steves_77vette

I'm at a bit of a loss as to where my problem is with this. I've been through several trouble shooting guides and they all lead me to the same point.....scratching my head. To set the stage, this is a new battery, less than 5 days old. The alternator is relatively new as well. I can hook up a voltmeter and verify +12 at the batt terminal and the #2 terminal output when fully off. When I turn on the ignition I get the GEN light turn on and then when the engine starts the light goes off. As I understand it, when the GEN light goes off it indicates that the alternator is creating flow OUT and available to charge the battery. However, after I've run my motor for 20 or so minutes my battery is depleted of charge and the motor won't turn over, etc. I charge it up and repeat the process. My batt meter in the compartment shows the proper readings as well.

What I can't figure out is how this can happen? Where can the fault be that would have the gauge and GEN light work properly but still not charge the batt? To set more context, this is after I've re-assembled my motor from a body off resto. The alternator and charging were all working just fine before the body off. New battery though, the old one was DOA after sitting for 6 months.

Garys 68

Check the voltage at the battery and alternator. It should be ~12.5V (ignition off). Start the car. If alternator output reads ~14V, it's probably OK. If it's 14 at alternator and 12.5 at battery, it's probably a wiring issue. If it's 14 at the battery too, there's a battery problem (unlikely since you can charge it).

steves_77vette

I know it's a wiring problem. The question is WHERE would it likely be given the observations? The only way the GEN light will turn off is if the voltage out of the Alternator is higher than the batt, ie it's putting out more than +12.5. But, it was also verified with a meter to be true. Which also means that the #1 terminal from the alternator is making it to the gauge panel. But from there I'm at a loss of where the fault might be.

tallgirl

Just because the alternator is charging the battery doesn't mean the battery is being charged enough to make a difference. Twenty minutes of running time is not a lot of amp-hours of charge. They can be gobbled up very quickly by repeatedly starting the engine, especially if it takes more than a few seconds of cranking the engine before it actually starts.

All of this is a lot easier if you have a battery tester. Mine has two jumper cable sized clamps and can put a simulated starter load on a battery to see if it can actually stand a load.

What you want to do is measure the voltage at the battery terminals while the battery is disconnected, while the engine is being cranked, the engine is running at an idle and while the engine is running at something like 1,500 to 2,000 RPMs.

A charged 12 volt battery with no load should produce something on the order of 13.2 volts (6 cells, 2.2 volts per cell). It takes more than that -- somewhere around 14.5 volts -- to charge the battery. So start by disconnecting the cables from the posts and measuring the voltage at the battery. Anything between, say, 12.5 and 13.5 is right.

Next, reconnect the cables, and while someone else is turning the ignition key, measure the voltage at the battery terminals. Internal resistance in the battery will cause the voltage to drop (Ohm's Law -- V = I * R, where V is the voltage drop, I is starter current, R is battery internal resistance) to something in the 8 volt range. Anything less and (generally speaking) you have a bad battery, bad starter ("I" is too high due to bad starter) or your motor is brand new and tight as can be.

Next, with the engine at a normal idle (the car should have started about now) the voltage should increase to above what the no-load voltage was. This indicates that the alternator is providing current to the battery. Might not be enough to charge it, but at least you're not drawing current from the battery to run the car. If the voltage does not increase to above the no-load voltage, you either have a bad alternator or a wire leading from the alternator back to the battery (heh -- lots of wires ) is broken.

Finally, increase the throttle until you reach something on the order of 1,500 to 2,000 RPMs. You should see the voltage at the battery terminals slowly increase, and then plateau. This is the voltage regulator in the alternator limiting the voltage so you don't fry your battery. If the voltage doesn't increase, you likely have a marginal alternator. If the voltage keeps climbing, to 15 volts and beyond, you have a bad regulator. If the voltage increases, but stays below 14 volts, you either have a bad alternator or your regulator is messed up somehow. (Hey, never seen that happen)

If none of this works, helps, or make sense, let us know.

bobs77vet

check out this website go to the 1 wire vs 3 wire alternator it will give you good insight to tackle this issue.

http://www.madelectrical.com/electrical-tech.shtml

stepinup

why dont you just cut to the chase and while the motors running,pull off the Pos. Battery cable?if the motor dies the Alt. isnt charging the bat. or working at all,if it stays running,it is charging the Bat.,kind of old school but has always worked for me.

mrvette

Seems as if Tallgal is a nerd....

welcome, fellow geek....;-)))

nerd.....'=-))

GENE

fl_rider

As long as you are turning 1500 to 2000 rpm it should keep running on the Alternator. "If it is charging properly"

Chris B

tallgirl

Because that's not definitive? He seems to have a system that mostly works -- car starts, light goes out. But it doesn't charge. Which means that current isn't going to battery, not enough current is going to battery, the voltage at the battery isn't high enough, starter is killing battery, blah, blah. If the light didn't go out, yeah, start car, pull the big red cable, see if car dies.

Oh, and I freely embrace my nerdness. I worked my way through college doing various jobs other than flipping burgers. Things like motorcycle mechanicry (Hondas) and welding big stuff. Somewhere along the line I had a lot of physics and chemistry and rilly hard math.

Garys 68

Well so am I. I just dont type fast enough to write full explanations like her.

tallgirl

Two words -- touch type.

iNdigo

I would go through your system with a multimeter and test for ground shorts. A good multimeter with continuity testing should do the trick. disconnect all your fuses (after first labelling which ones go where) and then fish around a bit. Then you can plug in one fuse at a time and go from there.

Just a thought

tbw

steves_77vette

I think I found it. It's contact resistance related. Seems my batt terminal coming from the alt was too high resistance due to an old splice in it. Cleaned that up and doesn't seem to be a problem anymore.

I too am a major geek.....I actually understood everything tallgirl wrote. Now that I'm in the motor-head world I enjoy talking this stuff much more. People ask me politely what I do and I usually just say "software". If I really explained it their eyes would glaze over in under 10 seconds. I won't do that to you all.

tallgirl

Glad to hear that!

Now, how do I get my signature to show up? I've tried several times and I seem to be missing something (now watch it show up this time ...)

The_Dude

I had very similar symptoms with a '79 I had. Light would go out and gauge showed it charging. I had the alternator tested and it was bad. Once I replaced the alternator all was well. Apparently the gauge and idiot light are worthless if your alternator isn't completely dead.