How Do I Do This?
Updating the Charging System
of Your Older Tractor
by Tyler Woods

Older tractors have externally regulated generators or externally regulated alternators, in some instances. That means there is one device that turns energy from the fan belt into electrical energy,

Generator

Regulator

and another device, called a regulator, that keeps the voltage from overcharging the battery. (yellow box in this picture)

This is a very simple description but it will work for our needs. Modern alternators offer improvement to an older tractor's charging system and cost far less. This article addresses the electrical adaptation of a modern alternator to an older tractor. I reference the Delco alternator used in most GM late 70’s cars and trucks with a 350 cu. in. V8 engine. (Type #GM 10 SI, Model 7127, Cost is roughly $35), but nearly all internally regulated alternator’s have the same output wiring. I used a Datsun alternator for my Massey Ferguson because the diameter was the same as the original generator.

Before you begin you need to make sure your system is a 12v, negative ground charging system. If your battery is 12v and the negative (-) post is connected to the chassis or engine block, your system is 12v negative ground. If your system is something else, you must make this conversion before continuing with an alternator upgrade.

First, you need to remove the previous charging system (generator or alternator, including the associated voltage regulator.) You may wish to save these parts for a future classic restoration. A wiring diagram is VERY helpful here. Many manufacturers use the regulator terminals for convenient power distribution to other ignition parts (like the ignition switch). Note wires from instruments or parts other than the generator/alternator and be ready to replace their function either by routing them differently or by adding jumpers. My tractor needed only one jumper wire to supply power to the ignition switch after removing the regulator, generator and associated wiring. Take care not to leave dangling wires on your tractor. Remove unused wires completely.

Mechanical mounting is going to be a case by case issue. I replaced Massey Ferguson generator, with a Datsun alternator, and only needed to use a longer bolt for the mounting lugs and some ¼" galvanized pipe, cut for a spacer.

New Alternator


The top mounting lug fit the existing generator tension bar and I didn’t have to buy a smaller V belt. Other applications may need custom brackets.

Modern internally regulated alternators typically have three external electrical connections.

  1. The main regulated power lug.
  2. The exciter field tap
  3. The "sense" line.
(Delco alternator is shown)

  1. The main regulated power lug is typically wired to the ammeter. Connect to the plus side (+) which will be the terminal NOT connected to the positive post of the battery. If you don’t have an ammeter, connect the power lug to the starter, where the big wire from the battery connects. You can also connect to the plus side of the battery but it’s usually easier at the starter.

  2. The exciter field tap is the wire from the modular plug on the Delco alternator (marked #1). This wire lets you shut down the alternator when the motor is turned off. It is usually attached to the ignition switch where the wire goes to the hot side of the coil. You can also make this connection to the hot side of the coil if your coil has an internal resistor, or to the hot side of the resistor for external resistor configurations.

    NOTE: some alternators back-feed power through the alternator and out the exciter wire. If your tractor does not shut off after you turn the key off, this is probably the cause and you will have to either install a diode in that line or find another spot to attach it. Some diesel configurations connect the exciter wire through a diode to the oil-pressure sending unit.

  3. The "sense" line is a very useful feature. It’s the #2 wire on the Delco modular connector. Let me start by explaining DC power line loss. Wire has resistance measured in Ohms. This resistance increases with wire length and is decreased with wire gauge (thickness). Line loss, in volts, is the product of the current, measured in amps, and line resistance. Since the current output of an alternator fluctuates with RPM, the voltage loss over the line will also fluctuate. To put it simply, as a wire becomes longer and skinnier, line loss (voltage loss) will become an increasing problem. The sense line samples the output of the alternator and feeds this back to the regulator to make adjustments. Since the sense line carries very little current, line resistance does not affect it significantly. When the sense line is attached to the output lug of the alternator as shown (notice the red loop of wire)

regulation is only controlled at the alternator output post. When the sense line is attached at the battery or starter, the regulator will compensate for line loss and regulation will be controlled for the destination. You may have seen work lights that brighten as the engine RPM increases. This can be controlled with proper use of the sense line. I attached my sense line to the output post of my alternator anyway. I figure the lines are short on a tractor and I may not have enough line resistance to notice any fluctuation in my lights. If I do find my lights brighten with RPM, I’ll rewire my sense line to either the positive battery post or the starter solenoid.


Sense lines are very useful when you have a dedicated alternator for charging batteries located some distance from the alternator. Running the sense line to the battery end of a charge line allows the internal regulator to compensate for line loss and offer a full charge to something like RV batteries at the rear of a trailer.

NOTE: If you have some other internally regulated alternator than the Delco and aren't sure which lead is the sense lead and which is the field (or exciter), you can watch closely as you touch each to power. The sense lead will never draw enough current to cause a spark but the field will.