furious george

I have one hell of a weird problem and thought I'd throw it out there before I start chasing it down and pulling things apart.

My truck will show ~12 volts in the ACC position if turned to on (not start) it pegs under 8 on the gauge. For that matter in the ACC position if anything is turned on it shows the same reading. If I turn on the lights the blinker (in fuse block) buzzes.

It can be jump started and immediately turned off and will then start again just fine; everything will also work. After driving for a while we are back at step one. To add to my confusion this just started the other day with nothing added to the electrical system that would have caused this.

Soo is this a common or rare problem anyone has had/heard of before? Or any pointers anyone has that can aid me in my chase of this electrical gremlin.

abecedarian

get the voltmeter out and check the voltage at the battery- don't rely on the gauge for much more than a hint.
should be ~12v with everything off, ~14v with the engine running.
if you don't get 14v running, check the charge fuse.

furious george

I'm running all aftermarket gauges and not relying on the factory junk. The voltmeter is my next jump, have to get a new one after my was 'lost' in my last move. It just seemed odd that after a jump with no time for the truck to take a charge off the alternator that it could be started repeatedly with no problems.

Bigblock

Do the headlights work?
Bigblock

abecedarian

in that case, what's the voltage with the engine running? as before, if it's not around 14v you have a charging issue so check the fuses.
Getting 8 on the meter suggests the battery is only getting a surface charge and the electrical load from the ecu, etc. is enough to pull the voltage down quickly. That may be due to the charging system or a failing battery with one or more dead cells.

furious george

Bigblock - the headlights work, when running

System shows around 15V when running so it would seem that the alternator is charging. Going to check it after I get off work on Wednesday. I've never had a battery work fine when started, then when shut off and started 10 minutes later suddenly fail or drop multiple cells.

My fear is a very serious drain that will only destroy a replacement battery. Thanks for the help so far; I'm still open to ideas

abecedarian

the drop from 12 to 8 just by turning to on does suggest a significant load. do you have a stereo system with amp(s)? or possibly, like mentioned, the battery has a dead cell or is possibly sulfated or even overcharged and low on electrolyte. Then you'd get a surface charge that may be enough to start it again within a few minutes but deteriorate rapidly after that.

now though, when you say around 15 do you mean less than or more than 15? anything more than 14.7 or so will shorten the battery life considerably, and the higher the voltage the quicker the death of the battery.

furious george

I have an aftermarket head unit, but no amps or anything to draw that much power. As for the charge it is less than 15 however I will have to confirm that with a voltmeter. Thanks for all the help so far; to be truthful I would rather replace the battery than track down a ghost or stray ground in the wiring. I will let you all know what I find out when I get a chance to check out the truck again.

furious george

Believe I've found the problems. I had the battery and alt tested:
Battery - state of charge 12.32V, rated CCA 690 measured CCA 268, health 38%, charge 66%

Alternator - Cranking volts 10.4V, Load off 13.36V, Load on 12.96 (lights+blower), diode ripple .12V

So it seems both are bad/going. The tester they had at the hobby shop was pretty sweet, it prints out a little receipt with the readings.

Thanks for the help abecedarian

abecedarian

It looks more like the battery is a problem than the alternator, so me being me, that'd be the first thing I'd do. The ripple on the alternator's not enough to make a difference in charging so maybe the weak battery is what's drawing the charge voltage down to 13.36.
...and see what I mean about not trusting gauges ,