alternator is not charging battery. changed relay board today. here's the rub. ran the car for an hour or so . got home and battery was dead. jumped it from my other car . started right up. removed pos battery connection and it stalled. thought alternator was no good but it runs the car but will not charge.

Did you remove the battery from the circuit? That is a real no-no. The battery must always be hooked up when the alternator is running.

In addition, you may have a bad voltage regulator.

Charge the battery.

Start the car.

Put a volt meter on the battery.

Pull the regulator.

Jump the field lead to the D+ lead.

If your alternator is good, you should see 16 to 18 volts on the meter.


If it does show that, replace the regulator.

[
Jump the field lead to the D+ lead.

i am new at this. please explain above also i thought that if the regulator was bad it would overcharge the battery thanks

I see what he's saying. Jump the positive lead so you bypass the regulator and measure what your getting at the battery without it.

The regulator can fail to where there is no voltage applied to the field.

Ok.. back up... Alternator function 101.


In the early days of cars , there were generators. Generators have big permanent magnets that surround an armature. The armature spins, and the armature passes through the magnetic field to make DC electricity. They will function even if the battery is not present.

The alternator is more efficent. It uses a fixed set of windings in place of the permanent magnets, but it makes AC current. There are diodes to turn it back to DC current for the battery.

To make the alternator work, you have to apply power to the field. By raising and lowering the voltage to the field, you control the output level of the alternator. If you feed it 12v from the battery, it goes to "full field" or maximum output, which is about 18v.

The voltage regulator measures the voltage of the battery, and in response to that voltage level it increases or decreases the voltage to the field. The regulator can fail in two ways, one is completely open, where it never applies voltatage to the field, and completely closed, where it full fields the alternator.

Normally they fail open. By full fielding the alternator, you verify that it and all of wiring components work.


If you look at the diagram below... (diagram courtesy of Pelican Parts)...




You will see where the regulator plugs into the relay board. When the motor is running, use a jumper wire to connect D+ to DF. The alternator will go to full field if it is working. Most of the time you can hear it start to whine slightly under load. Don't run it too long like that or you risk boiling the battery acid out. (by too long I mean 30 minutes, 5 minutes or less is fine).


Hope that helps!

much clearer will try tomorrow thanks

Ok, so what if the belt was off the alternator and you disconnected the battery while the engine was running? Would that damage the alternator?
If it would Richie I will give you the freshly rebuilt one I have here.

Nope... the problem is that when you disconnect the battery with the engine running, you take away the load for the alternator. So it sees a completely dead battery and tries to charge it up. With no load, it burns the alternator out.

No belt, no alternator charge. So if you disconnect the battery with the engine running and the belt off the alternator, the engine will quit.