1942 Allis Chalmers Model B
12V Alternator Conversion
I decided I had enough of messing around with my original 6v generator, the mechanical cutout relay and that awful original 3 position charge/light switch. many times I would use that switch to fast charge after a start but forget to reset it back to normal position. When using the tractor for several hours mowing, the battery would overcharge. I had tried replacing the switch and cutout relay with a regular mechanical voltage regulator but could never really get the regulator adjusted properly to maintain a consistent charge that tapered to minimal automatically. My 6v battery finally got a dead cell and I did give some thought to replacing it with another 6v one. However, my B does not have the compression it really should have and crank starting (if necessary) is not that easy anymore. The tractor runs very well after starting so I am not that interested in tearing into the engine just to improve the compression. I decided to just convert to a 12v, negative ground alternator system with internal voltage regulator and be done with it. It's been about two years now and I am very pleased with the result. It has turned out to be extremely reliable. I should have done this years ago.
Here's what the installation looks like:
And here's a close up view:
I modified the original lower generator mount to permit moving it towards the tractor rear some. That was necessary to locate the alternator in line with the fan belt. There is a length of 3/8" IPS pipe between the bracket mounting ears to prevent the ears from bending when the alternator mounting through bolt is tightened. The stack of washers are used as a spacer to precisely line up the alternator pulley with the fan belt. The pulley is reused from the old generator. The original generator top mounting bracket is not used. I modified an old automotive alternator bracket to serve as the top adjusting bracket.
I used a Delco 10SI alternator. It's a 63amp, 3 wire unit with internal, electronic voltage regulator. The NAPA part number for this rebuilt unit is 213-4011. The plug with pigtail leads that is used to connect to the regulator spade terminals is NAPA Echlin part number VRC148.
I chose a 3 wire rather than a 1 wire version of this alternator because with the three wire unit I didn't have to get a special smaller diameter pulley in order to permit the alternator to charge at idle speed. The 1 wire units are not only more expensive but generally require a higher speed to get them energized enough to begin charging. I think the 3 wire ones are a much better choice and the wiring is not really very complicated.
For the wiring, the main alternator stud should be connected directly to the battery +post (unless you are using an ammeter). I changed out my ammeter to a voltmeter because I think a voltmeter is much more useful in this application. There are two control terminals on the alternator. They are spade terminals that the above noted plug with pigtail leads connects to. One terminal is marked #1. That is the terminal that needs to have battery positive voltage applied to it, via a 3 amp fuse, in order to energize the alternator and begin charging. The other terminal is marked #2. This is a remote voltage sensing terminal that, for a tractor operation, can be connected directly to the alternator main output stud.
I used a double pole, double throw, two position toggle switch for on/off control. One pole connects or disconnects the magneto from ground to start and stop the engine. The other pole connects or disconnects battery voltage to the energizing terminal of the alternator. The same pole also energizes the voltmeter function. When the toggle is in the "ON" position, the magneto ground stud is disconnected from ground, the alternator energizing terminal #1 is connected to battery +12volts and the voltmeter is energized. This allows the tractor to start, the alternator to begin charging as soon as the engine rpm gets up to idle speed and the voltmeter to register. When the toggle is in the "OFF" position, the magneto is grounded and the engine is stopped. Also in the "OFF" position, the alternator energizing voltage and the voltmeter are disconnected.
Here's a diagram of the switch connections:
And here's a view of my instrument panel with the toggle switch and voltmeter:
The toggle switch on the left is for on/off control of engine, alternator and voltmeter. The toggle switch in the center is for on/off control of headlights. The push button on the right is for push-to-start operation of the starter via a starter solenoid. I selected spade terminal type connections for both toggle switches as that makes it easier to connect up a bunch of wires inside the relatively small instrument panel.
With this installation, the battery needs to be connected such that the negative terminal is grounded instead of the original positive ground. The starter doesn't know or care about polarity and will work just fine albeit at a higher speed. Instant starting, even though the compression is not what it should be. Any lighting will also have to be changed out to work on 12 volts instead of six. I had to replace the existing generator fan belt (was NAPA 5L370W) with one an inch longer to accommodate the alternator installation. A NAPA 5L380W worked fine.
Please note that this installation is for a magneto ignition system. A distributor (battery) ignition system will have to be a little different to avoid engine run-on or battery drain.