How can i tell which cable to the battery is positive or negative on a 1990 f150? Thanks!

Look at the battery cable on the starter, it's touching the smaller switch stud. Or the starter relay is stuck.

Positives are red,black for negative and green/ yellow for ground!
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Disconnect the negative and positive battery cables from the terminals on the battery. Take that battery out. Now then buy another battery for you make and model. Put the positive and negative battery cables on the battery terminals of the new battery. MAKING SURE, you have them on the RIGHT CABLES ON THE RIGHT TERMINALS. POSITIVE --- (RED) as the NEGATIVE CABLE is ----(BLACK). If you get them mixed up and try to start the car or something. You will blow a ton of fuses. and no telling what else! Its not hard as long as you can read the (-) << NEGATIVE (BLACK) WIRE and (+) << POSITIVE --(RED) WIRE on the battery.

1990 Ford F150 Truck Car Radio Wiring Schematic
Car Radio Battery Constant 12v+ Wire: Green/Yellow
Car Radio Accessory Switched 12v+ Wire: Yellow/Black
Car Radio Ground Wire: Red or Black
Car Radio Illumination Wire: Blue/Red
Left Front Speaker Positive Wire (+): Orange/Green
Left Front Speaker Negative Wire (-): Black/White
Right Front Speaker Positive Wire (+): White/Green
Right Front Speaker Negative Wire (-): Black/White
Left Rear Speaker Positive Wire (+): Pink/Green
Left Rear Speaker Negative Wire (-): Pink/Blue
Right Rear Speaker Positive Wire (+): Pink/Blue
Right Rear Speaker Negative Wire (-): Green/Orange

one is by disconnecting negative cable from battery for an instant with car running, if car stops running, your alternator is bad, other way is to read voltage at the battery with car running, what your reading is alternator voltage, should be between 12v and 14.9v any reading off means your alt. could be bad, but if your not shure, if you can, take the alternator out and have an autoparts tested for you they will tell you if is good or not. thanks

https://s11.postimg.org/crrcun103/generator.jpg

Click on the link.
The diagram shows generator with internal voltage regulator and with external voltage regulator, at the moment, I don't know which one you have?
The bat terminal at the generator should be hot even with the key off, is it, use a test light. If no voltage either the fusible link is faulty or the wiring circuit to battery +. You can see fusible links in diagram.
Could be other issues, the generator probably needs excite voltage or it won't work. The excite voltage comes from charge indicator circuit. Does the charge lamp on the dash work?

+ POSITIVE IS THE RED BATTERY CABLE - BLACK CABLE IS THE BATTERY NEGATIVE GROUND CABLE.

You need to pull fuses to see,but it can still be the alternator,with a bad diode allowing the drain,remove the negative battery cable from the battery,then buy a test light,they are cheap,clip one end to the negative battery cable,and touch the tip of the test light to the battery post,and the test light will light up,cause there is a draw on the system,make sure the doors are shut,and the lights are off.Then unplug the connector on the alternator,if the test light light goes off,the alternator is the problem,if not,remove the dome light fuse,and start pulling fuses one,at a time,and putting them back in,as you go,if you pull a fuse,and the test light light goes out,that is the circuit that is drawing the voltage from the battery,leave it out,and hook cable back up,and see what does not work with the fuse still out,and that will be the problem,please rate if this was at all helpful,thank you.

Ther are two positive cables and only one negitive cable

Remove the negative battery cable and connect a 12 volt teat light between the terminal and the battery post. Make sure all doors and trunk are closed and all consumers are in the off position. The light should glow. Now pull one fuse at a time until the light goes out. When it goes out that the circuit thats causing the draw.