The battery in my '84 pickup isn't charging. The battery is only a few months old. How do I tell if the problem is the alternator or the regulator?

You can “full fill” the alternator. Remove the green wire and ground the brush with a jumper wire. This will bypass the regulator and get max juice (according with alt specs and speed) from alt. If you get juice, is the regulator, if not is the alternator.

Could be neither.


Get a voltmeter. measure the voltage at the alternator blue wire, key in run, engine off. Should be close to battery voltage of there is nothing there you have a wiring or ammeter issue.If you have something there then run the full field test Nacho mentioned.

I was writting about that blue wire test too, but if the engine runs, the posibility about a non energized blue wire at brush is hard to happen since the circuit feeding the reg and brush is the same than the ign system. EXCEPT if the wire between the ign circuit splice down the harness tape and the brush is broken. I only can see that fail right at the field terminal but not down the tape.

Ammeter circuit fail? Thinking on the year, it should be a shunted system, where the full load between alt and batt is not running anymore throught the ammeter. Not even into the cab.

Nothing wrong on check the blue wire status thought!

I usually test it by seeing what the voltage is at the battery running. It should usually be 14-14.5 or so but more then normal battery voltage at the least if the alternator is working.
Every time I've had a voltage regulator fail the lights would get brighter and dimmer as you're driving and speed up and slow down. I haven't had one that's caused a no charge but have had the plug or wiring st the plug anyway be bad.

A volt meter is helpful to see if 14+ volts are at the battery running, but if you want a "poor man's/backyard/shade tree" troubleshoot....to rule out the alt or voltage reg, with the engine at idle, use a flat tip screwdriver and touch the rear bearing at the center of the alt with the flat of the screwdriver, if you have a slight to moderate magnetic pull the alt is making a charge, if you only have 12 volts or less at the battery, chances are the voltage reg is culprit, no magnetic pull on the rear bearing to the screwdiver blade, chances are the alt.....if you find the alt at fault replace both the alt and reg for insurance


MikeG

The magnetic field test is a good idea, but still could be a bad test if the fail is at the diodes bank, because the alt could be creating magnetic field and still no output due the diodes failure.

So you could think the reg and alt are good, but the failure elsewhere on the run to the batt.

This needs to looked at before any other tests. The "amp meter/shunt" goes between the alternator and the battery. IF there is a issue there you won't be able to do the other tests.

Thanks. That's a new one on me.

Not sure when Dodge changed from in line amp meters to shunts then to volt meters. If you have a in line amp meter or shunt you should check it.

The shunt thing happened in cars in the late 79s I haven't looked what my 83 truck is like but I'd guess trucks made the switch along with the other changes for 1981.

Dunno on trucks, but the shunt deal began on earliers 70s C bodies (maybe even 69? With the fuseleage body design?), on 75 was a new add for B bodies and on 76, on its last year for A bodies.

Then all F, M etc..bodies were all “shunted”.