I have a 81 f250 auto with a 351m. i just bought the truck about 6 months ago after it had been sitting for 4 years. it has a gm alternator on it. i got it tested from a guy and it was no good so he rebuilt it for me. i put it back on the truck. nows theres a cable that runs from the alternator to the battery pos terminal. theres a small wire, probably 14 or 12 gauge that should run to switched power. i ran the small wire to a toggle switch then back down to the battery positive so when i start the engine i can turn the switch on to charge. i did that and with the switch on i was getting 14.2v at the battery. So my question is, when the switch is turned off and not charging the engine idles fine. when i turn the switch on to charge the battery the engine really starts to die down. it doesnt shut off but pretty close. Im wondering what could do this? and should i connect this small wire to another place? and what size inline fuse should i have. this doesnt have to look pretty or anything its just a weekend mudtruck but i want it to be reliable and safe. thanks for your help...

 

If you want to get rid of the switch, you can hook terminal #1 to the green/red that goes to the Ford regulator "s" terminal. This is a switched power source that was for the original Ford charging system. If that's gone, I would run a wire from a switched voltage source in the fuse box to a dash light that you install somewhere. Make sure the light has a plastic socket with two wires. Run from the switched fuse box power to one wire on the light, and then run the other wire from the light out to the alternator #1 terminal. Now the light will light up when the key is on, but the engine is not running or the alt is not charging. This can be handy if you are out wheeling and lose a fan belt, the light will come on right away and you can shut the engine down and check things out.

Also, you do have a short wire going from terminal #2 to the large battery output terminal correct? Terminal #1 brings the alternator online, terminal #2 is the voltage feedback for the internal regulator inside the alt.

 

thanks for all your help. all there is, is a big wire goin to batt positive. theres a connector on the alternator with 4 pins, but only one wire coming out of it. the guy who rebuilt the alternator said it just had to be hooked to switch battey power which makes perfect sence.. but ya there is only 2 wires coming out of the alternator. Any ideas about the engine dieing down when the system is charging?

 

If your battery is low, the load on the alternator will be substantial, and it could lower the rpms some.

 

okay thanks alot im gunna try charging the battery overnight and try again tomorow.