Good battery, dead truck

[Muzzy5.0]

OK, here goes ..
1972 C10, 307 V8 3 on the tree.
Been trying to chase down a suspected vacuum leak, truck runs OK but will not tickover off choke. Anyway, it cut out the other week and was dead when I tried to restart. Thought the battery had died, so hooked up a jump pack, fired up and continued on. Used it again another day, no power problems.
Left it a couple of weeks, went to start it to carry on investigating the vacuum leak, totally dead. No start, no crank, no lights, no dome light, nothing.
Checked battery, good. Replaced voltage regulator and ignition switch.
Got power across from the battery to the multi splice connection. Got power to the bulkhead connector. Got power into the ignition switch. The “always on” parts of the fuse box are showing 12v as well. BUT, when I switch the ignition to the on position, the “always on” parts go dead …..
What am I missing ?

[HO455]

I would be going through all the ground connections. Starting at the negative post and follow it to the frame or block and then check the rest of them. Disconnect each one and clean with a wire brush removing all corrosion and paint. I like to put copper seize on them when I assemble them.

For example my Burban has the negative lead to the block on the water pump. (Photo 1) Then there's one on the drivers side head that goes to the frame. (Photos 2 ) Sharing the bolt on the frame is a lead to the core support. (Photo 3) Then on the back of drivers side head is a ground strap that goes to the firewall of the cab. Sorry to say no photo of the one at the firewall.

Arrows indicate the ground connections.

[Lee H]

How old is the battery? Did you do a load test on the battery? The battery may show 12 volts but may not have enough amps to run anything.

['68OrangeSunshine]

Check your Fusible Link. It runs from the Positive cable lug to a terminal on the Passenger Side fender.

[PbFut]

Check the connections on battery post, and starter. Especially the purple wire on the starter solinode.

[Willshook]

First check for me would be for a parasitic drain somewhere. All good advice for other stuff to check, but if the battery is draining while it's just sitting, this is the first check IMHO.

Easy, quick, and just takes a DVOM; basically put the meter between the battery cable and the battery and see if there's current flow. Then pull fuses until you find out which circuit, then trace the circuit.

FWIW, if you have an aftermarket stereo...99% of the time that's the source.

https://youtu.be/zdIKNnwEjIs?si=r9R1eQfyOIHEGLMk

[Stocker]

Originally Posted by Willshook

First check for me would be for a parasitic drain somewhere. All good advice for other stuff to check, but if the battery is draining while it's just sitting, this is the first check IMHO.

Easy, quick, and just takes a DVOM; basically put the meter between the battery cable and the battery and see if there's current flow. Then pull fuses until you find out which circuit, then trace the circuit.

FWIW, if you have an aftermarket stereo...99% of the time that's the source.

Good advice -- I would add you can use a simple 12V test lamp if you don't have a VOM. If it lights up, there's a drain on the circuit. You can do the same on each fuse position to pinpoint the drain.

[Muzzy5.0]

I’ve checked the earth connections, they all seem to be OK.
The battery is fine, there is no drain on it. Everything is the same even if I connect a jump pack to it as well.

I’m getting power to the ignition switch and the ‘always on’ parts of the fuse box.
However, when I cycle the ignition to the ON position, the ‘always on’ connections in the fuse box loose power.
What is also confusing is that even with the ignition off so there IS power to those parts of the fuse box, I still don’t have any lights or the interior dome light.
They should work at any time as they are independent of the ignition side yes ?

[Steeveedee]

Have you put the battery in another vehicle and verified that it works? I've had batteries that showed a full charge but when put to the test, wouldn't do anything.

[Rust_never_sleeps]

It sounds like something's wrong at your ignition switch. Maybe grounding out something when the switch goes to "on"?

[franken]

Measure the battery voltage with the ignition off, and with the key in the start position. Post the voltage, such as 12.6V here.
I'd follow the wiring from the battery through the connector on the fender all the way to the solenoid.
If the voltage goes away then the key is switched on indicates it a dead battery, or a bad connection.
Time to go back to square one and start over.

The thing that draws a lot of current, requiring large cables is the starter. The starter bolts to the block. Therefore the only large ground cable needed should go to the block.