cloudmaster_321

Hi all, well, took her out for the first time, and i had a couple of bugs. My alternator is not always putting out voltage for some reason. I can watch my volt meter go between 12 and then jump to 14 volts. If i try while running the engine at home, i can duplicate the problem, the alternator will actually go offline, then comes alive and whines because there is a load on it, probably due to running off the battery. Is the regulator in the alternator? Anyone seen problems like this before? I was thinking of just pulling the alternator and taking it to the local automotive shop for a test. What do you all think?

'90 merc 7.4L

hillbilly24

Is the problem happening at low RPM's ? the Merc alternators have an excite circuit that doesn't activate until about 1200 RPM. If that is the case this is normal operation.

Steve H

Be sure and check your belt. You should not be able to make it slip by turning the alt fan by hand. Another way is to run the engine while it is acting up, then check the alt pulley and see if it is hot.

cloudmaster_321

any rpm, can be wfo as well

cloudmaster_321

well, i went out on saturday and sunday, did i both days. If the belt was sliping, i don't think i would have much of a belt left after five hours of runnin. IMO

Lmarth

I think sutphen30 is on the right track. Unless you're bouncing and/or sticking the brushes, alternators are dumb-they either are putting out or they're not. Semi conductors, diodes, rectifiers and IC's, in general, again either work or don't. All the voltage regulator does is turn the alternator on and off. If you're occasionally seeing 14 volts, the alternator is capable of doing it's job. You could see an excessive voltage output situation, but that's for another day. Taking the 'exciter' feature out of the conversation for a moment, you used to be able to take the GM alternator with integral regulator and cheat the regulator while the motor was idling. You didn't need a meter, you could hear the idle drop as soon as the alternator started making output. Don't know how many hours are on the alternator, but I'd look at all the connections, evidence of worn brushes (worn brushes don't allow for strong contact on the armature) and at the regulator. I'd have the unit tested somewhere If you're not comfortable diving into it yourself. We have an alternator expert here on oso up in the Pacific Northwest also, Good luck!

cloudmaster_321

i haven't dipped my hands into an alternator yet, but i'm not afraid to. I take it the regulator and brushes you are referring to are part of the alternator? There are no external boxes are there?

Steve H

We have an alternator expert here on oso up in the Pacific Northwest also, Good luck!

I think I know him!

cloudmaster_321

How does the alternator get excited, if i do have the three lugs on the outside? Do the brushes actually transfer the outvoltage from rotor to wires? thanks Just trying to get a better understanding. Where can i get the regulator and brushes, that would almost constitute a rebuild by replacing those items wouldn't it?

Steve H

The alternator gets excited when the key is turned on in most applications. The exciter circuit needs to see at least three volts engine off key on. sometimes a oil pressure switch is used.

The regulator supplies current to the brushes which magnetize the spinning rotor. The lower your battery the more current the rotor gets. This induces voltage to the stator. Then the a/c current flows from the stator thru the rectifier (converting voltage to DC) to the battery stud and ground on the back of your alternator.