Had a dead battery, after the truck sat a couple a days.
What is the easiest/best way to determine the cause of drain ?
The battery is good, it is in the electrical.

 

Well there's one way to check whats going on and thats by pulling one fuse at a time while measuring amps or ohms (I'm spacing on which one it is, but someone else may know) on the battery. If you have a load on the battery then once you pull a certain fuse, for example the domelight fuse, then the load should drop off of your battery if that was the problem. But thats the way I suggest trying to do it is pull one fuse at a time, check the load on the battery and if it doesn't drop with the first fuse you pull, move on to the second on, and so on. If it is this that is causing the problem and you find the fuse then check what that fuse controls and if it's a light then that light may be staying on even though your closing the door or turning the switch off for that light. That's my suggestion, but it may not work, just throwing it out there!

 

Mashed Pwntato was close with the answer. You need to disconnect the ground cable from the battery and put an amp gauge between the cable and the battery. It will tell you if there is a power loss from the battery. Follow what Mashed Pwntato said about pulling fuses and the gauge will go to zero when you have the correct circuit that is draining the battery. With a truck that is 41 years old, look for any wires that may have rubbed through and making contact with the sheet metal or frame. Or maybe over the years some one cobbled the wires. I have found that sometimes the glove box door light may stay on even with the door closed or the cigarette lighter won't disengage. There are a lot of sharp edges in these trucks that wires pass by or through.

 

I have a RV with 2 optima battery's that would drain in 24 hr with nothing on, after months of troubleshooting I discovered that the solenoid had developed a green film on the inside of the case {much like you see on copper pipes} this film went from the brass fitting over the isolator to the case wall and was allowing voltage drain. changed the solenoid and have not had a problem since, that was 2 years ago

 

Thanks, looks like time to get that ohm tester out and try to figure it out again. Never have had any success with that thing before.
It is crazy what some electrical problems diagnose as, like the starter relay that caused the battery drain.
One time, a truck i used to have died while it was going down the road. The mechanic at the shop i worked at diagnosed it as a bad starter.
That one still baffles me.

 

I have the exact same issue but, turn on my heater now that it's winter and the battery dies in a 2 hours even while the engine is running. Had every component checked and they're all brand new from Ford and working perfectly...but the power still drains faster than the alternator can keep up!!! I seriously need help with this if anyone can suggest more ideas to try or look for? Been chasing this for almost a year now and replaced almost everything down to the wire connectors...found nothing to solve the issue.

I'll try the meter idea, but is there any other ideas floating around by chance? Mine is a stock 1980 F350 Dually with no mods at all save the dually setup and was a an auto trans and now is a 4 spd. Didn't touch wiring tho!!!

Any help would seriously be appreciated, getting desperate to have the heat on this winter while I drive,lol

 

If checking the solenoid remember to unbolt it and have it suspended so it's not already grounded when checking it

 

Thanks Allwthrrider, Always good to remind guys of those obvious things,lol, to often we forget the simplest stuff...at least I do,lol.

Thanks again!!!

 

Twistguns;

Something has to be wrong with your alternator/charging system. The alternator should be capable of putting out 45 amps. If your heater fan motor was drawing that much or more, it would have melted or caught fire in a few minutes! The heater fan motor isn't that big in power and current draw.

I suspect that you have some problem with your voltage regulator, or the wiring connecting it to the alternator. Your charging system is being lazy.

 

After bench testing, my alternator is putting out 72 amps at 1800rpm...so no issue there. But after all the excellent suggestions and help ive received here, I have gotten things down to an under dash wiring issue. Turned the stereo off and have the heater and headlites running, and the battery lasted nearly 14 hours before going to flat to run. Recharged it in under 4 hours on trickle charge of 2 amps input. So I think the guy who wired the stereo in for my Pops, the only other and previous owner of my truck, has really screwed up somewhere besides. Maybe it's the issue itself? I've got the stereo out and completely disconnected and am chasing around under the dash wiring.

Thanks again for the excellent hints and advice. Hopefully I find success and can get this repaired and be done with it.