I just bought a 89 XLT w/300 inline that has an electrical problem. The Battery won't charge so a new altenator was put in place and it still won't charge the altenator was check out as being good? Also I have a check engine light that goes off when the truck pass a higher RPM? And I found a voltage drop when the heater motor is engaged? Also I found the rear anti-lock light on?

 

Have you tried a new battery? Sometimes you can have a dead cell in the battery that will permit the truck to run normal for a few weeks and then go dead again.

------------------
1997 XLT Reg. Cab, 4.2L V6, 5spd, Superchipped, K&N Air Filter, Lund Deflector, Dash Mat, MagnaFlow Muffler, Hellwig Rear Sway Bar, Edelbrock Shocks, Duraliner, Extang Saber Tonneau, Painted Tailgate Handle, Bell-Tech 2" Rear Drop, Amer. Rac. Atlas Wheels Clad in 275/60/17 Eagle GTII. Likes straightaways, loves twisties.
1996 Shovie Cavalier (Don't laugh, it can lay down 30' of posi-rubber if you dump the clutch just right!)

 

I'm having the exact same problem with my 88 f150 . Rear anti lock stays on voltage drops when I turn the fan on if you figure it out please let me know what you find.

 

You could have a bad batt. cable or battery. I think the 89 have a fusable link between the alt. and batt.