First Time posting. Recently purchased a 79 f150 w/351m for a father/son project. The motor runs but is pretty well worn out but we've found a replacement for $250.

The immediate problem is that it keeps draining the battery. I read the several posts that describe how to track down a short but it is not showing a short when I disconnect the negative battery terminal and connect a test light. I've replaced the regulator (checked bad at Autozone). They also checked the alternator and it was putting out 13.5. After my son put the new regulator on he arched across the two battery terminals as he was tightening them up. Could this have blown the new regulator?

This site is a great resource. Any help would be appreciated.

 

Moved to the proper forum where the electrical gurus hang.

 

Sorry about posting in the wrong forum. I'll get the hang of it.

A little more info - it's a 1979 f150 custom, 351m, 2wd AT, has a bad rod or piston. Brake lights not working but tail lights and emergency lights work-seems to be a bad brake light socket.

Doesn't test for short when I disconnect the negative battery cable and hook a test light to it and touch the negative terminal. Battery is only @ 1mo old and is showing 13.5V wile running at idle. Already replaced the regulator. I feel like it has to be a short somewhere but don't know how to proceed given the test lite doesn't indicate a short. Any help would be appreciated. Thanks.

 

13.5 should be giving you a little charge, though I have seen better.
When you go out to try to start it, is it completely dead, or does it give a clicking sound and go dead, or does it just crank slow. If you are doing a lot of troubleshooting (starting and stopping the engine) and not getting much run time, and plus if the engine is a little hard to start, I am wondering if all this activity may be draining the battery low. How long does it take before it goes dead?

 

Thanks for the reply. It's usually completely dead. I can jump it off another vehicle but takes @ 4 or 5 min to charge. It will run then until I turn it off. Even with full charge on the battery it's slow to crank. Haven't been able to do a lot of trouble shooting yet since the battery won't stay charged.

Even when it won't start the headligts come on and don't seem to be very weak. As I'm typing this I'm beginning to think it may be a weak starter or a bad battery. Other than the regulator are there any other types of shorts that would not show up when you put the test light on the negative terminal?

 

Your test with the negative battery terminal will pretty much show any battery drain, since you have it placed right there at the battery.
Your headlights being bright is a good clue that it might be something else. As part of your project, I would replace the battery cables and the cable to the starter. If you want to do some troubleshooting, you can do this:

While you try to crank the engine, take your voltmeter and measure the voltage at the big terminal of the starter relay that goes to the starter.
Then measure the voltage while cranking on the other side of the starter relay (big terminal).

You can then measure the voltage on the battery cable clamp at the battery, and then the battery post itself. As you are working your way up the line, see if you have a large voltage difference between two points. If you do, then you have found whatever is causing the resistance in the circuit, and you can replace that component. It may be a bad battery cable, starter relay, or if all the voltage readings are fairly close, then I would start suspecting the starter.
A worn out engine ought to crank over fairly easily.

 

Thanks for the detailed instructions. We'll go on the "hunt" this weekend following your suggestions. I'll post the results when we finish.

 

YOU HAVE HIT THE NAIL ON THE HEAD! I have an 84' F-250 that does pretty much the same thing. I am trying to track it down as well. Same thing, I tested it at Checkers, and the Alternator was putting out about 13+ and the regulator (recently replaced) was bad I have since replaced my Neg cable and Starter (for GP) and it seems to have helped some. But I can still go out after it has set 24 hours and get good lights but slow crank and clicking. I haven't found it yet, so if you do please post it here, this is one of my last "GREMLINS" since I have rewired the entire truck.

 

Didn't get a chance to get into it this weekend. Wound up picking up a used 351m to replace the one that's in it. I'll replace the battery wires and check the starter and alternator when I switch the motors. If I find out what it was I'll post what I find.

 

Well, after doing a lot of reading and on this site, and in my Haynes manual (as well as talking to a couple of mechanics) I replaced my Alternator. That was it!! It was tested at PEP Boys less than a month ago and it put out 13+ volts, but numerous people have commented that when an Alternators' Diodes go bad it can still charge, but it drains the battery when it's not running. I also had another clue in that every time i switched my Blower motor from High-Medium-Low, it caused my TAC to spike... I couldn't think where a short could cross between the blower motor and the TAC until a mechanic I know mentioned that when your blower motor kicks on/changes, it puts a different load on the Alt. If the Alt is bad this could reflect in output/Tac. I tried it and that was the answer. Hope it works for you.

 

Thanks. It sounds like our problems were almost identicle. Autozone tested my alternator 2 weeks ago and it was producing the same as yours. I'd almost bet it's the same problem.