Tech Q&A - Electrical
12 VOLT OR 6 VOLT?
Question: I have owned Model A's for 40 years and I just purchased a 1930 Model A Dlx. Coupe all restored & beautiful. The only thing is they put a 12 volt alternator that reads 12 volt negative ground. I have the car at a friend's house waiting to make space to bring it home so I have not gone through it yet. My question is that it appears that they put a 12 volt alt. & battery because the car spins over very fast but it seems like the wiring is correct looking for 6 volt? Could someone convert to 12 volt without changing the wires? and if so I assume the bulbs must be 12 volt as well. I am not good when it comes to electrical. Would the stock wiring hold up for 12 volt?
Thanks, Joe Todaro, Bedford Hills New York
Answer: One of the reasons the auto industry converted to 12-volts in the '50's was to reduce the cost of the automobile. When the electrical system is changed to 12-volts the amount of current (amps) is reduced to maintain the same electrical power (watts). With the reduction in current in the system the size of the wire can be reduced, therefore saving money.
As the original system wiring is designed for the 6-volt system the wire size is actually heavier than necessary. This doesn't cause any problems in the system. As for the light bulbs, these would also have been changed to 12-volts. This will also affect the horn, either it has been re-wired for 12-volts or a voltage reducer has been installed to reduce the voltage to the horn to 6-volts. The ignition coil would also have been changed or a voltage reducer installed in series for it also. You might wish to contact the previous owner and determine what was actually done when the conversion was done. - Chuck Christensen, 2011 MAFCA Technical Director - posted 12/10/11
I went to drive our 1929 town sedan and the battery was completely dead. This has happened before, the last time was about 4 weeks ago. The car runs great, then the next day or so - no battery. The ammeter is at 0 when I push in the pop out switch and release it (take my finger off, I do not see it move to discharge). The distributor is between cylinders. Do I suspect the switch or a bad spot in the alternator that is letting the current flow backwards? Thank you for taking your time to answer this question.
Make sure the black wire from your coil is connected to the terminal box stud that has the yellow wire with black tracer. This should be the (+) side of the Ammeter. The (-) side of the ammeter (yellow wire) should connect to the other terminal box stud, along with the yellow wire from the starter motor. No other wires should be connected to the Yellow wire stud at the terminal box. All other wires should be connected to the terminal box stud with the yellow wires with black tracer. This will ensure that all electrical circuits will pass through the ammeter and all current drain will be detected on the ammeter. The ignition switch is connected between the coil and the ignition points and is used only to apply voltage to the points.
No other electrical circuits are affected or connected by the ignition switch. The ammeter should always show any current drain in the circuit. Watch the ammeter while disconnecting the battery (fully charged battery) to determine if a slight discharge is indicated. Disconnect the electric windshield wiper and the single wire from the generator cutout (or the alternator) to see if it causes any slight movement of the ammeter to indicate a slight discharge. If you have an alternator installed, and getting a current drain through the alternator, it indicates there is a bad diode in the alternator. If you have a generator with a cutout installed, the cutout relay may be bad and not opening, allowing the battery to drain through the generator. I hope this allows you to isolate the source of drainage. - Les Andrews, 1998 Technical Director
Do you have any experience with 6 volt alternators? I have one and use an 8 volt battery. I have changed 3 batteries since April and can't find the source of the problem. I use a safety cutoff switch so there is no battery drain during idle times. One associate from Birmingham said he had to go back to a 6 volt battery to alleviate this condition. I can't buy that either. I wrote the NuRex company but have not heard a reply to date.
Regarding the 6 volt alternator, it should be putting out at least 7 volts to maintain a charge for a 6 volt battery. A 6 volt alternator cannot maintain a proper charge for a 8 volt battery. It would have to put out about 9.5 volts to maintain a proper charge on the 8 volt battery. The 8 volt battery is signaling the 6 volt alternator regulator not to put out any voltage because it thinks the battery is fully charged. And they do not make battery chargers for 8 volt batteries. I suggest you go back to a 6 volt battery. If you are using a 8v battery because it is easier to start, try adding a second ground Cable. Connect it from the same bolt that connects the battery ground strap to the frame cross member and the other end to one of the bell housing bolts just behind the starter motor. This provides a more positive ground for the starter motor and you will find that the starter will turn over easier. Use a standard GM battery cable about 30" long with eye connections at both ends. 6 volt batteries should be readily available at any automotive parts store. It uses a Group 1 battery. - Les Andrews, 1998 Technical Director
Quick question: What could cause the amp meter to indicate charge when turning on the headlights or blowing of horn?
It sounds like the leads on the Ammeter are reversed, or they may have been reversed at the terminal box posts. An easy fix. Pull the instrument panel out and reverse the leads on the back side of the ammeter. (not uncommon for someone to hookup the ammeter leads backwards).
My ammeter on my 31 coupe is hot or warm to the touch when charging 8 amps. Is this normal?
This is not a normal situation you have with your ammeter. This is usually caused by the two nuts on the back of the ammeter being loose and causing a high resistance at the connection. The resistance generates heat. Remove the instrument panel and tighten the two nuts on the back of the ammeter. Also make sure the nuts on the firewall terminal box studs are tight. Remove the two brass wing nuts on the terminal box cover, remove the cover and tighten the nuts on the two stud bolts. Also check the two wires that connect to the two terminal box studs to the two connections on the ammeter (yellow and yellow/black wires). Make sure they have not been over heated due to loose connections. This should solve the problem. If not I would change the ammeter. - Les Andrews, 1998 Technical Director
When I attempt to connect the battery cables to the 6 Volt battery in my 1929 Model A Ford, it seems to generate an excessive amount of sparking when I touch the cables to the posts. I don't remember happening before. Can you suggest any reasons for this? Might I damage the electrical system if I secure the cables anyway? Thank you for any help you might provide.
Make sure you are not connecting the positive to the negative ground. Also check to be sure there is nothing touching either of the two battery posts, such has the battery hold down clamps, shorts or other ground contact. Let me know what you find. - Lyle Meek, 1997 Technical Director
Thanks for the information on the amp. I am reluctant to tell you that your book. MODEL A FORD- MECHANICS HANDBOOK "clued me in" when in scanning same under the battery section I discovered, much to my surprise. that you were supposed to connect the battery cables in reverse of what I had always done on other cars --- red to negative etc. I did this after pulling the battery for a charge and then reconnecting - Pretty dumb ! Incidentally, when using a battery charger to charge your battery, do you also hook up the red to the negative post and ground to the plus pole ??
Remember that the Model A has a positive ground electrical system. Ford was the first automaker to do this and for good reason. Due to gasses from the battery, the positive terminal always has a tendency to collect corrosion and cause poor connection. Ford engineers thought if the applied the positive terminal of the battery to the ground connection (reversing current flow) it would reduce the terminal corrosion and the corrosion that would form would not have as much effect on reduced voltage. ...a little bit of trivia. When connecting a battery charger always connect the charger (+) red lead to the Battery (+) side.
-- Les Andrews, 1998 Technical Director
I recently purchased a 1930 Roadster. When I went to remove the battery I found it connected with the negative terminal to the chassis?? When I install it in the spring should I connect it correctly, with the positive terminal to chassis, and will the generator and starter operate as they should? Is there anything I should do like shocking the generator?
I would reconnect the battery for positive (+) ground connection. The starter will operate just fine with either negative of positive connection. If you reverse the battery polarity connection you will have to shock the generator to reverse the polarity output. To reverse polarity (shock) of the generator, connect a cable to the negative terminal of the battery (ungrounded side) and momentarily touch the end of the cable to the output post of the generator (not the cutout). This will change the polarity out put of the generator. If you change polarity you will also need to reverse the wires on the back of the ammeter. Start the engine and check the output of the generator with a volt meter to make sure it is outputting about 7.2 volts negative voltage at about 1300 rpm engine speed. Everything else should be OK. - Les Andrews, 1998 Technical Director
In the Snyder's Antique Auto Parts, -97, page A-38 is a device listed as "polarity tester" which is inserted twixt coil and distributor. It clearly shows arrows for "up" (coil). Text reads "..it indicates if you have the wires on your coil properly (installed)..." Is there a "coil only end" for a coil wire? New to me. Been trying to start my A Model (30) after some years in storage and found that the ignition switch was faulty (new one on the way). Tried jumping the switch, but no contact. Why? Could my installation of coil wire been a problem also?
New battery, starter spun engine nicely, but no "pop." Plenty of gas, no spark to plugs. Timed it, distributor shaft a bit loose (problem?) Car started and ran well some years ago (I have added top oil through plugs and turned engine with crank periodically). This time starter turned well, but then simply quit after few tries at starting engine, wouldn't contact. Cleaned contact again, no luck. Have disassembled it again, cleaned everything, ready for new ignition switch and attempt. Second time the charm?
I logically may have a series of electrical problems, each of which won't allow me to fire up my A. Any suggestions? Would be great to get it running again, before it hits -40F around here again. Thank you for any advice.
The coil wire itself is not your problem, unless its old and cracked. I would first replace your condenser and then break the points and check for spark. Also disconnect the ignition cable where it screws into the distributor, turn the ignition on and touch the end of the cable against the engine and see if you get a spark, if so this will eliminate an ignition wire problem. You could have a short between the lower distributor plate and the upper plate. Check for a frayed wire where it screws to the bottom of the upper plate. Also you could have a bad coil...if the car set for a long time it could have deteriorated. Sometimes when the condenser and or the end of the ignition cable is screwed into the lower plate area it gets pushed against the inside of the center of the distributor body and this will cause it to ground out. There maybe be a local model A chapter in your area and we can get someone to help you. - Lyle Meek, 1997 Technical Director
What were the original dimensions of the 1928-1931 Model A Ford battery?
Original Ford Battery Dimensions (at bottom):
Width: 7 1/8" to 7 1/4"
Length: 8 7/8" to 9"
Source: Restorer Mar/Apr 1992 Volume 36-6 page 13.
Enrique Klein, Los Altos, CA.
CONVERTING 6 VOLT TO 12 VOLT
What does it involve to change my 6V system to a 12V system?
You'll need an external resistor for your 6V coil, 12V battery, 12V alternator, 12 volt filament bulbs, 1 ohm resistor for your horn. Remember you are changing from 6V positive ground to 12V negative ground so be sure and switch the amp meter wires. Your starter motor will be wired for 6V and will work on 12V, BUT you run the risk of breaking your starter Bendix bolts. No problem just have a spare set....Years ago I did mine and went for 2 years before breaking one...
It would be best to convert the starter to 12V...You can do it yourself or have a local shop do it... Just remember it will take two Model A starter motors to make the change. The original motors have a right and left hand field connect in parallel. You must remove the fields from BOTH starter motors and reassemble with one motor having two left field and the other having the two right fields...both sets connected in series. Or just buy the 12V fields from the parts dealer. - Lyle Meek, 1997 Technical Director
On a recent trip to the Pacific Northwest I found myself with a defective cutout many miles from help. After some thought, I devised a wire (10 gauge) loop which I connected to both sides of the cutout and ran into the cockpit. I interrupted this loop with a switch mounted to the drivers side kick panel oriented so that if it inadvertently touched by my leg when dismounting, it would be turned off rather than on. Once the car is started, simply turn on the switch and you are "genning". Be vigilant in turning it off at each stop. A battery will go down pretty fast if this is left on. Maybe this is worth passing on.
Good information. I've carried a cutout jumper under my seat for years...Have since gone to the alternator for dependability...Not recommended for the purist. - Lyle Meek, 1997 Technical Director
I would like to know how to rewire a Model A Sparton Horn from 6 volt to 12 volt. What size wire? Number of turns?
To change the Model A horn from 6 volts to 12 volts, you must rewind the two field coils. Leave the armature as it is. First disassemble the horn to remove the brushes and the armature. Unsolder the two wires at the connector clip ( one from each field coil.) Note the direction of winding on the coils. The two coils are wound in opposite directions. The rule of thumb is that when you double the voltage, you use 1/2 the wire size and double the turns. So going from 6v to 12v use 24 gauge wire (original wire is 20 gauge, 1/2 that size is 23 gauge, but almost impossible to find 23 gauge so I have always used 24 gauge readily available at Radio Shack). Six volt coils have 45 turns and I have been using 100 turns with 24 gauge wire with great success. Be sure to use coated wire, normally used to wind RF radio coils or speaker coils. After winding the horn coils I usually brush a coat of light varnish to help insulate. The windings do not need to be real tight or in neat rows. In fact I found that if I wrapped just tight enough to form the wire around the core, and laid about 6 or 7 turns per layer, without being too careful how straight each wind was, I got better results on horn operation. In looking at a lot of original horns, some were manufactured with very precise and straight windings and some were wound very haphazardly.
I recently purchased a 29 coupe and I am having a electrical problem. First my horn and lights did not work....now I keep blowing fuses as soon as I put a new one in the fuse protector circuit on top of the starter....I am new to the model A and I cannot find any info on trouble shooting the electrical system.
Well you are going to have to become an electrical detective. There could be a number of places that you could have a short. Disconnect the wiring harness from the bottom of the steering column and try a fuse. If it blows check the wiring at the fire wall terminal box and behind your dash panel. If it doesn't blow, follow the wires from where the wiring harness connected at the bottom of the steering column. Follow each wire, checking for frayed edges that may be in contact with metal. The ferrules that cover the head light and horn wires are a source of shorts. Also check inside your headlight buckets. If you have a metal tube with wire in it running from the generator to the terminal box on the fire wall, check it carefully. Sorry I have no magic solution. Let me know how you make out. - Lyle Meek, 1997 Technical Director
I am about ready to install the wiring on my 1931 Victoria. Can you inform me of any reference material that depicts or describes how and where the wiring should be run along the frame, and where the wire clips should be located. That is; are clips attached along the upper or lower edge of the frame rail, over and/or under the frame, etc. From the light switch to the head lights and to the tail lights and stop light switch. I have the schematic layout, but I am unable to locate any drawings or photos of the physical locations. Your help will be greatly appreciated.
From the light switch bulb, the headlight loom is held in place with a loom clip that attaches to the bottom of the front most hood bracket (Hood latch rear most screw). This is identified in the Ford Service bulletins. From there the left headlight and horn loom goes under and behind the lower radiator to shell bracket and out the shell grommet holes. The right headlight loom runs along the bottom of the radiator and attaches to the radiator with three bend up tabs. These tabs were soldered to the top edge of the bottom radiator tank. Then route these wires under and behind the lower radiator to shell bracket. The wires for the rear tail lights go under and around the motor mounts and then routed along the bottom inside of the frame rail and fastened with a clip to the bottom frame rail. Route through the center cross member and use three wire loom clips to attach the harness along the inside of the bottom frame rail until it gets to the rear cross member. Goes over the rear cross member to the frame upper rear corner bracket where it is fastened with a clip then routed to the fender light.
We have a 1930 Sport Coupe and a 1931 Pickup. This spring, the Coupe developed a discharging problem when the ignition was off. We found it to be a hung cutout and replaced it. Unfortunately there must have been a short somewhere else in the system and we almost had a fire as the wiring harness "melted down" from the starter to the ignition switch and amp gauge. All wiring was replaced and a new coil was added as a precaution along with a fuse between the starter and the wiring harness. The car started up immediately. However, when taking it for a test run of about 3 miles the car just "shut down" and could not be restarted.
We thought the problem might be in the distributor and replaced all components as a precaution. (They hadn't been updated since the early '70's other than the usual condenser and points). The car was re timed and started again. It fired and ran briefly but just shut down once again. We thought there may have been heat damage to the ignition switch and replaced it. Still nothing. We tested the wiring with a tester and found we were getting power as far as the coil. We replaced the coil with several others and the same results: we were not getting power to the ignition beyond the coil. We thought the coil was wired in reverse but the car would have had no power when it did run. (I'm not outside to look at it, and for my own satisfaction, which terminal does the red wire attach to, positive or negative?)
We also thought the wire from the ignition attaching to the distributor may have been damaged so we exchanged it with the pickup's wire. Still nothing! Everything in the electrical system has been replaced. Is there a chance that three coils could be dead (two of them new)? Is there a test for the coil? I know that the car has been rewired properly because it was checked against the wiring of the pickup - but you never know. Could the new ignition switch be defective?
Sounds like you have a problem in the ignition circuit. Here are the checks to make:
- Yellow wire connected from post on starter switch to terminal box post (post on passenger side of terminal box).
- Ammeter (-) side connects to passenger side post on terminal box.
- Ammeter (+) side connects to driver side post on terminal box.
- Black wire connects from driver side post on terminal box to coil (-) terminal.
- Red wire connects from coil (+) terminal to ignition switch.
- Yellow/Black wire also connects from driver side terminal box post to the cut out terminal. That completes the ignition wiring except for the ignition cable and distributor plate.
- Here is a voltage check of the circuit:
- Connect the (+) side of your volt meter to a good ground point on the engine or frame.
- Touch the (-) probe to the passenger side terminal box wing nut. Read 6 volts.
- Touch the probe to the driver side terminal box wing nut. Read 6 volts.
- Touch the probe to the (-) terminal on coil. Read 6 volts.
- Touch the probe to the (+) terminal on coil. Read 6 volts.
- Place a piece of paper between the point contacts.
- NOW TURN IGNITION KEY ON.
- Touch the probe to the end of the points arm, read 6 volts.
- Remove paper between points. Open and close points and look for spark each time points open, (no spark means bad condenser, replace condenser).
- If points are sparking then disconnect the coil center (high tension wire) from the distributor cap (leave connected at distributor end). Place the free end of the coil wire about 1/8" from one of the engine head nuts. Crank the engine over with the ignition key on. There should be a bright blue arc from the coil wire to the nut (ground point). No arc means bad coil.
Having said all of that, there are several things to check for. New condensers can go bad and new coils can go bad. But I would first look for something that may be shorting the circuit or causing an open in the circuit. Some of the other things to check are: Bad Ammeter. If ammeter has open circuit, all voltage is lost to the coil. You probably burned the wiring up because of loose connections on the back of the ammeter, or missing rubber grommet through center of terminal box, causing the ignition cable to short against terminal box posts, causing large current draw to burn wires. Do not screw ignition cable into the distributor too far. It can short out if screwed in too far. Make sure all nuts on the terminal box posts are tight. Also the two rivets on the bottom distributor plate sometimes get loose (especially on the reproduction plate) and short out the bracket it attaches. - Les Andrews, 1998 Technical Director
Where can I find wiring diagrams for my Model A?
There are several books and publications that have wiring diagrams. The Model A Service Bulletins is always a good place to check. In addition, you can find them in many "How to Restore" books.
Back in the early 1990s, MAFCA member Jim Parcelluzzi created wiring diagrams for the MAFCA Clip Art disk. You can find copies of those diagrams here on our website:
- Rick Black, 09/05/14