Battery Light Never Comes On

1997 F-350 - Headed to work this morning I notice the lights were dim and the volt gage was lower than normal. It was way after the glow plug relay would have kicked off, so I was suspicious. The alternator was definitely not charging. However, the battery light wasn't on, which is odd because they relatively notoriously come on frequently.

I take a little time over lunch to troubleshoot, and the first thing I do is turn the key on without starting to confirm the battery light comes on. Nope, no battery light with the key on and engine off. I figure thats my problem, no signal to the alternator to charge.

The only related fuse I saw in the owners manual was underhood maxi fuse 17. I pulled it and it looked OK (I didn't check resistance across it, and should have). I pulled the plug off the alternator, and I'll be darned if the grn/r wire and yellow wire didn't both have battery voltage. I know the grn/r is the signal wire for the alternator.

Well, I am not sure why the battery light won't come on but I figure I'll check the fusable links anyways, and they appeared to be good.

All I can really think of at this point, with power at the plug on the signal wire, but no battery light at any point, is that something is failed in the alternator causing that light to never come on.

I did see one person that said to ground the grn/r wire and see if the bulb comes on to confirm that circuit is working, so I might try that this evening. Does anyone else have thoughts on what might be causing this or similar experiences? I used the search function but most people have the light coming on when they don't want it, not the light never coming on.

Is the bulb bad?

I've never quite had the inclination to verify on these trucks, but many times the bulb circuit serves as the circuit to excite the field windings on the alternator. Alternators are not "self-exciting" as are old-style generators, so if the field winding are not provided with voltage the alternator will not charge. This was the case on other cars I've worked on. As Magnum PD posted, check the bulb. Cheers!

Thanks for the replys guys. I have not checked the bulb yet, as I have done minimum troubleshooting in the parking lot at work over lunch.

What I do know is that the wire that I am pretty sure runs through the indicator bulb (green/red tracer) at the alternator has battery voltage (or close to it) with the key on. I would have thought if the bulb was bad it would have no voltage. I will definitely check the bulb this evening though, as I have heard that the excitation voltage comes through the indicator bulb.

Patrick's post is right on. The voltage to excite the alternator goes through the battery light in the dash

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Here is the wiring diagram for the charging system for our trucks.

Update on what I found this weekend. First thing I did was ground the exciter (not sure if thats a word, but I am going to use it) wire, and sure enough, the battery light comes on. This means the bulb and that wiring is functional. I am not too surprised by this, as I was getting battery voltage on it when checked the wire with a DVM. I had another alternator sitting around, so I put it on and everything functions normally again.

There was some sort of failure in the alternator that caused it not to charge, as well as the battery light not to come on (at any point). The windings appeared burnt to me, and the guy checking the alternator confirmed my thoughts. I am not sure if there was some other internal failure or not.

Thanks for everyone's input and information on my problem. I couldn't find much information on if the battery light never came on (especially if the bulb was NOT burnt out), so maybe some of this information will help someone down the road.