Just purchased a 89 F-150. Got a great deal, runs and drives! But the battery drains when turned off. There are cut wires to the doors (windows and doors) and all the fuses are good. I tried to connect a OB I reader and it did not connect. Also there is an unpluged wire by the parking break, see picture. Not sure if it matters, but it is a 5.0 5 speed.

Any suggestions on where to start? I do have good grounds in body, engine and chassie.

 

Battery draining is not an OBD issue to the code reader is not going to help you with that. If it drains a charged battery overnight, it is most likely the alternator voltage regulator being stuck. Unplug the alternator and see if the draw stops.

 

That is the Electronic Brake Controller Connection.

Red/Light Green; Trailer Brake Lamps.
Blue/Black; Trailer Electric Brakes.
Red; Fuse Link "F", from the St. Relay, +12vdc, (Always Hot)

Looks like your truck has the factory towing harness.

 

Thanks. With disconecting the alternator the battery stayed charged. Looks like a new alternator voltagge regulator.

Step one, more problems to be revealed.