Charging system problems, not sure it's the alternator

Hey Gang,

I have a 2001 SR5 / TRD, V8 engine, around 220K miles. The charging system is not functioning, and I need some guidance. I've poked and prodded and measured, and here's what I know so far.

1. Engine starts fine. I've been keeping the battery charged with a solar trickle charger.

2. With engine running, voltage across the battery terminals is around 12.3 V. This is with nothing on (no lights, no fan, etc.). When I turn the headlights on, fan, etc., voltage drops to 11-ish.

3. Possibly some weirdness: with the keys in the accessory position, the check engine light, airbag, etc. indicators light up for the lamp test, however once the engine starts and the lamp test is over, only the typical dash lights are on (P R N D light, door light if door is open, etc.). I never see the battery / charging light turn on. (More on that later).

4. Under the hood, serpentine belt looks OK, all pulleys appear to be spinning, no horrible grinding or squeaking noises.

5. I checked for continuity between the battery cable coming out of the alternator and the positive terminal of the battery, seems to be OK. (I may have messed this up, though, as I only have one of those $10 multimeters from like Harbor Freight).

6. I read somewhere that if the battery / charging light bulb in the dash burns out, that it actually prevents the charging system from working. So I got the dash panel out, and tried to measure some stuff. With the key in the position to do the lamp test, I measured across the copper pads where the battery light goes, and didn't get a voltage (but, for instance, the check engine light was on for the lamp test). But I'm not sure if there should be a voltage at the battery light or not. I did replace the bulb anyway, but it didn't fix the issue.

7. The Tundra is parked outside, we live out in the country, and I have had some rodent damage in the past - I had to solder my tail light assembly harness back together after some little buck-toothed critter chewed through the whole bundle. I did my best to look around under the hood but didn't see any obvious damage to any electrical harnesses, but I can't rule that out.

...I think that's everything. What else can I check / do? If I have to, I think I can get the alternator out and take it somewhere for testing, but if there are other checks I can do first I'd much rather do that! This truck isn't my commuter, so it can sit in pieces for a while if necessary. I do probably need to check some grounds, so if you have any advice on how to do that, I'll give it a go.

Thanks a ton, I appreciate your help!

 

While running have you checked the voltage on the alternator?

There is a 3 wire harness coming off the alternator. One wire is an alternator signal to tell it to charge. Another one is the low voltage wire. I believe the 3rd is a ground.
I would recommend following those to the fuse box and make sure those aren’t shorted out.

I had a good alternator that wasn’t charging because those wires had failed where they plug in to the fuse box.
The battery light on the dash wasn’t coming on either.
I’ll try and post a picture if you need it but check those first.

 

Thanks, I will try to check those tomorrow - I was wondering about them when I checked the battery cable, I wasn't sure where they went.

That reminds me - I did look at the large 120A fuse in the engine fuse box. It looked OK (to my untrained eye...). Still a shiny, unbroken metal band inside that didn't look smoked or melted. (I attempted to remove the fuse, which very quickly turned into an "adventure"...but I got the fuse box put back together after 45 minutes of trying to get a single hex-head screw put back in...why, Toyota, why?)

 

Does your BATTERY light on your dash light up? It should be lite up when your key is turned to ON but not started.

That battery light is in fact, a resistor, without which your alternator will not produce a charge (which its not, running voltage should be at least 13.5 volts)

That lamp also has to be a regular bulb, not and LED. If its not lighting up and the bulb tests good that indicates a faulty wire somewhere in the dash or to the alternator.

With the engine stopped one side of the battery light circuit should have 12volts. The other side should be a ground. With the alternator working, BOTH sides should read 12volts to body ground (which is why the light goes out with the engine running, the ground becomes a hot and with two hots there is no circuit)

The charging wire, the one that establishes a charging field, is the Yellow and Green wire on the alternator.

You can test the system by running a wire from the positive side of your battery, through small lightbulb to this yellow green wire on the alternator. This should cause the alternator to begin charging (with the engine running of course.)

There is also an alternator fuse and your main gauge fuse which are part of the system.

 

Thanks for the diagram, Aerindle! I've checked all the fuses (they look fine) and a few of the wires. I need a little more basic (really, stupidly, embarrassingly basic) help.

1. <edit: nevermind!>

2. On the schematic you attached, I can follow it logically, but need some help with the markings. For instance, I'm assuming that the 10A GAUGE fuse connects to pins 2, 5, and 7 in the fuse box (the pin numbers in this case aren't particularly important, I just want to make sure I'm reading this diagram correctly). Are the 1I, 1F, and 1E connector designations? At any rate, the battery bulb looks like it connects to pins 3 and 4, maybe connector C12?

 

Yes. That is correct. All those connectors are located under the driver side finish panel.

 

hello just had the same problem
Battery light won’t come on with key in on position, replaced battery and alternator still the charging system isn’t working.
Where you able to figure out the source of your problem?

 

A quick and dirty test for the alternator. While the engine is running hold a metal screwdriver over the small cap in the center of the rear alternator housing(it covers the rotating shaft). If it's magnetic it's working.