Charging system, voltage regulator?

[Wall 'o text. Sorry, I'm thorough and wordy.]

Vehicle: 2009 Tacoma Prerunner, 4.0 V6, no tow package.

After six years and a month my OEM battery kicked the bucket. It just 'buzzed' the solenoid when trying to start, didn't turn the engine over at all.

I jumped it, and tthe voltage by my Scanguage was running about 13.1 volts at idle, and it would quickly drop down to 12, 11, 10 volts after shutting off the ignition. if I turned on the headlights and all of the accessories, it would drop to about 9.9 volts and then the Scangauge would shut off. I didn't do a lot of additional diagnostics, because I knew it was ancient in battery years and I figured either the summer heat or winter cold was doing to kill it this year.

After replacing the battery, I hadn't changed the Scangauge back to another reading, and I noticed it was reading 12.7 volts while running. It would climb up to 13.1 or 13.2, then slowly drop back down again. Of course I notice this when my wife and I are halfway to another town. So, a 70 mile round-trip later, while watching voltage and doing things like turning on the headalmps to see if the alternator was able to compensate (it did), I get home.

When I pull the alternator control plug from the alternator, the voltage on the new battery as indicated by the Scangauge would drop to the low 12, then high 11 volts. Plug it back in, and it would jump up at least a volt to a volt and a quarter (to high 12/low 13 volts). Interestingly, the alternator/charging system light never illuminated when I pulled the plug.

I went by Autozone and asked them to run a test on the charging system. Their device bled off a surface charge (indicating, to me, that the battery had been charging at least some), load-tested the battery, said it was fine, and then I started the engine. The device indicated the starter was fine, then waited for the voltage to stabilize. It eventually said that the diodes were fine, but diagnosed a bad voltage regulator.

They claimed the voltage regulator is inside the alternator, but I had been under the impression that the alternator field current was controlled by the engine control computer. I haven't seen a voltage regulator listed for the 4.0 V6.

Once home, I poke around with a voltmeter, and find that the battery terminal voltage is 13.3-13.4 at idle, and 13.5 at about 2000 rpm. FSM says that's AOK.

So, I have three questions.

1. Where is the voltage regulator on the 2009 Tacoma V6? (Or is it regulated by the ECM?)

2. Should my charging system light, which lights before starting, have come on when I pulled the alternator field plug?

3. Is it possible that the low output voltage is normal, and that the Autozone tester was just expecting it to have a higher voltage?

I also welcome any diagnostic advice or experiences you've had.

 

The alternator controls the output depending on load and engine speed. You should test your alternator, because on my 13 Taco the voltage always about 13.5 volts which is normal. It shouldnt drop down to 11-10v when just turning headlights om

 

Most alternators have built in voltage regulators. The Tacoma doesn't use any secret engineered parts. Voltage regulators fail like any other wear and tear part. When they fail, you can see voltage fluctuations. The tester at AutoZone was right. Your alternator needs replacing, unless you're handy enough to rebuild the alternator yourself. I would suggest buying a remanned alternator and call it a day.

 

The voltage regulator is inside the alternator, attached to both the brush assembly and rectifier. You can test the regulator while the alternator is attached to the engine by testing its voltage when your truck is running at higher than idle RPM and several accessories are on. Pretty simple, if the voltage regulator is faulty, the voltage will be higher than around 15volts and.or fluctuate quickly. YOu can also test the voltage regulators current/resistance while the alternator is detached from the truck and taken apart. The regulator should show current between the metal posts.

My hunch is that you had bad brushes or a bad rectifier, based on the voltage readings you were receiving while the engine was running.

 

Please see attach. The light only comes on when a large difference in voltage potential between the battery and the alternator output. If it's slowly dying the light won't come on, and if you disconnect the plug the light also won't come on, it needs to see a path to ground. You probably have a bad brush assembly in there or one of the SCRs isn't firing. You can rebuild it yourself or just buy a new one and take old one back for the core charge. The VR is built into the alternator assembly.

 

Can ya'll help please. Ok so truck never have battery light come on or nothing on dash (I'm think'n sumones done shit under dash) so truck won't start just clicks so get started go down they test it say oh it's your starter said no it's cracking engine fine ok it's your alt I said sure it's not battery? Do I put alt. in truck won't start same as before huffin n puffin go back down they say it's battery and voltage regulator then machine just said voltage reg n not batt then the opposite GRRRRR can u please tell me if I need to. Change batt, regulator or both?! Any advise would really. Be appreciated

 

Was it the alternator?

 

https://www.tacomaworld.com/threads/2nd-gen-information-diagnostics.784008/#post-28042155