Alternator good, not charging battery

[mrybas]

My neighbor came over to help me diagnose the situation. With the engine running, and nothing changed, as he probed around with a test light, suddenly the pitch of the engine changed as the alternator started working.
He's not sure what contact he touched when it started working. I'm pissed their there isn't a 'duh' problem.

[mrybas]

Nix that. Not charging now that I shut off and restarted

[mrybas]

Ok, some progress. When engine is running, alternator not charging....connect purple wire (D+) to power (B+) and alternator starts charging.

[DeepFrz]

OK, progress. Either he hit a bad connection or he supplied exciter voltage to the alternator through the VOM. In that case trace back the wire for the excitation, probably back to the key switch(?) on the panel and see if there is bad connection in there. First though try putting the pos. meter lead on the pos. terminal of the alternator and the other lead on the exciter terminal and see what happens. Engine has to be running of course.

Edit: there are some pretty dirty connections on the back of that alternator that could use a good cleaning.

[mrybas]

Originally Posted by DeepFrz

OK, progress. Either he hit a bad connection or he supplied exciter voltage to the alternator through the VOM. In that case trace back the wire for the excitation, probably back to the key switch(?) on the panel and see if there is bad connection in there. First though try putting the pos. meter lead on the pos. terminal of the alternator and the other lead on the exciter terminal and see what happens. Engine has to be running of course.

See above

[DeepFrz]

Cross post at same time.

[David M]

On many alternators there is a terminal separate from the voltage sensor terminal that runs to the key switch which effectively turns on the alternator when you start the engine. This terminal excites the magnetic field which is necessary to charge the battery. Make sure this terminal is getting 12+ volts of DC.

Also, run an ohm meter between the hot side of the alternator and the battery terminal. It should have very minimal resistance. Crimped terminals can go bad. Others have told you that your terminals need to be clean and tightened down.

Also run an ohm meter between the negative terminal of the alternator and the battery's negative terminal, the resistance there should also be minimal.

[mrybas]

So, the purple wire (D+) has a problem somewhere between the key and alternator?

[mrybas]

Originally Posted by DeepFrz

OK, progress. Either he hit a bad connection or he supplied exciter voltage to the alternator through the VOM. In that case trace back the wire for the excitation, probably back to the key switch(?) on the panel and see if there is bad connection in there. First though try putting the pos. meter lead on the pos. terminal of the alternator and the other lead on the exciter terminal and see what happens. Engine has to be running of course.

Edit: there are some pretty dirty connections on the back of that alternator that could use a good cleaning.

It might just be the light, the connections have all been hit with a scotch brite, elec contact cleaner, and paper towel.

[DeepFrz]

Originally Posted by mrybas

So, the purple wire (D+) has a problem somewhere between the key and alternator?

Exactly

[David M]

Check the positive wire between the key switch and the field wire terminal on the alternator when the key is in the Engine Run position. It should be hot.

If it is not hot, then there is your problem.

[mrybas]

I am assuming purple wire at the alternator should get power when key is switched on, that doesn't happen.
At least I found a place to start. I was going nuts and wasted half my day looking for bad connections!

[DeepFrz]

Yes, and you will probably find one.

[redbeard1]

Hi,

We chased a similar problem for three months. Finally traced it to spider webs in the socket for the light that indicates the alternator is working (most consoles have this light as is serves to provide the current to excite the alternator). The circuit is designed so that the resistance of the bulb will draw enough current to excite the alternator. If the circuit has too much resistance (spider web in our case, usually corrosion for others), the amps drop and not enough current is delivered to excite your alternator. There is often a parallel circuit with a resistor to supply the current if your bulb burns out but in our case, this extra circuit was broken).

If you have a multimeter, check for resistance at each leg of the circuit. Remember to isolate the circuit before checking for resistance.

edit: sorry took a while to type my reply, did not see the replies after 5:04

[redbeard1]

Originally Posted by mrybas

I am assuming purple wire at the alternator should get power when key is switched on, that doesn't happen.
At least I found a place to start. I was going nuts and wasted half my day looking for bad connections!

With your multimeter set to volts, what are you getting when you place one end on the purple wire at alternator and the other end at a good ground?

With the ignition on, you should get less than 12v but more than 2V. On our set-up, we get about 3V. If you are getting 0v, it is less likely corrosion and more likely a break in the circuit (is your alternator bulb on when the ignition is on?)