yo Ferrochel,
WELCOME!
I have this Starter/Relay, Alt etc. Wiring Diagram in 84 Bronco & F-Series (partial); "...Similar to 80-91 Bronco & F-Series..."
Source: by Steve
Some checks;
Are batty post, terminal & cable strands clean? Also check main batty grounds to frame, etc.
Is battery fully charged"?
"...a normal battery that is fully charged produces 12.6 volts, not just 12.0 V. (Remember, when measuring battery voltage, everything in the car should be off, or the battery should be disconnected. Voltage measurements are always "no load" measurements unless the battery is being tested for it's performance under load)..."
Source: by Gordon
Is starter relay attached tightly to inner fender liner? Relay body is grounded via liner. See if there is ruat around relay; if so, remove and inspect; remove rust and reattach.
No Crank, Low State Of Battery Charge TSB 91-10-8 for 85-91 Bronco, Bronco II, Econoline, F-150-350 Series, Ranger; 86-91 Aerostar; 88-91 F Super Duty, F47, F-53, F-59; 91 Explorer, etc.
Source: by Ford via
http://www.performanceprobe.com/misc/tsb.php?article=91-10-08