Good evening. I'm looking for a little help to diagnose a battery charging issue. When I'm running no extra electronics, the voltmeter reads low (between 10/12) but seemingly high enough to not drain the battery. If I turn on the headlights, the voltmeter drops to 10 or below and will drain the battery. Ask me how I know. I have attached photos. It's a 1982.

Im an amateur wrencher at best but this seems to likely be a problem with the alternator. My only hesitation is the one item of maintenance that the PO actually did. Probably 5 years ago (or more) he replaced the alternator with a rebuilt one. That certainly doesn't mean that this one didn't go bad. The belt seems to be turning fine.

So two questions. Is there any other troubleshooting I should do or should I just replace the alternator? If I should replace it, I have an 84 parts car that I could pull one of off. How difficult is it to replace the alternator and should I try the one off the '84 or just order a new one?

Thanks in advance for thoughts.

Tim

 

Don't rely on the volt gauge on the dash. They are notorious for being in accurate. It has to do with the way is it grounded through multiple systems and the accumulated resistance, or something like that. Do some archive searching if you really want more info.

To verify the accuracy of your volt gauge and verify the volts being supplied by your alternator, you should be testing at the alternator, or more conveniently at the battery. Get your multimeter out and measure the volts being returned to the battery.



Do the test with the car running and not accessories on. You should see about 14.4. Anything below 13.7v would indicate insufficient charging. Compare the values on the alternator to what you see on the gauge. Turn on a bunch of high load accessories and compare the readings on your multimeter versus your dash.

Your dash gauge is just as low as mine even though the volts being charged to my battery good.

You may find you do have insufficient volts being returned by your alternator. You can replace the voltage regulator on your alternator and that will help too.

 

This is great. Thanks. I'll try and then report back.

 

Reporting back. I finally had a chance to break out the multimeter and test the alternator. Running without any extra load, it reads 12.33-12.39. With lights and stereo, it runs at 12.13. Clearly it is not charging. The question is why.

The car is now running really uneven. I'll post a couple of videos shortly to demonstrate. I don't know if that's related or not. As always, your wisdom and guidance is appreciated.

https://youtu.be/pHJ1hd0XsQs

https://youtu.be/aWpmq4OYng4

 

I trust you are aware that the alternator cannot supply all the juice that is needed at idle/low rpm condtions. The alternator does not need too many revs to address the balance but it sure needs some.

Suggest you run your tests again and report the numbers at say 2500 and 3000 rpm and take it from there. If you hae a spare alternator changing it out is not a huge job.

First thing I would check is your earth strap at the back of the car. A lot of folks seem to think this is a non consumable item but it is anything but. Look specifically at each end to check the condition of the braid where it enters the fastener and under the heat shrink covering- you may find it is discoloured and the wires badly frayed. If so time for a new strap. Amazing what difference a new strap can make to the feel of the car during starting.

Your battery state of play may also be in the reckoning at idle and when starting but when running above 2k rpm you sould see 13.5 volts typically displayed once the alternator is controlling things.

Rgds

Fred

 

First things first. As mentioned, the dash gauge is very unreliable. But, when you turn the key to the "ON" position...is there a small red light illuminating just below the 10 volt mark? If not, you could have a couple different issues, one in the dash cluster or the alternator.

Mine was out and ended up being the alternator.

Just jack the front of the car up, add jack stands, test the small wire (exciter/field wire) going to the alternator...you should have around 11.7 to 12.4...depending on quality of connections and resistances. If you have that and still get nothing more than battery voltage while running...it's most likely the alternator. If you have no voltage or extremely low voltage at the exciter wire...then you'll have to backtrack towards the ignition to find the problem.

Brian.

 

Thanks for the feedback. I'll run the tests again above idle speed though I don't expect that to make a difference. Driving the car for about 35 miles at highway speeds still drained the battery to the point that I had to have the car towed the last 15 miles home the day the issue was discovered.

I do have a spare alternator in my parts car. I do not know if it is any good but I can swap that in if I get to that point after running through the further diagnostic tests mentioned above.

 

Hey V, been following the thread.
I think you are on the right track.
I would confirm that the volt readings aren't higher at higher rpm's, as Fred mentioned above, but I would also expect readings higher then what you are getting even at idle.
Where did you take the readings from? Hot post, CE, battery?

You may want to also pull your battery before you go further, have it load tested on a big machine -- not just the handheld units.

Hey BTW, I love the concept of you jumping in on the car and having a whole parts car as back up -- that's something I wish I had done early in the game.

 

Thanks, Dave. I took the readings at the battery. The battery is brand new. It's been in the car for about 6 weeks.

I paid $800 for the parts car and it has easily paid for itself at this point. I need to finish stripping out the interior (dash and seats--my difficulties in removing the seats are an issue for another post) and then get it out of the garage (per my lovely wife's gentle requests). So far, I've pulled off about $1500 (retail cost) in wheels, lenses, interior pieces, minor engine pieces. It also came with the spare, tool kit (though rusty and w/o sunroof tool), jack and emergency kit, none of which I had.

 

i will bet you need a new voltage regulator as the brushes are probably worn,
only use bosch parts