Sorry all, Its another alternator question.

[2Kblacksleeper]

So i finally get my other issue taken care of. Now when i am driving the car, when i get above 4,000 rpm the battery light comes on and the volt gage starts dropping.

My friend and i took a volt meter and at idle the car is running at 12-13 volts. The battery alone is at 12volts. When you rev the car how ever it starts dropping down to 8-9 volts.

Does this sound like the voltage regulator?

I previously had taken the alt to auto zone and it passed the test.

What else is there to check for?

Thanks again for helping.

John

[1992MustangDW]

kinda sounds like a volt reg problem, but usually it will over charge?:dunno:

[2Kblacksleeper]

My friends dad still seems to think that it is the alt again. I am not sure. It passed the test at Autozone. I hope that someone else has had the issue also and can help.

Thanks for your response though.

John

[A05C]

have the battery tested.

check all the cables for corrosion.

you might have to do a charging and alternator test while the car is running.

[2Kblacksleeper]

I had already done the 2nd and 3rd items you suggested. I will take the battery in the next few days and have it tested though.

I am asking this beacuse i have no clue, but why would it only drop the voltage above 4k rpms?

Thanks again.

John

[A05C]

have you done an alt. test while the vehicle was running? and check the actual output

it could be numerous things or something very minor. its real hard to try and diagnose an electrical problem over the internet lol

[2Kblacksleeper]

When we did the tests the car was running.
First we just checked the battery, It came out fine. 12volts ish.
2nd we started the car and the meter read out at 12-13 volts.
3rd we revved the car and it starts dropping down as low as 8ish and then would return down to 12-13ish.

His dad said that it should be at like 14ish when it is running. Does that sound right?

Thanks again, I am new to all this and i want to make sure it is completely road safe and wont leave me stranded some where.

John

[WARMACHINE]

Is the belt slipping?

[mustangjon]

13 volt or below means battery is discharging the alt is not working. I vote for the voltage regulator, they are notorious for going out and cheap parts generally quality that is. even at idle it should be low 14's at lowest, now it could be possible dead short internally and a dead cell in the battery causing it to not take charge when its running, but highly unlikely, take the battery loose and take into autozone ect for a load test to test that..

[2Kblacksleeper]

Thanks for you info.

It was a new belt on the car. I replaced it when the I took off the alt to replace it the 1st time. You know i wanted to do things right the first time, and i am still having issues. I guess it is just life. I will take the battery tomorrow to have it tested.

John

[1992MustangDW]

the alternator is only tested at a low rpm if it has some coil issue on the inside the faster it spins the harder it is for it to generate volts

[A05C]

the proper range is 13.5-14.1v

the proper way to test a battery or alt. is to have the car at ~2-3000rpm with the lights on, as many internal electrical devices as possible running to put a good load on the battery and alternator.

a voltage drop test is a good way to find out if the item your testing is bad. IE: alternator, battery, so on.

the right way to do a voltage test on an alternator is to put your positive lead on the output terminal of the alt (while having loads on the charging system, and at ~2-3000rpm) and go to the positive post on the battery with the negative lead. that takes care of the positive side of the charging system.

the way you check the negative side is by putting the positive lead on the case and the other lead on the battery negative (loading the battery also)

thats a quick way to check the alternator and charging system to see if its the cause of the problem.

on your meter
positive side you should see no more than .5v
negative side you should see no more than .2v

less is good for either one