The Ford Barn

rbhunsaker · 02-21-2013, 04:22 PM

I recently purchased a very nicely restored '31 Coupe. Between the restoration and now, it has gone through two hands. I have discovered that if I leave the battery connected over night, it is drained by an unknown short. One previous owner thought it still had a 6 volt system (converted to 12v),so he installed a new 6 volt battery. Maybe this did some damage somewhere ?

Where should I began looking? I have been told the alternator may be the first place to start; or the ammeter ... ??

I am fairly savvy with electricity, having been in amateur radio for over 50 years, but I am a greenhorn with this car.

Many thanks for any help or guidance you guys can offer. I am also new to this web site.

Brian

P.S. · 02-21-2013, 04:34 PM

Welcome!

I have an extra class ticket. Interesting stuff.

Anyway, the first place to look for sure is the generator. Most likely a cutout problem.

Before we go any further, please verify if your car has an original type generator or a modern type alternator. Second, we need to know if it has positive or negative ground. Next, we need to know (if it still has a generator) what type of cutout it has (relay type or diode type).

If your car's generator has been upgraded to an alternator, it is imperative to check the polarity of the car's wiring. They came from Ford as positive ground. Some people convert to negative ground.

Since you're a licensed amateur radio guy, I assume you have a voltmeter around the shack somewhere. Please let us know if your meter has a current (amp) setting on it for use as an inline ammeter. Otherwise, do you have, or have access to a DC clamp-on style current meter?

Phil · 02-21-2013, 04:40 PM

I would suspect the alternator diodes. To check: rig a 12 v bulb and socket with alligator clips. disconnect the battery cable and clip the bulb in series, cable one clip, battery post the other clip. If you have a load the bulb will glow. When you clear the load build will go off. I would hook up test lamp then remove alternator wire. To see if it clears the load.

Hicktick · 02-21-2013, 04:44 PM

If its draining a 12 volt overnite I would also check the brake light switch .

Brake lights hang up sometimes and kill the battery

Mike V. Florida · 02-21-2013, 04:52 PM

When the engine is off what does the ammeter read?

Zero or near zero have the alt checked for bad diodes and mentioned.
More that zero chack the brake light. Actually check the brake light first as it is the easiest.

Does it have a fuse? Remove the fuse and see if the battery goes dead.

Let us know what you find.

newshirt · 02-21-2013, 04:59 PM

Sponsored Links (Register now to hide all advertisements)

Do you have an electrical schematic of the Model A? If not, print this out for reference.

You could either slip an ammeter inline to measure current drain, or disconnect suspected connections to see if your still battery drains overnight. If not, you've located the current drain path.

MikeK · 02-21-2013, 05:12 PM

If you were in amateur radio this should be a piece of cake. Start by disconnecting the battery ground strap and place an ammeter in series. If you are flattening the battery overnight, the parasitic draw is likely greater than 1000ma, so choose the meter range appropriately.

1) Isolate major branches of the wiring. At the gen/alt output there will be three wires. One to the horn, one to the light switch, one to the terminal box on the firewall. There is also another lead to the terminal box from the starter switch. Disconnect all four, make sure they are isolated and do not touch the gen/alt output stud.

2)With a jumper from the battery apply power to five points: the gen/alt output stud itself, the light lead, the horn lead, each of the two leads to the terminal box. Your meter will tell you which branch has the fault. Proceed from there.

BILL WILLIAMSON · 02-21-2013, 08:35 PM

A low draw buzzer in series from disconnected ground cable to positive + battery post works. Just disconnect stuff here & there until buzzer stops, then you're close to finding the draw. If you're hard of hearing, use a buzzer from an old smoke alarm & hope somebody doesn't call the FIRE DEPT! Bill W.

02-21-2013, 04:22 PM	#1
rbhunsaker Junior Member Join Date: Feb 2013 Location: HArlingen, Texas Posts: 1	1931 A Coupe - electrical short I recently purchased a very nicely restored '31 Coupe. Between the restoration and now, it has gone through two hands. I have discovered that if I leave the battery connected over night, it is drained by an unknown short. One previous owner thought it still had a 6 volt system (converted to 12v),so he installed a new 6 volt battery. Maybe this did some damage somewhere ? Where should I began looking? I have been told the alternator may be the first place to start; or the ammeter ... ?? I am fairly savvy with electricity, having been in amateur radio for over 50 years, but I am a greenhorn with this car. Many thanks for any help or guidance you guys can offer. I am also new to this web site. Brian

02-21-2013, 04:34 PM	#2
P.S. Senior Member Join Date: Jul 2012 Location: California Posts: 1,695	Re: 1931 A Coupe - electrical short Welcome! I have an extra class ticket. Interesting stuff. Anyway, the first place to look for sure is the generator. Most likely a cutout problem. Before we go any further, please verify if your car has an original type generator or a modern type alternator. Second, we need to know if it has positive or negative ground. Next, we need to know (if it still has a generator) what type of cutout it has (relay type or diode type). If your car's generator has been upgraded to an alternator, it is imperative to check the polarity of the car's wiring. They came from Ford as positive ground. Some people convert to negative ground. Since you're a licensed amateur radio guy, I assume you have a voltmeter around the shack somewhere. Please let us know if your meter has a current (amp) setting on it for use as an inline ammeter. Otherwise, do you have, or have access to a DC clamp-on style current meter?

02-21-2013, 04:40 PM	#3
Phil Senior Member Join Date: May 2010 Location: Bethany, Ok Posts: 382	Re: 1931 A Coupe - electrical short I would suspect the alternator diodes. To check: rig a 12 v bulb and socket with alligator clips. disconnect the battery cable and clip the bulb in series, cable one clip, battery post the other clip. If you have a load the bulb will glow. When you clear the load build will go off. I would hook up test lamp then remove alternator wire. To see if it clears the load.

02-21-2013, 04:44 PM	#4
Hicktick Senior Member Join Date: Aug 2011 Location: Hickory Tavern , SC Posts: 422	Re: 1931 A Coupe - electrical short If its draining a 12 volt overnite I would also check the brake light switch . Brake lights hang up sometimes and kill the battery

02-21-2013, 04:52 PM	#5
Mike V. Florida Senior Member Join Date: May 2010 Location: South Florida Posts: 14,054	Re: 1931 A Coupe - electrical short When the engine is off what does the ammeter read? Zero or near zero have the alt checked for bad diodes and mentioned. More that zero chack the brake light. Actually check the brake light first as it is the easiest. Does it have a fuse? Remove the fuse and see if the battery goes dead. Let us know what you find. __________________ What's right about America is that although we have a mess of problems, we have great capacity - intellect and resources - to do some thing about them. - Henry Ford II

02-21-2013, 04:59 PM	#6
newshirt Senior Member Join Date: Jan 2011 Location: Colorado Springs Posts: 868	Re: 1931 A Coupe - electrical short Sponsored Links (Register now to hide all advertisements) Do you have an electrical schematic of the Model A? If not, print this out for reference. You could either slip an ammeter inline to measure current drain, or disconnect suspected connections to see if your still battery drains overnight. If not, you've located the current drain path. __________________ Ray White

02-21-2013, 05:12 PM	#7
MikeK Senior Member Join Date: May 2010 Location: Windy City Posts: 2,919	Re: 1931 A Coupe - electrical short If you were in amateur radio this should be a piece of cake. Start by disconnecting the battery ground strap and place an ammeter in series. If you are flattening the battery overnight, the parasitic draw is likely greater than 1000ma, so choose the meter range appropriately. 1) Isolate major branches of the wiring. At the gen/alt output there will be three wires. One to the horn, one to the light switch, one to the terminal box on the firewall. There is also another lead to the terminal box from the starter switch. Disconnect all four, make sure they are isolated and do not touch the gen/alt output stud. 2)With a jumper from the battery apply power to five points: the gen/alt output stud itself, the light lead, the horn lead, each of the two leads to the terminal box. Your meter will tell you which branch has the fault. Proceed from there.

02-21-2013, 08:35 PM	#8
BILL WILLIAMSON Senior Member Join Date: Dec 2010 Location: FRESNO, CA Posts: 12,560	Re: 1931 A Coupe - electrical short A low draw buzzer in series from disconnected ground cable to positive + battery post works. Just disconnect stuff here & there until buzzer stops, then you're close to finding the draw. If you're hard of hearing, use a buzzer from an old smoke alarm & hope somebody doesn't call the FIRE DEPT! Bill W. __________________ "THE ASSISTANT GURU OF STUFF"