I have an 87 b2 and it has killed about 3 batterys, now it's on its 4th, I don't know why, I checked all fuses and wiring, it's all working, but for some reason, when im driving the battery light will flash, but if I turn the fan on, it stops flashing..
And now, no matter what battery, even if I charge it, it wont start nor will it even hold enough of a charge to turn the head lights on, and im not leaving any lights or the radio or anything on.

 

A few questions.
Is it discharging while sitting in the vehicle, or is it never getting charged?
Have you had the battery load tested? Alternator output verified?
Can you jump start the truck?
At this point, it's obvious something is wrong that just replacing the battery isn't going to fix. There's a short somewhere, bad battery cables, bad switch/relay, bad starter, etc. Need to go through it carefully and see what the problem is.

 

I had a similar problem .I had a short in the dome light and it would end up running down the battery.It may be worth checking

 

i had an 86 b2 do this to me. never did figure it out either. the 86 i have now had started to do that and i ended up going to a bigger alt. but myne was dicharging because of my stereo (it pulled 50 amps from a 50 amp alt) this seems to be a commom prob. on myne i replaced the alt v,r and bat and the prob still haunted me. so i would suggest taking a weekend and going over all the wiring and taking the alt bat and starter to an auto parts dealter and have them checked. check the bat cables for corosion and resistance. if you know how or have the manual check all of you relays. i had an arostar do this aswell. it was because someone had taken the bat loose while it was running. fried the relays (fuel pump would circulat all the time and the vr stayed grounded) so in 10 mins the bat would be fried aswell. but i would mostly focus on wiring for a day and include the relays. a faster aproach is to hook a load tester (i think thats the name. its just a little lite thing) up and pull fuses until the bulb dies. then go over that circut. but the doors must be shut vanity mirrors off and anything else that can use power off. i had to use a volt meter so i was forced to also take the computer loose in that van. i figured it out after a few hours but i forget what caused it. didnt care to fix it either since the trans kept dying on it. but go over your wiring with an eagle eye. if its cut fix it if its fraid replace it with a new section. if you have to do wire work get a crimper and boat full (3 packs is what i used on my b2 when i had to straighten it out if you have extras return them unopend.

 

I disconnected the cd player, speakers, dome lights, and the inside lights like rmp meter and stuff, it still did it, I finally disconnected the fan, and it holds a charge now, but I don't know for how long.
And yes I could jump it, and it would hold a charge, but after it ran for about 2 miles, the battery light would flash like crazy, than wouldn't start again, but as allways the light would stop when I turned the fan on.
but its all disconnected now so I still got my bronco.

 

It could be the diode inside of the alternator. Trying removing each of the huge fuses from the fuse box inside engine compartment next to the battery. Once each fuse is removed... try slowly putting it back and see if you get a spark. This helps you diagnose whereabout the problem exists. If it's the 2nd fuse counting from the back of the truck then it's more likely it's the alternator's diode.

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Quang
89 BroncoII