Last 2 or 3 weeks, the battery light on the instrument panel has been coming on occasionally. It might stay on for 2 seconds - at other times maybe two minutes (or longer). Can't seem to find a pattern here.

A week ago the light stayed on all the way home from work (about 45 minutes).

I have not driven the truck for about a week.

Yesterday, I had to charge both “new” batteries sine each was down to about 5 volts. Not surprising if they're not charging since I have an inverter that runs 24/7 (I've pulled the fuse).
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1. Cranked it up 30 minutes ago, checked voltage on both and found 12 volts only.
2. Checked Alternator output and initially no voltage.
3. Re-checked batteries and found 14.2 volts.
4. Re-checked alternator - found 14.2 volts
5. Opened the door to check if bat light was on – no light.
6. Rechecked the bats – “0” (zero volts)!
7. Rechecked the alternator – no voltage.
8. Checked the bat light – it was now ON!
9. Rechecked the bats – 14.2 volts
10. Rechecked the bat light – now OFF!
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What the heck is going on? Alternator? Computer? Something else?
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Thanks in advance to the forum.
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Clif

 

I would say the alternator is on the way out.

Hook Em'

 

How did you check the battery voltage ? on the lug or the connector ? I can't believe you would have "0" volts at the batteries.. Maybe your connectors are loose/dirty?

 

When you checked the battery and you got no volts - how did you check it? With a meter on the battery posts? Voltmeter gauge?

 

Checked voltage with engine running. When bat
light is on, no voltage from alternator and "0" at
terminals. With engine off after driving 35 miles,
standing voltage is 12.20. Only when the bat light
is off does the voltage read 14.20 at the terminals.

 

By the way - I'm using a Fluke DVOM for all measurements. Made the 35-mile trip back home tonight with no problems. I had shut off everything I could ( no a/c, radio, dash lights, low beams only). Truck ran fine and the bats are showing about 11.8 volts after I got home. I'll put the charger on it in the morning. Hopefully I can "nurse" it along until I can narrow down the cause for this situation. Hopefully it's just the alternator. and YES!.........the voltage IS "0" with the engine running and the battery light is lit on the dash. No output from the alternator either! 12.20 volts (at the bats) with the engine shut off. Got me confused for sure.

 

Something doesn't add up, the motor can not run with zero volts on the battery terminals. That said I do think you have a serious problem with the alternator. My guess is that the brushes on the slip rings of the alternator have worn down and have become intermittent. I would take you truck to a local alternator rebuilder and have them test the system for you.

 

I agree with the guys above. Most of your local parts shops test alternators for free, that would be a good start.

 

Also check the little plug on the top pf the alternator. I had almost the same issues and that was where my problem was.

 

I've had the same issue Twice with my truck- Both times problem was an alternator.
I had Auto-Zone check charging system, purchased replacement from them, when that went bad, ( I'm guessing three years), they replaced for free. Keep your receipt

 

I would check your cables. It sounds like you are losing connection which would give you the "0" volts. Always check the simple things first before spending money. Disconnect your batteries and use the multimeter to check continuity from your negative cables to the engine block and the positive cables to the alternator battery terminal. I just seen another thread mentioning a similar problem and the fix was a new battery cable clamp.

 

If the alt is bad, you should at least see battery voltage at the alternator output terminal. Make sure all your connections are clean and solid. Take them all off and look -- the can appear good until you see the connection area itself. Cables can also look good until you look close. I had one (on my old Bronco) that looked fine, but was almost 100% corrosion inside the insulator. I honestly don't know how it ran...

 

I had the same problem. I went through 3 new alts. We finally figured out the 2 problems. First the 2 wire connector on the top of the alt was bad - installed a new pigtail. Then traced it further to a bad computer in the instrument cluster. Unlike the good old days when each instrument was it's own, they now operate electronically and interface with some motherboard in the panel. The light coming on is some type of computer error and when it happens it prevents the alt from charging properly and that in turn causes the alt to overwork trying to charge and then burn up.
It took over a month, 4 new batteries and 3 new alt's to get to this point. I'm having the instrument panel replaced at this time. I'll let you know what the results of this are.

 

sounds like the internal voltage regulator in your alternator is trying to take a dump on you. bad positive batery cable can cause this also. internal cable corrosion causes alternator to overwork and burn out prematurely. cable is about $95 from ford, just replaced all of my cables and alternator with a DB electrical 135 Amp hi output alternator.

 

First I want to thank all of you for your insight and willingness to offer possible causes/remedies for my dillema.

In short, as it turns out, the alternator was bad. Apparently the diode trio had starting going bad some time ago and simply took about 2 months to completely go south.

Apologies to the forum for my adamence regarding the "0" reading with the engine running. Turns out I had some bad leads on my high-dollar DVOM.

Bottom line: $209.00 at Advance Auto Parts with a brand-new "reman" alternator from Mexico [glad it's not from China]. Lifetime guarantee so the receipt went in to the owner's manual.

Thanks to RSB Diesel in Spicewood, TX for checking, cleaning all connections and diagnosing the connection on the alternator before settling in on the problem. Randy's the best and I'm priviledged to have him as my mechanic.

Thanks again to everyone. You're the tops!