Still trying to get my wiring straight. something isn't right. When switch is set to run, I'm getting a constant current draw of about 5A. Disconnect the three wire plug to the distributor and it drops to 3A.

Trying to find the cause of the other 3A, there is no radio. Disconnected All lights, wiper motor, heater blower, steering column. Disconnected the electric cooling fans.

What the heck else could be pulling 3A? Does it make sense that the distributor would pull 2A in RUN?

1977 F100. 4.9L I6, C4 transmission. Just did a 3G alternator upgrade but I believe that whatever is wrong was wrong prior to this alternator change.

 

Do you have duraspark II?

EDIT: Where are you measuring your draw? The coil will draw voltage when the key is in the "on" position

 

Measuring current between batt- and battery cable that ties to the block. I disconnected the line from the distributor to the coil and still had the drain, but I didn't disconnect the feed into the coil. I'll try that next.

I just keep killing batteries and am trying to find out why. I just realized the next obvious thing to do is to see if I still get a current draw with the key in the "off" position. If that goes to 0, I'll just assume a constant draw in "run" is normal.

 

I've got some electrical troubleshooting help here;
Ford Truck Enthusiasts Forums - Grubbworm's Album: Troubleshooting help, for anyone that needs it.

 

I have a 1977 F100 300I6 manual transmission. For almost 2 years I kept killing batteries and couldn't figure out why. The truck would discharge them within a few days of me charging them on the bench or not running the truck. The battery itself would die within a few months and I would have to exchange it. I re-wired the entire charging circuit, upgraded to a bigger alternator, changed the voltage regulator. I like you measured the current draw but couldn't figure out where it was leaking from. I pulled all the fuses and put them in 1 by 1 measuring the current draw each time and still couldn't figure it out.

I ended up fixing it completely by accident. The last battery I got came with a plastic tray on top to protect it. It was a nice tray, cross hatched and very strong. I looked at my rusty battery tray which had an aging insulator mat on it and figured I would use the battery tray on top of it to strengthen it up. It raised my battery an inch higher but I had room under the hood and the battery mounts were easily tall enough.

Low and behold suddenly my battery stopped discharging and dyeing all the time. I thought to myself seriously, that was the problem all along .

I'm not saying this is your problem but you never know... Otherwise to trouble shoot it this is what I would do. You have the right idea checking the current draw between the negative terminal on the battery and chassis ground. Disconnect everything from the battery and pull the fuses, connect one thing at a time and monitor the current draw. It could be as simple as some insulation failing somewhere in the wiring of the truck.