver side) is only used for starting
• Primary battery and secondary battery are charged when vehicle is running
• 4WD fuse in cabin fuse box is required

There maybe a solenoid and relay in play but haven't gotten around to finding those yet.

 

So in a 2007 Silverado with the 6.0, the TP2 option was set up in 2 ways like firebane stated.

There is a relay on the firewall near the primary battery by the transmission dip stick. That relay will only engage when the ignition is cycled to the on position prior to starting. Once the vehicle is started, the relay will close, charging both batteries.

So fuse position #68 controls the accessory wire to the trailer plug. With the fuse connected, it will run accessories off both batteries. When pulled, it will only provide power to the constant 12v pin on the trailer plug to power trailer accessories, without depleting the truck starting battery.

In your description of the issue, either the primary starting battery is weak, the terminals are compromised, loose ground somewhere or the alternator is not functioning properly. Once you start the vehicle, the charging output based on the starting battery voltage should be at least 14v and will stabilize as the battery charges. If not, I'd look into the alternator or to see if the BCM is functioning correctly.

There is a way to bypass the relay by providing a 12v source to keep the relay closed during starting, but I haven't come across someone who has done that. I believe the diesel engines have this feature incorporated into their wiring.

Hope this helps..

 

I think the diesels just run the batteries in parallel, for the extra umph needed to start the engine.

 

Correct, but I'm not sure how the wiring is configured for the diesel battery set up.

 

Remember OP has an 07 classic - GMT800. Your info is pasted from a GMT900.

 

You're right. My body builder's guide has a range from 2007-2013..on the GMT900 platform.

If it's on the GMT800, there is a wire in the under block fuse panel, I believe a pink wire that is connected to the relay / solenoid on the firewall. This wire triggers the relay / solenoid when the ignition is cycled and is closed when after the vehicle is started. You can change this wire connection with a constant 12v source to have both batteries start the truck if desired.

When I upfitted my 03 Yukon 2500 using factory parts, the schematic showed 2 ways to wire.