Alternator/Regulator Explained and Tested
Thread Starter
Le Mans Master
Joined: Apr 2000
Posts: 5,098
Likes: 1,351
From: New York New York
2024 C1 of the Year Finalist - Unmodified
2024 C2 of the Year Finalist - Unmodified
2023 C2 of the Year Finalist - Unmodified
2023 C1 of the Year Finalist - Unmodified
Alternator/Regulator Explained and Tested
I just posted this on my website. I hope it is helpful.
This is for systems with separate voltage regulators.
Here is a basic explanation of how an alternator/regulator works:
When you start the engine a voltage is sent to the number ‘4’ terminal of the regulator, then it goes through the regulator, out the blue wire to the ‘F’ terminal of the alternator.
This energizes the field of the rotor and creates a spinning magnetic field inside the stator windings of the alternator. This starts the charging cycle. Now a small amount of voltage is sent back out the ‘R’ terminal of the alternator (white wire) to the number ‘2’ terminal of the regulator. This closes a set of points and allows the battery voltage from the red/orange wire to power the field circuit. If you had an idiot light it would now turn off. The amount of field current depends on the electrical load that is placed on the system. Now that the stator is producing current it will maintain the battery and take care of all the loads that are put on the electrical system.
Remember the battery is there to start the car and add current to the system only when the alternator is not producing enough to cover the electrical loads.
So if you have 37 amp alternator and you have 40 amps of electrical loads, then 3 amps will be supplied from the battery. This means the alternator has no current left to charge the battery and the voltage reading across the battery will be below 12.66v.
A properly working system should maintain a battery voltage of approximately 13.8-14.8 volts depending on temperature.
Always check charging volts with a fully charged battery and engine above 1000 RPM.
If the battery is very low when tested, then the charging voltage will be low. As the battery begins to charge then the charging voltage will rise.
A fully charged battery, after any surface charge is removed, will read 12.66v.
Alternator/ regulator testing:
First, check for battery voltage at the stud where the large red or orange wire connects to the alternator. You should have battery volts. If not, repair the wire or circuit.
Then disconnect the plugs from the regulator and alternator.
Check the continuity of the blue (field) and the white (relay) wires between the plugs.
Repair any wire if they do not have continuity.
Reinstall the alternator plug and leave the regulator plug disconnected.
With a voltmeter, check voltage at the number 3 terminal on the regulator plug, it should read battery voltage. There should not be any voltage at the other three terminals at this time.
If you have voltage at the number 2 terminal, R on the alternator, you have a leaky diode in the alternator, replace the alternator. This will cause a battery drain and the regulator with be hot to the touch with the key off.
Now turn the key on, you should have voltage at the number 4 terminal, this wire comes from the fuse panel. If you had an “idiot light”, this would be from the bulb.
This is what is called the exciter circuit, without voltage to this terminal the charging system will not begin to charge.
If no faults were found so far, leave the regulator plug disconnected, make sure all wires are clear of the fan and start the engine.
With a voltmeter connected to the battery and the RPM at approximately 1000-1200, jump from the ‘F’ terminal of the regulator plug to the ‘3’ (orange or red wire) terminal.
You should see an increase in voltage at the battery and hear the alternator working. If you hear a growling noise from the alternator you have either a bad diode or a bad stator winding.
Don’t leave the jumper connected too long because you don’t want the voltage to go over 15 volts for an extended period of time.
If you don’t see the voltage go above battery voltage after a few seconds then you have a bad alternator.
If the voltage rose quickly and then decreased this means you have a slipping fan belt.
Now connect a voltmeter to the number ‘2’ terminal (white wire) of the regulator connector and again momentarily jump from ‘F’ to 3. You should see about 8-10 volts.
If not, you have a bad diode in the alternator.
If all is OK up to this point reconnect the regulator connector.
If it is still not charging, run a separate ground wire from a good ground to the base of the regulator. Now if no there is still not charging, replace the regulator.
If you have an overcharging condition, it is either a bad ground at the regulator base or a bad regulator.
This is the basic testing procedure that I have been using for many years. There are other possible problems that can cause a no charge condition but they are rare and harder to diagnose.
Any questions or comments you can email me.
Joe
This is for systems with separate voltage regulators.
Here is a basic explanation of how an alternator/regulator works:
When you start the engine a voltage is sent to the number ‘4’ terminal of the regulator, then it goes through the regulator, out the blue wire to the ‘F’ terminal of the alternator.
This energizes the field of the rotor and creates a spinning magnetic field inside the stator windings of the alternator. This starts the charging cycle. Now a small amount of voltage is sent back out the ‘R’ terminal of the alternator (white wire) to the number ‘2’ terminal of the regulator. This closes a set of points and allows the battery voltage from the red/orange wire to power the field circuit. If you had an idiot light it would now turn off. The amount of field current depends on the electrical load that is placed on the system. Now that the stator is producing current it will maintain the battery and take care of all the loads that are put on the electrical system.
Remember the battery is there to start the car and add current to the system only when the alternator is not producing enough to cover the electrical loads.
So if you have 37 amp alternator and you have 40 amps of electrical loads, then 3 amps will be supplied from the battery. This means the alternator has no current left to charge the battery and the voltage reading across the battery will be below 12.66v.
A properly working system should maintain a battery voltage of approximately 13.8-14.8 volts depending on temperature.
Always check charging volts with a fully charged battery and engine above 1000 RPM.
If the battery is very low when tested, then the charging voltage will be low. As the battery begins to charge then the charging voltage will rise.
A fully charged battery, after any surface charge is removed, will read 12.66v.
Alternator/ regulator testing:
First, check for battery voltage at the stud where the large red or orange wire connects to the alternator. You should have battery volts. If not, repair the wire or circuit.
Then disconnect the plugs from the regulator and alternator.
Check the continuity of the blue (field) and the white (relay) wires between the plugs.
Repair any wire if they do not have continuity.
Reinstall the alternator plug and leave the regulator plug disconnected.
With a voltmeter, check voltage at the number 3 terminal on the regulator plug, it should read battery voltage. There should not be any voltage at the other three terminals at this time.
If you have voltage at the number 2 terminal, R on the alternator, you have a leaky diode in the alternator, replace the alternator. This will cause a battery drain and the regulator with be hot to the touch with the key off.
Now turn the key on, you should have voltage at the number 4 terminal, this wire comes from the fuse panel. If you had an “idiot light”, this would be from the bulb.
This is what is called the exciter circuit, without voltage to this terminal the charging system will not begin to charge.
If no faults were found so far, leave the regulator plug disconnected, make sure all wires are clear of the fan and start the engine.
With a voltmeter connected to the battery and the RPM at approximately 1000-1200, jump from the ‘F’ terminal of the regulator plug to the ‘3’ (orange or red wire) terminal.
You should see an increase in voltage at the battery and hear the alternator working. If you hear a growling noise from the alternator you have either a bad diode or a bad stator winding.
Don’t leave the jumper connected too long because you don’t want the voltage to go over 15 volts for an extended period of time.
If you don’t see the voltage go above battery voltage after a few seconds then you have a bad alternator.
If the voltage rose quickly and then decreased this means you have a slipping fan belt.
Now connect a voltmeter to the number ‘2’ terminal (white wire) of the regulator connector and again momentarily jump from ‘F’ to 3. You should see about 8-10 volts.
If not, you have a bad diode in the alternator.
If all is OK up to this point reconnect the regulator connector.
If it is still not charging, run a separate ground wire from a good ground to the base of the regulator. Now if no there is still not charging, replace the regulator.
If you have an overcharging condition, it is either a bad ground at the regulator base or a bad regulator.
This is the basic testing procedure that I have been using for many years. There are other possible problems that can cause a no charge condition but they are rare and harder to diagnose.
Any questions or comments you can email me.
Joe
