'96 5.8 Auto 4x4 >8600lb.. Was driving last week and the batt light came on briefly but no issues because of it. Battery stays charged and all acessories work. Went for a 180 mile trip today and the same thing happened mid-way into the trip, only this time it stayed on & is still on. Truck starts right back up after shut down so I don't think it's draining the battery. Any ideas on what to do about this?

 

I would be inclined to pull the alternator and check the brushes.

When my ex-wife's 2000 Expedition started this, she let it go long enough to let the brush chew through the slip ring the brushes ride on.

It would be fine for short periods, but she didn't tell me till the light was staying lit for her entire commute.

 

Check the voltage at the battery with the truck running and the charge light on. Should be high 13s, maybe 14. Check with brights and fan on high.

Check the battery voltage after the truck sits overnight or longer. Should be 12.6 or more.

Load test the battery.

Auto parts or walmart will test it for you.

 

My 96 F350 w/460 did the same thing last week, had to replace alternator.

 

OK, here's a quick follow-up:
Truck sitting overnight-key off, not running, Voltage is 11.8
Truck running, no accesories, Voltage is 14.1
Truck running, A/C, lights & fan on HI, Voltage is 13.8
Shut truck of, reading was 12.8, then gradually dropped within 2 minutes to 12.2
FWIW, the alt was slightly squealing when under full load...

 

Load test your battery. Check for drains when sitting.

 

This indicates that your charging is fine.

 

This indicates that your alternator is working hard to overcome the load - probably weak battery ... But again, check the current draw.

A fully charged battery is 12.6 or better. Your battery is not holding a charge. 12.2 is near dead. One of my batteries holds more than 13 volts overnight.

 

I'd check the specific gravity in all the cells first.

One is dead or near dead I bet.

 

Battery is junk sounds like. How old is it? Sealed maintenance free type batteries when fully charged measure 12.80 volts @ 70°F. 12.65 volts for old time batteries that had caps on top for refilling with distilled water. Keep in mind a defective battery isn't doing the charging system any favors. In order for a battery to pass testing it not only has to take a charge but it has to be able to hold it.

 

Good point, may be a parasitic or "phantom load" drawing battery down. This will shorten battery life as it nevers gets charged. Check current draw 1 hour after shutdown, should not exceed 50 milliamps or so.

 

OK, follow-up to the follow-up....Battery was toast-1 or possibly more dead cells. Full 100% replacement since it was under 3 years old. Thought they were going to pro-rate, but they simply swapped out for a new one. Now it reads 12.9V not running, 14.2 w?all access. on and running. I'm assuming this means the alt is still healthy...thanks again for all who chimed in here!

 

Make sure the grounds and all electrical connections are clean, and tight. That means actually disconnecting them and grinding down to bright shiny metal at frame and block, etc. Voltage is one thing, but current (amperes) is just as important. If they've never been cracked apart after 20 years they are choking off alternator current output, guaranteed. It's not noticeable at idle but under load it will not allow rated current to pass through.

Check for excessive parasitic load after shutdown, wait about an hour. This kills batteries before their time as well.