Subject line basically covers it. I've tried two different alternators with the same result. With the key in and at the first position, all dash lights are on as normal. Start the engine and everything goes out, as normal, except that my voltage at the battery is around 12.45v, down from 13.4v with the engine off. I think I may have a broken exciter wire because the dash light sometimes comes on if I mess around with the wire loom, but I don't think that covers why the light normally goes out like the alternator is charging properly.

The battery got so low the other day that the 3 amigos paid me a visit and I couldn't shift out of P (luckily after I pulled into my own driveway), but after a day on the charger everything was back to happy so I know that everything works as it's supposed to except for the charging circuit.

Any pointers as to where I should be looking?

 

You've replaced the battery?

I had a somewhat similar issue in a Bronco years back because the bolt holding the negative cable to the frame had fallen out. The truck got enough ground to run thru the small wires running off the battery terminal, but after a coupla hours some resistance would build up and the voltage would drop under 12. My point is definitely check all your battery connections both at the battery and where they connect to the frame and starter. Then check the wiring to the alternator.

In fact, it's probly a good idea to put a multimeter on the alternator to see what the output voltage is at the source.

 

Battery tests good, and holds a charge. I'm not sure how to test the alternator directly.

 

take it auto-parts store and have 'em do a test. they're usually happy to test in hopes of selling you a new alternator.

 

Think you said where to start looking. Besides that connections and gnd. May have two of three diodes shot in alt. Fairly simple system.

Taking one of your alts to store for bench test is one option.
.......

 

Well, I was hoping for a really simple fix of not having reattached the heavy ground lead to the auxilliary bracket after I replaced the head gaskets, but I did reattach the wire so I'm going to have to dig further. Haven't had the alternators tested yet, but I'm not hopeful there since both of them exhibit the same symptom where the light goes out but the battery doesn't charge.

 

It kind of sounds like the computer isn't controlling the alternator. You can try grounding the control wire manually. If it starts charging, you'll hear it. Just don't full field the alternator for too long. If that works, you can check the control wire at the ECU and see if it has a pulsed ground when the engine is running. I usually use an incandescent test light for that test. It should flicker if the ECU is trying to control the alternator.

 

Okay, so I remove the control wire from the alt, start the engine, then while it's running I ground the wire and I should hear... ?
If I hear ?? then I check the control wire at the ECU by hooking in a bulb in sequence?

 

I would back-probe the control wire with the connector still plugged into the alternator. You can then connect a test light to battery positive and touch the test light to the probe in the control wire. If it flickers there, the computer output is getting to the connector and there's no point in checking at the ECU.
Don't put the test light in series because it would just be ground to ground so it would not light. You will have to be connected to 12 volts on the other end of the test light.
I hope that makes sense. It's easier than I can probably explain it.
Basically leave everything plugged in and back probe the control wire at any connection you can find from the alternator back. Hopefully you won't have to go back to the ECU.
While you have the connector back-probed on the alternator you can use a jumper wire to ground out the control wire. If the alternator starts charging you'll hear it whine. If it doesn't there may be a problem with the connector like corrosion or sometimes the connectors can get spread out.
Just make sure you're on the control wire. If you're not sure which one it is, you'll need a wiring diagram.

 

The alternator only has two wires going to it -- heavy amperage positive, and control. It grounds to the engine block.
If I have my voltmeter reading across the battery terminals, I should expect to see an increase if the alternator starts charging when I ground the control wire, correct?