I have a '79 Bronco with a 351M and have been through 4 batteries in the last year. Fortunately they've all been warranty replacements but the root problem is still there. So I need your guys' help.

Here's what I've done so far:

I tested the battery (at the beginning of this process) across the posts with a Voltmeter and it said it was about 12-13 volts. That's good.

Then I hooked up the positive cable to the battery and touched a test light from the negative post to the ground cable and the light came on. That's bad right? It means I have a short somewhere?

Next I unplugged each fuse, one by one, from the fuse box and checked the test light in a simialr fashion. It still lights up no matter what fuse I remove.

Then I unhooked all the leads going to the alternator and again checked the test light. It still lights up.

At that point I decided I would start the engine and see what the voltage across the battery was while running. Well, the battery would barely roll the engine and I couldn't get it started before the battery went dead. So I got out and checked the voltage across the battery (key off) and it said it was 5 volts.

Does anybody have any advice on this one? I need to have my truck reliable for the winter months and this electrical stuff isn't my strong point. Based on my manual and the tests I decribed, there is a short somewhere but the alternator is good. What should I try next to locate the short?

Thanks a lot for any help you can give me. In the meantime, I guess it's off to the automotive store for another new battery.

 

Since the fuse deal didn't help, I think I would start at the starter solenoid & work from there. In other words, unhook main cable from solenoid, obviously should be ok as positive cable is only thing hot. Then unhook at firewall, if that doesn't stop the short then sounds like short is in that wiring. Also try unhooking voltage regulator if it has one, (not familiar with that year if it's in alternator or not).

 

Yes, pull the small wire(s) off point #3 in the diagram below and see if the light goes out.

 

Sorry guys, I forgot to mention that I've already replaced the starter relay solenoid. I'm still going to try unhooking wires there though to see what happens. Also, I'll move on to the voltage regulator and see what that does for me. I'm going to get the current battery checked/charged/replaced when I go to get the new voltage regulator too, so I'll be right back at square one after that. I'll post back later after I try these things and let you know how I came out. Thanks for the advice!

 

I found the draw on my battery! The previous owner wired the tach and the radio power supplies directly to the starter solenoid positive terminal. I unhooked the wire and my test light went out. So, for now I just have it unhooked. Any advice on where I should wire in those power supply wires so they are only on when the key is on? Should I wire them to the fuse box somewhere? I'm not too great at this electrical stuff. Thanks again for the help everybody!

 

Look behind the radio in the dash. There should be a factory hot wire back there that goes to the ignition power that powered the original radio. You can hook your tach to this wire too.

 

Okay, it looks like I'm all set up finally. I found the old radio supply wires and tapped into that for power to my new radio, tach, fog light switch, etc.. I ran the test light 'test' again and the light does not come on. With the key on/truck running everything works like expected. My radio will reset each time I turn the truck off (no constant 12V for memory now!), but I'll trade that minor inconvenience for the hassles of a dead battery and getting stranded with no juice to start the truck!

I guess my advice for anybody reading this thread would be to look closely at any aftermarket electronics that have been installed in the vehicle. Once I focused on that I found my problem very quickly.

Thanks again Franklin2 and Rancheroracer for your help!

 

You're welcome, glad it works. You could always check your new radio wires, might have one wire to connect to "hot" to maintain memory.

 

I spliced into my cig. lighter for my constant.