I'll try to make this short. Truck in signature Got off work Friday truck fires up as usual and I head home. Decide to stop for some things at the grocery store - park, turn it off, and set the alarm fine. When I came out the FOB wasn't unlocking the truck. I remembered how to unlock it manually and was ready to start it to keep the alarm from going off too long. That's when I notice nothing's powered up - no dome, no alarm LED, no dash lights, nothing. It's like the batteries were removed. Popped the hood and checked the batteries. They're at ~4.5 volts. Had my son-in-law swing by to give me a jump and we're getting very hot jumperr cables. So, we decide to yank the batteries and take them in to test. One battery is shot, so I decided after 4 years it's probably best to just replace both. We bring the new batteries back and drop them in and the truck fires right up and I drive it home with no issues.
I get up yesterday and figure I would check the alternator to make sure that's OK. Truck fires with one turn of the key no problem. The alternator's putting out 12.15 volts at the battery. Alright, I'll grab a new one and my issues will be done. Picked up a stock NAPA replacement and put it on, hook the batteries up and go to start the truck and I have the No-Power issue again. Check the battery voltage and they're back at 5V. Disconnect the batteries and start searching for a short. We pull the starter and recheck the the battery cable - no short. So, we figure maybe the starter is the suspect. Tried a backyard bench test and we get nothing. Back to NAPA. New starter installed - same issue. 🤔 We now have the short with the starter connected or disconnected.
We start disconnecting everything (one by one) and rechecking for the short - starter , GPR, alternator, etc. What we have found is with the little wires disconnect from the pos battery terminals, everything is fine. The voltage from the + battery post to the little wire passenger side is ~.12V, but on the drivers side it's ~10.5V. So, it would appear the problem is on the little wire off the drivers side + terminal.
I've pulled the relays on the fender well and have the same problem. Pulled the 30A breaker in the fuse panel and the 60A breaker(?) - no change. Any suggestions on my next move so that I won't have the truck completely disassembled in my driveway on Christmas eve? What I don't understand is why it worked fine (with the exception of the low alternator output) from the time we dropped the new batteries in until we swapped out the alternator. Or why it ran fine until I parked at the store locked it up When the batteries were attached, the voltage dropped to ~5V really fast. Disconnect the cables and the voltage slowly climbs back to ~10.5V-11V.
One more thing - for about 3 weeks prior to this, most of the time starting it, I had to double cycle the ignition for it to start. Turn once, it turns over fine but no start. On the second turn, it fires right up. I was attributing this to the cold weather we've been having. Maybe not. Thanks for any suggestions and sorry I lied about making this short. 😔

 

For it to flatten 2 batteries that quickly, something has to be getting hot. Connecting or disconnecting battery cables would throw some sparks.

What little wires are you talking about on the drivers side?

 

I have the same question as Paul, I don't recall a smaller wire on the drivers side battery, but I could be wrong. Also, not sure what year your truck is, but some of them came with a connection that sits on top of the drivers side front wheel well that has been known to corrode and cause all sorts of problems.

Here is a picture of it I stole from Google... Of course, it does have a cover over it, but just so you know what you might be looking for.

 

It's the small wire that comes off the pos post to the harness. This is what I'm looking at now. This is with the neg wire hooked up. The smaller wire with the neg lead is usaully connected to the stud on the pos post. So this looks like a direct short from the smaller wire to ground. I shouldn't be reading any voltage, should I?

 

I was just looking at diging into this guy.

After check: We're on to something Sous. It's not corroded or anything - just a little dirty, but disconnecting it drops the voltage to 0. That wire goes through the firewall. Time to look under the dash. 👍

 

So, the wire is the main power to the fuse block in the dash. If I pull most of the top connections on the back of the fuse block and the big oblong connection, I eliminate the short. That doesn't narrow it down much, unless it's the fuse panel itself. I pulled each of the fuses out before pulling the connections with no luck. Any chance it's the tuning chip? Didn't want to go through the hassle of pulling it if there wasn't a chance it was the culprit. More digging...

 

Your on the right track, but the only way to be sure is to continue doing what you were doing. I would remove the chip, that is just one more thing that you can roll out.

 

I just hooked the fuse box back up and brought the short back. I was going to pull the chip to eliminate that. Think that would be worth the time? I'm thinking bringing it back to stock before tearing into the fuse box anymore. There's a lot of connectors I pulled to make the short go away. Following that, it could be a number of things.

Follow up thoughts: Sorry, Sous, didn't catch your last sentence.

So, I don't think it's the chip. I pulled the firewall connector from the computer - completely eliminating it from the truck in essense - and the short remains. Back to the fuse panel. 😒

 

I decided to pull the fuse box out and tear into it to see if I notice any glaring issues inside before dropping a couple bills for a used one. No burn marks - clean as a whistle, all connectors seem to have continuity, resistors and the one diode check out OK. I cleaned it up and put it all back together and reinstalled the relays and fuses after giving the blades a good sanding to make sure I have good connections. I don't think the fuse box is the issue, so I'm slapping it back in and continuing from there. I'll have time this afternoon through Tuesday to dig back into it.

 

If the short/load went away when you pulled the fuse block then you know it is a circuit within the fuse block. You should be able to pull fuses one at a time to determine where the load is. If it is draining the batteries as fast as you say it is then you should be able to see and arc when connecting the fuse. An inline 10A meter would be best if you have one handy.

 

That's what was initially leading me to think it was the fuse block. I had pulled the fuses and relays before with no luck. I was thinking maybe it was further down the line - I can pull 5 of the connectors on the back and get the short to go away. When I get home today, I'm going to start from scratch. I may have been too rushed trying to get things done on Christmas Eve and overlooked something. The whole thing is weird - running fine one minute, completely dead the next, and then back and fourth. Hopefully, I'll find something today. Thanks, Bill.

 

"The whole thing is weird - running fine one minute, completely dead the next, and then back and fourth. Hopefully, I'll find something today."
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This really smells like a bad connection somewhere and if there is a resistance short to ground in the process it will drain the batteries in no time.
BTW, That photo of the cable in post #5 looks very corroded to me with all of that green fung on/in it.

 

That one's not mine! I'm back working on it a again. I agree on the short somewhere. It's just a matter of where. I just pulled the chip and put the computer back. I'm about to throw the fuse box back in and see what I can find. I've got 5 days to get it. Hopefully, it won't take that long.

 

Does your starter sound weak or is intermittent? The solenoid can short, which can seem to come and go.

 

It's a new starter. But what I have found is the GPR is a direct short across across the 2 bit lugs - 0 resistance. Question: with the key off, should the one lug have 12V to it? Or should they both be dead until the key is turned?