O.K., so here are the facts:

Battery was dead but under warranty so I replaced it. Now reading 12.1 volts off the battery terminals with voltmeter and engine not running. According to all the info I could find it should be reading at least 12.5 with the engine not running. I have put about 15 miles on the truck since battery replacement which I think would account for the voltage being low. I actually thought it might have been lower if there was no charge at all coming from the alternator.

Alternator tested bad at AZ. All they could tell me was that it was not regulating properly. They could not tell me any specifics past that. I changed the brushes the cheapo way by soldering the new ones in. Going to check my work on that job now. With engine running, the voltmeter reads 11.4/5 volts.

I can only assume that the brush replacement did not work because at the terminals the voltmeter should read 13.5 to 14.5 volts with the motor running according to my readings here and elsewhere.

If my brush installation checks out is there any way to test the voltage regulator? From what I have read, if the regulator is bad the alternator will send TOO MUCH current to the battery and bulbs will begin to blow as well as other issues. As far as I can tell the only issues right now look like low voltage such as the dash lights dimming during turn signal use. My guess is that the regulator is fine. How do I test the diodes? If anyone can help out with these questions I would be eternally grateful. Thanks.

 

I was told by a Toyota tech that you could test the diodes with a volt/ohm meter. He said to unhook all the wire from the alt.Then set the meter to ohms and put one lead to the housing and one lead on the pos stud on the alt. Check for continuity then switch the leads and check again. If the diodes are good you should have continuity one way but not the other.
When the brushes where replaced did they move freely in the holder? One of them might not be making good contact.

 

Just checked the brushes and they appear to be fine. Solder has held and the are contacting the spindle perfectly. The spindles did have some carbonization on them which I scrubbed off with some 2000 grit sand paper. I have figured out how to test the voltage regulator and will have that info next. Still don't know how to test the diodes.

Thanks Swampfox. I will try that.

Just for some further info, this alt has an external regulator.

 

I was assuming it was a internally regulated alt , I am not sure if this will work on an externally regulated alt .The guy never said . You can try and see what happens.

 

New info. When I reassembled the alternator and did a voltage regulator test, and revved the engine the voltage went down but only by about .1 from 11.5 at idle to 11.4 at about 2k rpm. From my readings I was told that if it was the voltage regulator, the volts would have only risen, not fallen. So from this I deduce that the voltage regulator is probably fine. The damn thing is simply not charging. I am going to redo the brush job next. Even though I think the job is good now, I am going to make sure that I have perfectly clean contacts and use high temp solder. I don't know if it's possibly the diodes or something else but I really, really doubt it. If anyone can confirm a diode test for an externally regulated alt or any other ideas, please chime in.

 

Check all your big grounds, & I mean take them off, clean, & reassemble. Then start working your way back from the battery testing voltage. I am thinking short in the harness or bad ground.

 

You have to remove the diodes to test them( depending on the design you might have to even unsolder them).
You can test the diodes( without removing them) for ripple (AC) and that will tell you if you have a bad one. Run the engine at about 2000 rpm, turn your meter to AC , black probe to ground and red to Batt terminal on back of alternator. you should see no more than .5 volts , if you do, then one or more of your diodes are bad. But it sounds like your alternator is just not charging at all.

And whatever you do, don't disconnect the battery while the engine is running to test your alternator!

 

why don't you just get another alt at the wrecking yard it would be cheaper than trying to fix the old one. another way to check if your alternator is charging is to set a screwdriver on the rear center bearing while the engine is running if it is charging it should be magnetized.

 

He probably already got rid of the truck because there was no sign of it getting fixed since " 01-13-2011, 11:53 PM"

 

Scary that someone can think a voltage regulator being bad only produces a over voltage condition

 

yes usually they produce an undervoltage unless it full fields it then it boils out the battery and blows out the lights.