I have a 78 LT with stock 305, the battery has been dyeing lately after running it for about 30 min. Not sure if bad alternator or not. Battery is new. After I charge battery fully the volt gage in cluster will show around 12-13 volts and runs fine, after running for about 20 min or so I noticed its been dropping to around 8 volts. If i keep engine to about 2000rpms it will increase and after about 5 min at 2k rpms it stays at 13 volts for a bit and drops again. Anyone have any ideas or way to confirm if alternator is not working properly? Whats throwing me off is it does charge when i hold rpms up for a few minutes.

 

You might wire brush all of the red wire connections, i.e., the junction block, and other attachment points before replacing alternator with a bad regulator.

 

So you think the regulator on alternator may be bad?

 

Have the alternator bench test .Then the wiring ,In a step by step method.Good luck.

 

Are you using the dash volt meter. They should not be used to diagnose. Only thing they are good for is to tell you to go get a real volt meter. Is the voltage the only symptom? is your car shutting off? As a general rule most HEI distributer will stop functioning around 8 volts. Once a system really hits 8 volt it will not crank the motor with out help from a charger or jumper cable.

If your battery is near dead you should be seeing closer to 15 volts when the alternator full fields. Try check the voltages at the battery with a volt meter, then check it at the large wire and engine block. Then lets us know.

 

Thanks for the reply, I think it was alternator,Before when I ran car at idle for about 20 min i would see it slowly drop at battery starting out 13.8-14.2 after i fully charged battery. when I put A/C on it would drop alot faster. I replaced alternator and then without A/C on it stayed 13.8.14.2 constantly no dropping. Put A/C and voltage would start dropping just a little now down to lowest 12.8-13.2. The idle was running at about 500-600 rpm's I increased the idle without A/C on to about 800 rpms. No when A/C kicks on the voltage is staying about 13.8. So I think that issue is resolved now.

Now I need to figure out why battery is slowly draining when car is off. I check it and after about 5 hours it goes from 12.47 to about 12.15. What would best steps be to start trying to locate the ghost draw on it?

Thank you and sorry for so many questions and issues.

 

There are some tricks to find a draw but you still need to understand what you are doing. It not the kind of thing you can teach in a thread.

Lets call this tips and tricks.

Step one disconnect the battery for the same 5 hours of time and verify it does not draw down on its own. Even new batteries can get small internal shorts.

If your car has a trunk light open the trunk and quickly feel the bulb as long as you are quick the bulb should be warm but hot. It it burn you then it is not turning off. If it is on all the time even the metal around it will be hot.

Next do that same process with the glove box light.

Assuming both of those are good you should pull the lighter fuse then just do the 5 hour test to see if still draws.

Next do the same thing with the accessory fuse.

Then I would go around the car and look for anything aftermarket and disconnect that. Radio, Alarm, fog lights or even a trailer connection. If it look like any of that was install by an amateur then you may want to start there.

After that you need a AMP meter. Some way to measure the voltage flow.

I have an inductive DC amp meter. I can just hook that around and battery cable look at the draw and just start disconnecting things. I don't think you will find that list in any manual but it use to find about 70% of the drains I would get back in the day. Those inductive amp meters used to be pricey but I think you can find them pretty cheap. Careful the regular multi meters that say they can measure DC AMP draw. The are very limited on the amps they can handle. Your draw many be small enough that the multi meter will work but if you open a door or the glove box you can fry the meter. Those meters are design to work on computer like circuitry. Milli-amps in most cases.

I did find this.
https://www.harborfreight.com/automo...ter-67724.html

You could test every fuse in the car just remember if the door is open then then the lights are drawing power and they will show a draw.

 

Well it may not be good on an alternator but one sure thing is if the car is idling and you pull the pos bat cable off and it dies the alt is not putting out what it should.
Your car should stay running on just the alt.

 

Thank you guys, I may have figured last part of puzzle out. I disconnected cluster and reconnected it all and cleaned up the lighter plug and reconnected it all. I also Wired in a glove box light instead of leaving disconnected. Its now been two days in a row not loosing any battery sat overnight and battery still at 12.41 volts not running. So so far so good will keep everyone updated.

 

Well the power to the cluster should turn off with the key off and a disconnected a glove box light will not cause a draw either. So the issue was the lighter. If it was rust just replace it. You can break the rust up but its just a matter of time before it comes back. I assume you do not use the lighter so just get a cover for it. If you do use the lighter then replace the element as rust tends to jump from one to the other. It used to **** customer off then they came back 8 month later with the same issue, explaining to them that it was the socket the first time and the element the second time does not make the customer you just had their car towed happy.