Voltage regulator/charging problem



Sorry for the delay and thanks to everybody who shared their expertise. I talked to my local mechanic and he said without hesitation he agreed with the remote mechanic’s diagnosis (bad reman alternator right out of the box). I had the remote mechanic proceed with installing a new alternator. After that, it worked fine. Consistent, sustained 14.2 V across the battery terminals. Picked it up last week and drove it 5 hours home no problem.

The mechanic in Springfield, Va was great. I’m new to this forum so I don’t want to break any rules by endorsing a particular business, but if you want the name, PM me.


Click on the "Mechanics button and post a review.


Some good info here about charging systems. Especially @Tester web link. As far as the battery supplying power TO the alternator, it really does. Power is needed for exciter field windings of the alternator to get the alternator action working. If the field windings have no initial charge then the alternator will have no output no matter what RPM it is running at.


There no fundamental reason why a properly rebuilt alternator wouldn’t perform as well as a new one. So your idea of using a rebuilt unit wasn’t a bad decision. You apparently just got unlucky with the rebuild & testing job on that sample is all. If you suspect you have a bad alternator, many parts stores have a fixture that they’ll use to test it for you as a customer service, free. You have to remove the alternator from the car and take it into the store. As posted above, alternators can be tested on the car too, but sometimes w/that method there can be other problems in the vehicle that can make an alternator look faulty when the alternator isn’t actually the problem. So a parts store alternator test can be very worthwhile.

Another common cause of what seems to be an alternator problem is just that the drive belt is slipping. And with newer cars and their fancy computerized charging management system, that can be where the fault lies too, in the car’s computer system.


If I am not mistaken this is only true for synchronous motors. You don’t need a battery to bench test an alternator, only get the motor turned by mechanical means to produce voltage.


Unless the exciter field has some residual magnetism so a charge can build up, there will be no charging happening if there is no current flow in the exciter, which the battery provides. Remove the wire for the exciter field and see what happens when you start up the alternator from a no turning position.


You are mistaken.

Edit: Well, these are synchronous alternators. I would think you’d run the risk of damaging the internal voltage regulator without a battery attached. Have you tested automobile alternators without using a battery?


I’ve never tried it myself, but my guess they are made with some kind of iron-based core, so there would be enough residual magnetism in that to get the current flow process started once it starts to spin.


another general statement about exciter (sense) wires. Not all alternators have one… as a matter of fact many aftermarket alternators are self-starting.


Yes you most certainly can ruin a perfectly good alternator, IF you remove the load while the alternator is running.I.e disconnecting the positive battery cable. It can generate a surge of up to 150V. Starting an alternator without a load and slowly bringing it up to speed generally does not damage it.

Edit: No, I have not bench tested an alternator. Why take it out when it can be tested while in the car?