I have a 1977 F100.

Recently, out of the blue, I had a short/drain start. I didn't do anything mechanically to the car. It was driving fine, but suddenly it started dying after sitting overnight.

In this period that the problem started, I noticed that the neutral safety switch wasn't working as great, having to move the shifter back and forth and one headlight is dimmer.

When I check across the battery leads, resistance is zero. When I test the amperage, the meter reads over the limit and then slowly comes down to 30 mA. A small spark can be seen when you reconnect the battery terminal.

I pulled all the fuses, one-by-one, and none of them showed to be the cause of the drain. When I disconnect the dim headlight, I still get a drain. Only thing I can think of is maybe the ignition coil? I also replaced the ignition control module, because I suspected that, it was easy, and it had been replaced 10+ years ago.

My next idea is to get a new multimeter and pull the fuses again.

Any ideas?

 

You could have a shorted diode in the alternator. To check, disconnect the heavy lead from the battery post of the alternator. Disconnect the negative cable from the battery when you are doing this as this lead is battery positive at all times. Tape up the wire to prevent a short to ground, reconnect the battery and leave overnight. There are other ways to test, but none as cheap.

 

I had the drain with the old alternator and the new.

 

The ignition coil is only hot in RUN (and START, indirectly by the starter solenoid). Unless you're leaving the key on, the ignition coil is completely unrelated.

The regulator can also be responsible for a drain that persists across fuses being pulled. Disconnect the negative battery cable and re-connect it with a test light in series. Don't use a multimeter for now since the magnitude of the drain is unknown. Assuming the drain is still present, the light will be on. Disconnect the regulator and see if the light shuts off. Make sure you don't have any "fake" drains during the test, like the door open keeping the dome light on, hood lamp, etc. Not sure if some trucks also had a glove box lamp, etc like the sedans did.

 

I've already replaced the voltage regulator.

I think it's got a bad starter solenoid that why the battery appears to be dead. Gonna whack it good/short it next time it won't start.

Short seems to be on lead on wire from batter side of starter solenoid to alternator.

 

Just because things are new doesn't mean they're good. If everything was good, you wouldn't be having this problem. It's highly unlikely the starter solenoid is responsible; you'd be seeing smoke. Stick to the tests given to you.

 

And the winner is....voltage regulator. When I replaced the old one, I did it in haste on the street, trying to get the truck off the main drag before a snowstorm. I just plugged in the voltage regulator and didn't bolt it to the body. Afterwards I thought that probably wasn't a good idea. I did eventually bolt it on. I knew the drain was coming somewhere from the alternator area.

I used the continuity tester in series across the battery and then disconnected/connected the voltage regulator.

 

The voltage regulator has to be bolted to the body, otherwise it won't operate at all. It uses the fenderwell sheetmetal as its ground. When requesting for help with an electrical issue (or any issue, really), you should list out any exceptions or special cases like this early on. Otherwise we would have directed you there right away.

An ungrounded (and therefore non-operational voltage regulator) will definitely prevent the battery from charging properly, but it won't induce a drain. There's no path to ground if it's not bolted down, except through the field windings of the alternator I guess. That won't matter. So I'm not really sure what fixed the problem. But it sounds like you're back up and running, so I'm glad to hear you got it (whatever "it" was) worked out. I only mention all this because I personally am hesitant to walk away from something and call it "done" if there are lingering mysteries, otherwise I find myself stuck with the same problem 3 weeks later. Perhaps it's not a mystery to you, but it is to me based on the information I've been given.