'05 3.0 not charging but no charge light?

[franklin2]

Confirmed its the brushes. I mounted the VRM with washers under it to try to space it back onto a better spot on the slip rings. It charged for about 20 seconds then cut out. I figured it would wear through pretty quickly on a corner like that but it's a proof of concept. Now to figure out if it's fixable.

That's strange because if you have a brush/slip ring problem, it would not have passed the full field test.

The field is the rotor turning in the middle. So when you full field a alternator, you are putting a full 12v through one of the brushes, through the slip ring, through the windings in the rotor, back out the other slip ring to the other brush.

Think of the rotor as just a big magnet turning inside the windings on the outside of the alternator case(those are called the stator). They vary the voltage on this spinning field, which varies the amount of magnetism, and that varies the output of the alternator.

The outside stator windings(think of the word "stationary" to remember) are configured into groups of 3 coils. So the alternator actually generates 3 phase A/C power from the windings on the outside as the rotor spins in the middle. They take this 3 phase power and send it to a pack of 6 diodes. These diodes are arranged to take the 3 phase A/C and convert it to single phase DC, which is then sent out on the output lug of the alternator.

When you are rebuilding your alternator, check these diodes. You can have one bad, and the alternator will charge but not as good. And sometimes one of these diodes will short across. This causes the famous "drain" and will run your battery down as the power from the battery runs backwards through one of the stator coils.

 

[pentode]

That's strange because if you have a brush/slip ring problem, it would not have passed the full field test.

The field is the rotor turning in the middle. So when you full field a alternator, you are putting a full 12v through one of the brushes, through the slip ring, through the windings in the rotor, back out the other slip ring to the other brush.

Think of the rotor as just a big magnet turning inside the windings on the outside of the alternator case(those are called the stator). They vary the voltage on this spinning field, which varies the amount of magnetism, and that varies the output of the alternator.

The outside stator windings(think of the word "stationary" to remember) are configured into groups of 3 coils. So the alternator actually generates 3 phase A/C power from the windings on the outside as the rotor spins in the middle. They take this 3 phase power and send it to a pack of 6 diodes. These diodes are arranged to take the 3 phase A/C and convert it to single phase DC, which is then sent out on the output lug of the alternator.

When you are rebuilding your alternator, check these diodes. You can have one bad, and the alternator will charge but not as good. And sometimes one of these diodes will short across. This causes the famous "drain" and will run your battery down as the power from the battery runs backwards through one of the stator coils.

Yes, you're completely right, I knew I was being "dyslexic" for lack of a better word, I realized my logic was backwards once I de - stressed enough to think clearly.

So it was easy enough to test for brush contact without tearing anything down.

However, the brushes were totally shot and the slip rings are really bad. What I can't explain is why i was able to get it charging again for 30 seconds by moving where the brushes were landing. I did change the brushes, and it still doesn't charge. So now, having thought it through a bit more, I'm back to thinking it's the VRM... Which is unavailable locally and $130 shipped from Rock Auto. So I guess I'll be making a trip to the u pull tomorrow.

I appreciate the logical feedback. I know how this stuff works but under stress, the mind tends to fixate on the answer it wants. I knew I was missing something.

Edit: oh, I'm sort of afraid I'm missing something obvious again. Any chance this could be a PCM problem? My wife did say every light came on on the cluster a couple times... I'm wondering if something failed in there

 

[pentode]

OK, total reset here. Appreciate you guys bearing with me. When it's other peoples' problems, I'm calm and rational, but apparently when it's my truck broken down my brain just shuts down and I start looking for silver bullets.

Back to square one, the basics, with some data and proper diagnostics.

Tore out the cluster and the batt light bulb is definitely good - I even swapped it with another good one just because. I get no charge light ever, not key on engine off, not running, never. I'm thinking this must be relevant, but I don't know exactly how the ECU is controlling the VRM.

I have good 12.6+ voltage at both the B+ stud and the white/yellow, which should both be hot at all times. There are supposed to be fusible links in those but so far, I haven't found them (haven't looked too hard, but I figure the links must be good if there's voltage there unless one is backfeeding the other somehow). Grounds look good in general, 0 ohms from battery to block, etc, but I'm lacking in tools to load test them. Still, it did charge yesterday briefly, so I don't think that's it.

Edit: full field test passed means stator and stator wiring must be fine, rotor wiring and brushes must be fine, red B+ and grounds must be fine if they can put that much load on the alternator.

Full field test yields 16V and you can hear the load on the alternator so I think the B+ (red) wire and grounds must be good.

Here's where I really need someone to get into the weeds. I have very limited experience with PCM controlled alternators, but I believe what I've sussed out is other than the heavier gauge white/yellow hot at all times wire, which I'm guessing actually supplies power to the VRM, you have a sense wire and a control wire (the other 2 on that 3 pin connector. The sense, I'm guessing, sends a voltage back to the computer, and the control as far as I can tell sends a command every few seconds to the VRM.

I don't know what the sense voltage should be and I don't know what to look for on the control wire. I imagine I'll probably have to put the scope on it. I don't have piercing probes nor decent jumper wires for automotive so I'll have to go a bit caveman and pierce those wires with a needle. Anyone happen to know a) what the sense voltage is supposed to be and b) what waveform to look for on the control wire?

I'm just trying to establish that it's the VRM and not the ECU or wiring before I throw an alternator at it. The fact that the batt light never comes on is my concern at this point, but I don't know the conditions for setting it. I've seen a blurb on one of the schematics about how it's supposed to work but I couldn't make any sense of it.

Edit 2: oh, and I was informed by the wife that just before the system went nuts, she hit a pothole "very hard". I'm thinking it's just a coincidence but just in case it triggers an idea I haven't thought of. I haven't found any signs of damage in the harness or anywhere else.

 

[Curious Hound]

but I don't know exactly how the ECU is controlling the VRM.

It's not. That's only on much newer vehicles.