Project Geronimo: Generator To Powermaster Alternator Conversion

Our project truck, Geronimo, is a first year, first-generation C/K series Chevy truck that we saved hours before being crushed. This project is a study in how to create a reliable daily driver from a vehicle that was discarded. Most of the work, so far, has been routine maintenance to ensure that all the fluids and mechanical items are roadworthy. Now, we can actually start making upgrades that will update the lovable old truck. We decided to convert our electrical charging system from the stock generator to a modern one-wire 100 amp PowerMaster Alternator (part #7294).

Powermaster's 12si family of alternators is not only a great high-output replacement for early GM 10dn and 10si alternators, but they work remarkably well in generator-to-alternator conversions. The one-wire alternator, identified by the single post in the rear of the unit, is the easiest to install and wire into an existing charging system.

Everywhere we take the old Apache, it gets smiles and waves from other drivers on the road. Even with the rusty and haggard exterior, the odd looking eyebrow truck makes people happy. The areas where we wanted to improve the performance of the truck center on reliability so we could keep getting happy response from our fans. The truck, like most work trucks of this period, came with a generator for maintaining the electrical current.

To understand why an alternator is preferred over a generator for daily driver dependability, we need to have some basic automotive electricity-101 knowledge. The common response most experts give when asked what the difference is between generators and alternators is, “generators produce voltage all the time when turning. Alternators produce voltage only when needed.” This is not necessarily correct and we’re going to explain why.

Many people still believe that alternators do not charge at idle.         – JR Richmond

Basic Electricity 101

Anytime you move a wire through a magnetic field, a small amount of electricity is generated in the wire. Anytime you have an electrical conductor moving through a magnetic field, there is voltage generated and current beginning to flow.

A generator creates this flow of electricity by moving a wire armature within a fixed magnetic field. The amount of electricity created is controlled by the speed at which the armature moves in the magnetic field or by changing the strength of the magnetic field.

A generator creates energy by rotating a conductor inside a stationary magnetic field. Alternators create their movable energy more efficiently  by rotating a magnetic field inside conductors.

An alternator has an arrangement of magnets that creates a magnetic field that spins inside a series of windings that surround the magnets. The winding element is called a stator, and the magnet component is the rotor. While the result of the alternator’s process is the same as a generator, it is much more efficient. Alternators usually have three separate windings that allow for a smoother electrical output, can be brought up to speed more quickly, and can develop good power at lower RPM.

“The older style one-wire alternators used to require revving up the engine to excite the alternator to start the charging,” said Powermaster’s J.R. Richmond. “Many people still believe that alternators do not charge at idle or that the engine has to be revved up to turn on the alternator. Both of these are not true with Powermaster products. These begin charging at much lower RPM than the older-style alternators,” he added.

Generator vs. Alternator

Gen: Windings spin in magnetic field

Alt: Magnetic field spins within windings


Gen: Narrow range of RPM (low RPM = low output)

Alt: Wide range of RPM


Gen: Magnetic strength has low variability

Alt: High variability


Gen: Needs to turn to work

Alt: Needs field current to work


Gen: Heavier in weight

Alt: Lighter weight


Gen: Simple design

Alt: More complicated design

Choosing An Alternator

We decided that an alternator was the most cost effective upgrade to restore the reliability in our daily driver project truck. We asked Richmond what should we consider when choosing an alternator to replace the OE generator. “The first thing anyone should do is figure out what electrical demands are in the system, then size the alternator to your system,” he advised. “For what we are dealing with here, a stock system with no add-ons, a 100-amp alternator will work well and live a long life.”

“If you have some added demands like a banging sound system, air conditioning, or other major components – especially electric cooling fans – you may need to step up to a 150-amp alternator,” he added. With no real additions to our system – not even a radio – we decided that Powermaster’s 12si-type 100-amp alternator (part #7294), with a single-groove pulley and a natural finish, would fit our needs.

This is a high-output alternator designed as a replacement for early 10DN or 10si alternators. It has the appearance and look of an original GM alternator with seriously upgraded internals. “Selecting the right size alternator is about getting a component that can do the job correctly and last a long time. It’s about efficiency,” said Richmond. “You can buy a larger output alternator, and it will cost more, but won’t necessarily do anymore than a 100-amp unit, if that is all your system needs. It is a supply and demand system. The alternator is only going to supply the energy that the system demands.”

To summarize, if your electrical demand at peek is 80-amps, and you have a 100-amp alternator, it will provide 80-amps to the battery. If you have a 150-amp alternator, it will still provide 80-amps to the alternator because that is all the system needs. In this case, the 100-amp alternator is cost effective and efficient for the application.

One-Wire Or Three-Wire?

The critical difference between a one-wire and a three wire alternator is the way the alternator is energized to begin working. The original equipment manufacture has designed the modern alternators (3-wire) to begin working when the ignition switch is turned on.

A one wire alternator energizes when the sensing circuit built in the internal voltage regulator senses rotation of the alternator’s rotor. The rotor spins and trips the circuit which starts the charging process. For PowerMaster alternators, the alternator will need to get between 1,200 and 1,400 rpms to turn on. With a typical 3:1 pulley ratio, this can be as low as 400 – 500 engine RPM.

Once the charging process has started, the alternator will charge at all speeds, even very low RPM, until the rotor comes to a complete stop. The circuit is shut off and the process will need to start again for charging to begin again.

Many of the Powermaster one-wire alternators can be used with the OE style plug if the removable cap is removed from the OE hook up slot. The one-wire charging wire is connected in the single pole sticking out of the back of the alternator. Notice the optional grounding strap threaded receptacle in the alternator’s rear case. This is not as optional as the name would imply. We were reminded to use this feature as an insurance policy to ensure that a good ground was achieved.

Outside of the energizing of the alternator, the other major difference between the one-wire and three-wire alternators is the amount of wires to connect the unit to the electrical system. Three-wire alternators use an OE style connector that is plugged into the back of the unit. A one-wire alternator uses a single wire that runs from the charge post on the back of the alternator to the positive pole on the storage battery or starter solenoid, whichever is closest to avoid voltage drop.

With none to little difference in performance, the choice to use a one-wire alternator rests in the ease of installation. If you are building the electrical system from ground up, the easiest installation is the one-wire alternator. In this case, a complete swap from generator to alternator means the one-wire alternator is a no-brainer. PowerMaster’s PowerGen unit is another simple option to solve the reliability issue and still use the existing brackets while retaining the original look. We opted to go with the Powermaster alternator. A three-wire alternator remains a popular choice as an OE stock replacement upgrade as well.

Sizing An Alternator

Consider total electrical demand:

CDI Ignition 6-36 amps

HEI Ignition 6-10 amps

Electric fuel pump  7-15 each

Electric water pump 12-25

Electric fans 10-50 each

Headlights/tail lights 10-20 amps

Trans Brake 12-20 amps

Nitrous Solenoid 5-30 amps each

Gauges 2-6 amps

Sound system 10-50 amps

intercooler pump 10-20 amps

line lock 4-8 amps

Select Charge Wire (4-7 feet max):

30-70 amps = 8 gauge

70-100 amps = 6 gauge

100-150 amps = 4 gauge

150-200 amps = 2 gauge


As we have already mentioned, and it does bear mentioning again, plan ahead before starting the installation. “The alternator is a major player in the electrical system,” said Richmond. “Make sure you have taken into consideration all of the upgrades you are planning to do in the next couple of years and size the alternator accordingly.”

It’s a smart move to inspect all of the original wiring in the area, and the wiring that you will be removing or reusing. In the case of the one-wire alternator, you will be adding a new charging wire to the battery or the starter/starter solenoid, but you should take a hard look at the wiring you are removing from the external voltage regulator and other wires to make sure it is in good shape. If it is dry rotted, has broken ends, or the insulation is bad, chances are the rest of the system is like that as well.

The generator that you will be removing is much longer than the alternator you will be installing. You will need a generator to alternator conversion mounting bracket. We chose to use one from Alan Grove Components Inc., for the 235 and 261 straight six Chevy engines. They have a great reputation for their products, and Richmond seconded that selection.

Richmond also noted that “Powermaster alternators are tested with a 3:1 pulley ratio in mind, because this is the street ratio that most OE manufacturers use in their applications. An underdrive pulley will slow down the rotor RPM and may keep the alternator from cutting on or producing enough power to satisfy electrical demands at idle.”

We used the Alan Grove Components, Inc. Alternator mounting bracket (part #224L) designed specifically for the 6-cylinder 235/261 Chevy engines from 1955 through 1962.

Keeping things simple, we decided to keep everything as stock as possible, using the same accessories and wiring, we would only need a 100 amp alternator to support the truck’s electrical needs. Because the alternator has an internal regulator, there is no need to keep the external regulator and wiring going to the regulator.

Many enthusiasts performing this conversion will leave the external voltage regulator in place, with the wires disconnected and taped off, but we decided to remove the unit completely. Future plans include cleaning and painting the engine bay and the voltage regulator is just one less thing to remove later.

Sometimes builders leave the voltage regulator in place with the wires unhooked and taped off with electrical tape in case they want revert back to a generator charging system in the future. Keeping in mind that our objective is to make our project truck a dependable daily driver, there was no chance that we were going to be needing the stock voltage regulator again. We gave it the old “heave ho” and taped up the wires.

Things To Keep In Mind

All-in-all, the conversion from generator to alternator is a simple one. Removal is a matter of unbolting the generator, removing the mount, and removing the voltage regulator. Installation is simply installing the new mount, installing the alternator, attaching a charging wire to the battery or starter/starter solenoid, from the alternator, and adding a secondary ground wire from the alternator to the block.

The Powermaster alternator bolted into place, leaving a lot more room than the original generator took up. The drive belt was checked to ensure the pulleys were in alignment and the belt was tightened “alternator tight.”

According to Richmond, there are a couple of common issues to watch for. “One key area is belt tightness,” he warned. “You don’t want it generator loose so that the belt slips, but you don’t want it so alternator tight that it kills bearings either.” According to Richmond, if the belt is too tight, it can transfer that pressure to the bearings which will wick away the grease and lead to bearing failure. So, tight enough to prevent slippage but not overly tight.

“A simple and foolproof way to check alternator belt tightness is to place a socket on the alternator pulley nut and turn clockwise with a ratchet. The belt isn’t tight enough if the pulley slips,” said Richmond. “When checking the belt tension this way, the pulley should try to turn the engine and not slip at all.”

It is important to use the correct size charging wire to get the full movable energy from the unit to the storage battery. We opted to use an 8-gauge charging wire from Powermaster.

Car owners that make this upgrade are generally proud owners that take care of their engines. Most likely they keep their engines painted and looking as good as they run. Richmond also warned us of the ground issue. “If you are relying on the mount as the only grounding source, you may not be doing your charging system any favors,” he said. “It would be best if a secondary grounding source from the alternator case to the engine block is added to ensure a good ground and complete circuit.”

Following JR Richmond’s instructions, we added a ground wire from the case to the engine. Unfortunately our local parts store only stocked red wire in 8-gauge. Much like the charging wire, size matters, so we opted to use the right gauge wire in the wrong color. This will ensure that we don’t cheat ourselves on amps.

Final Thoughts

Richmond reminded us that alternators are consumables. “If you have sized your alternator correctly, and installed it properly, you can expect 60,000 to 80,000 miles on a daily driver,” he stated. Correctly sizing the alternator to your system and planned upgrades, is the key to ensuring that your daily driver will have a long charging system life, and not stuck on the road in the middle of nowhere without power. Our installation went smoothly and our truck couldn’t be happier.

We were very pleased with the ease of installation and the electrical power the system provides. No more dim lights at night slowing down for a stop sign. This is a necessary safety upgrade for vintage cars and trucks that are intended to be daily drivers.

Article Sources

About the author

Bobby Kimbrough

Bobby grew up in the heart of Illinois, becoming an avid dirt track race fan which has developed into a life long passion. Taking a break from the Midwest dirt tracks to fight evil doers in the world, he completed a full 21 year career in the Marine Corps.
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