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Discussion Starter #1
Seems like every morning I get a weak crank on my truck. Rest of the day is fine. Installed new set of AC Delco’s a couple weeks ago and same thing. Put a volt meter on the batteries in the morning and they are fully charged at 12.6V. Truck charging system appears to be working fine. Charging at ~14.2V. Seems like my batteries need to be at 13V+ in order to get s healthy sounding crank on the motor. Why cant I get a healthy crank at 12.6V? (fully charged resting voltage)
 

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2009 Sierra 2500hd, CCSB, 2wd, Duramax
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Seems like every morning I get a weak crank on my truck. Rest of the day is fine. Installed new set of AC Delco’s a couple weeks ago and same thing. Put a volt meter on the batteries in the morning and they are fully charged at 12.6V. Truck charging system appears to be working fine. Charging at ~14.2V. Seems like my batteries need to be at 13V+ in order to get s healthy sounding crank on the motor. Why cant I get a healthy crank at 12.6V? (fully charged resting voltage)
maybe try a load tester instead of a volt meter?
 

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Temperature (cold) can be a contributor, as can bad grounds, battery to engine, engine to frame, engine to firewall, etc. And as bawskee306 suggested, you may have a new, but 'bad', battery.

When you replaced your battery, were you having a consistent problem starting your truck? Low battery voltage, long crank times, high amperage draws, can damage the starter, so it's possible that it could be the starter, or starter connections.
 
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Miles? My 2010 LMM started acting up, left me stranded once but fired with a boost pack. New battery (just the main) replaced under warranty and was good for a month then happened again. Never realized how slow it had gotten. Popped in a new delco starter and wow, didnt realize how whooped it was. Assume original starter at 190k.
PS I did the new starter after checking/cleaning all my grounds. Honestly thought maybe cables or ground. Starter cured it and 15k miles later still going strong. A parts store load tester can see if you are over drawing indicating might be starter or bad ground. It'll draw more load compensating.
 

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I had the same experience with an LML. I put 2 new batteries in and a few months later found the passenger side battery dead. The cable had lost connection to the battery terminal so I was only working with one battery. Never realized how slow the cranking had gotten until I was back to running on both batteries.

You can check ground path faults by running a jumper cable from the (-) battery terminal directly to the starter. If that helps, look for a ground issue. Probably check/clean the (+) terminals again if you haven't already. If none of that helps, like someone else said, you might need a starter. Diesel starters can have a pretty rough life.
 

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Discussion Starter #6
Truck has 130K mi and yes I had been dealing with these weak morning cranks on my old batteries for some time before changing them out. I'm definatley running on both batteries and both are getting charged as well. I'll check all my grounds but the starter going out sounds like a real possibility. Probably just go back to Autozone and see if they can run a load test while starting.
 

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I think you have got some good advice to follow up on here. Put a load test on the batteries, probably fine since they are new. Check the grounds, I have seen the grounds cause issues with starters until it killed them. If that doesn’t fix it, then spend the money on a starter.
 
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Discussion Starter #8
Are the ground locations on these trucks documented, or do I need to just get in there and chase wires?
 

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If it only happens the first time you start it in the morning, it could be a slight hydrolock from a leaking head gasket.
I really hope I'm wrong. But I have seen starters replaced when it was hydrolock.
Good luck man!
 

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Discussion Starter #10
If it only happens the first time you start it in the morning, it could be a slight hydrolock from a leaking head gasket.
I really hope I'm wrong. But I have seen starters replaced when it was hydrolock.
Good luck man!

Its whenever the truck has sat and batteries are allowed to settle back at 12.6V. Typically in the mornings. Wouldn't I be loosing coolant if it was a head gasket/hydrolock?
 

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It doesn't take much coolant to hydrolock.
I just threw it out there because I have seen it. Usually it will turn over hard one time, then it will be good until it sits overnight.
I don't want to scare you. I'm probably wrong in your case.
I hope you figure it out and it is something simple.
 

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Are the ground locations on these trucks documented, or do I need to just get in there and chase wires?
Most Chevy trucks in the last 20 years or so have very similar ground points, though the circuits attaching to the ground points may have changed.
At the bottom of the radiator support, both sides;
at the side near the front of the block, under the exhaust manifold, both sides;
At the A-pillar, under the front door hinge, vertical support on the outside of the frame, both sides;
From the firewall to the back of the engine block, driver's side, inboard of the master cylinder;
inside the cab, at the dash, near the radio speakers, both sides.
Oher points are used, so if you don't think your particular electrical device is covered by the above, look at the wiring diagrams.

And to answer the documentation question, yes they are well documented in the wiring diagrams published and are free from Gm at GM UPFITTER - Body Builder Manuals. Find the ELECTRICAL MANUAL for your year model of truck. If your year doesn't have what you're looking for, go earlier or later, likely earlier.
 

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If you placed new batteries, I’d load test at full charge. If that checks I would do a parasitic drain. If that checks out I would remove the starter, if you have a shop near you that rebuilds them have them test it if not get it rebuilt or replace new.
 

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Another thing to look at is your codes, just had the same issue on my truck, 05’ LLY, and it turned out to be a glow plug control module.
 
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You need an inductive amp meter to check for high starter draw. Also need to measure system voltage drop. Put a volt meter in series from the + battery post(s) to the starter terminal. Anything over about 1.5 volts while cranking and you have a high resistance point in the circuit. Locate it by moving from point to point back toward the battery and repeat the test. Do the same test on the ground side.
 

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Seems like every morning I get a weak crank on my truck. Rest of the day is fine. Installed new set of AC Delco’s a couple weeks ago and same thing. Put a volt meter on the batteries in the morning and they are fully charged at 12.6V. Truck charging system appears to be working fine. Charging at ~14.2V. Seems like my batteries need to be at 13V+ in order to get s healthy sounding crank on the motor. Why cant I get a healthy crank at 12.6V? (fully charged resting voltage)
There is an isolator between the batteries so it is possible one is getting charged and the other is not. Need to isolate them, check voltages and do a load test to be sure.
 

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Where is this isolator and what does it do?
It is buried within the electronics of the vehicle. I initially thought that if I attached a charger to one battery it was connected to the other, not the case. You have to disconnect them from ground and then charge them seperately, but when the engine is running and everything works they both charge of course. Sounds like you need to disconnect the batteries, get them load tested and then check voltages with engine off and engine running.
 

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So what is the purpose of the isolator? My only assumption would be that it would isolate one battery from being drawn down in the key off scenario.

But on my 2012 when I had a draw that drained my batteries overnight, both were drained. So I guess maybe I'm confused.
 

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I assume it was to protect one from the other, but if one is drained when the batteries are connected the flat one will drain the other very quickly and likely not start the engine.
 
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