my 1976 f100 has a 302cid which is not the stock motor but im told had no problems before the previouse owner parked it two years ago. i purchased the truck about a month ago and the battery will not stay charged while its running. i replaced the alternator and the battery holds a good charge as long as its not running. ive checked all the grounds and traced all the wiring back as far as i can go. Any ideas or advice that anyone has would be great, thank you

 

Can't say what's wrong with yours, but I can tell you what was wrong with mine. That 45 Amp alternator just wasn't cutting it with the radio, radiator fans, and everything else. I was driving down the road with a working alternator and still drawing down on the battery.

A junkyard 3G alternator as people describe here was the smartest, easiest upgrade I've done to this truck. $20.

 

Does this truck have an ALT light, or an ammeter gauge in the dash? There are a few wiring items to check first; if they check out then the regulator is suspect.

 

i do have an ammeter and i have an ambulance grade alternator but if i disconnect the neg- battery cable while its running the engine dies, and i just took the alternator down and napa tested it for me and it read out good

 

"This truck will do the speed limit on the trailer to the scrap yard"

 

check the fusible link that comes from the alt harness to the battery side of the starter sel.

 

Never disconnect the battery from a running engine. If you do this, the alternator output can jump very high, and fry any onboard electronics like CB radios, ignition modules, etc - in some cases even burn out bulbs.

Check the fusible link in between the alternator output and the battery-side of the starter solenoid for continuity. If that's fine, then have the regulator tested.