Alternator-Upgrade Wiring Tips for Popular GM Charging Systems

A popular form of performance upgrade revolves around applying current technology to older machines. While everybody else focuses on four-digit-horsepower LS engines with giant turbos or superchargers, let’s keep our goals far more street-worthy and practical.

Among the most overlooked aspects of swapping late-model engines into early Chevys is upgrading the charging system. It was in the early ’60s when alternators replaced generators. Since then, a landslide of charging system and alternator enhancements have followed. We decided we needed to focus our attention on some of the more popular alternator conversions and wiring harness modifications necessary to accommodate them. We wanted to get input from a few professionals in the world of automotive charging, so we reached out to Tuff Stuff Performance and Painless Performance.

On the back of GM alternators, you will find a two- or four-wire plug along with a large output stud. The large stud is for the output wire connection that is connected to the battery positive post.

Variations On Charging

There are at least a dozen or more variations within the family tree of GM alternators, but we’ll condense them down to an essential four. The best way to upgrade the charging system on a ’60s or ’70s Chevy is to step-up to the latest model versions like the CS130D. Even a stock replacement CS130D will offer more power at low speeds than previous models. That’s just one idea. Other alternatives that also work well.

Before we get into the swap details, it’s beneficial to investigate charging-output numbers. In nearly all cases, alternators are rated by maximum potential-amperage output. This is not amperage delivered while at idle! Based on multiple factors — like alternator design and pulley ratios — an alternator’s output at idle can be far less than its maximum rating. The original 10-DN externally regulated alternator is probably not capable of much more than 35 amps at idle. Back in the days of AM radios, that was enough to maintain system voltage.

We spoke with Mike Stasko, marketing manager for Tuff Stuff Performance, and he has a recommendation. “Once you determine the amp requirements of your vehicle, check to see if there is a higher amp alternator in the same alternator series. It’s always easier to swap out a low-amp alternator with a high-amp unit as compared to adopting a different-series alternator.”

Not All Alternators Are Created Equal

Late-model alternators are far more efficient at idle, so a stock 100-amp alternator might be capable of 60 to 65 amps at idle. But let’s look a little closer. Alternator rating numbers are generally tested with the alternator at ambient temperature. Unfortunately, with the charging system at normal operating temperature, internal resistance increases with heat, and the output drops, typically by 15 to 20 percent.

If you have an alternator rated at 100 amp at idle, its normal operating temperature is probably capable of only around 75 to 80 amps. That’s something to think about if your twin electric fans and other electrical devices combine to pull more than 70 amps. The net result is a loss of system voltage at idle.

Alternator upgrade

These are the four most common alternator connectors. If you are not sure which alternator you have, the connector shape is a good indicator of alternator configuration. From left-to-right: 10DN, 10/12SI, CS130/CS144, and CS-130D.

“If changing to a different alternator series, make sure the belt(s) line up and the wiring is in good condition,” Mike states. “An alternator with a higher amp output than stock requires a heavier charge wire (the wire that connects the alternator to the battery) because of the increased amperage.”

To evaluate your charging system, try this simple experiment. With the engine idling at operating temperature, turn on all the electrical components such as the headlights, blower motor at full speed, four-way flashers, electric fans, and the stereo at a reasonable level. Then, note the electrical system operating voltage. If the voltmeter reads below 13 volts, none of the electrical devices — including the cooling fans — are running at peak efficiency. They need a minimum of 13.5 volts.

All About The Connections

Assuming you want to upgrade, we’ll take a couple of the more common options and run through the wiring variables. The original 1960’s GM alternator employs an external voltage regulator. This alternator (10-DN), uses a flat, two-prong connection at the back of the alternator. The other main connection on the alternator is the output terminal that charges the battery.

The least expensive upgrade from the 10-DN would be to step up to a 10-SI or 12-SI. The main advantage of either unit is they employ an internal voltage regulator (SI stands for system integrated). But this is not a simple bolt-on conversion. The 10- and 12-SI units use a different two-wire connector plug on the rear of the alternator. The Number 1 wire on the 10- or 12-SI is connected to the charge warning light on the dash. The Number 2 wire is what is called the voltage sensing wire.

When converting from an external voltage regulator to an internal such as the 12-SI, many enthusiasts merely connect the Number 2 voltage sensing wire directly to the output terminal. While this shortcut is simple and functional, it will not optimize the charging system. The voltage-sensing wire is best connected closer to the battery.

alternator upgrade

High-output alternators (greater than 100 amp) need a minimum of an 8-or-larger gauge charge wire to reduce resistance. The bottom wire is AWG 12, the middle is AWG 10, and the largest is AWG 6. The largest is always better (although clunky in appearance) for minimal resistance for alternators putting out more than 100 amps.

Here’s why using a remote voltage-sensing connection is a wise move. The main charge wire on the back of the alternator is eventually tied into the positive post on the battery. However, this connection is often a long wire. This cable length creates resistance that can be easily measured with a simple charging system efficiency test.

With the engine at idle — and several components like headlights, electric cooling fans, and perhaps the heater fan, operating, compare the voltage readings at the alternator to those at the battery. There will generally be a slight voltage drop at the battery of around 0.50 to 0.60 volt. By locating the voltage-sensing wire closer to the battery, the alternator can compensate for this slight drop in voltage and maintain the overall electrical system at around 14 volts. With the voltage-sensing wire connected to the output terminal, this half-volt drop is not measured and the entire charging system under-performs.

Singled Out

This is also a good place to mention one-wire alternators. These aftermarket alternators eliminate the warning light and voltage-sensing wire connections all OE alternators use. Voltage sensing is accomplished internally, which (as we just covered) is one reason why one-wire alternators are not as efficient as a remote-sensing alternator.

Another minor disadvantage to one-wire alternators is the rotor in the alternator must achieve a certain speed to self-excite. This usually requires the driver to rev the engine to increase internal voltage to sufficiently excite the alternator to begin charging. This isn’t a huge issue. But, you need to be aware of this and rev the engine after it starts to ensure the charging system is functioning. Remote-sensing alternators are capable of charging the moment the engine starts.

alternator upgrade

This is a Tuff Stuff illustration of how to convert a typical mid-’60s 10DN alternator to a much more robust CS-130. Tuff Stuff also sells a plug-in harness that will accomplish this based on whether the car has a warning light or not.

Among the available charging-system alternatives, you can choose to merely upgrade to a higher output alternator within the same design as your existing alternator, or update with a later model unit with more output. The simplest would be to upgrade your current alternator. For example, Tuff Stuff Performance offers a higher output 10-DN option. Retaining a 10-DN with the separate voltage regulator might be a good idea for those who want to retain the original appearance — for restoration purposes. If that’s not important, it’s usually better to increase output and efficiency by stepping up to a newer model alternator like a CS-130 or larger CS-144.

Some Resistance Required

Let’s go through an example of upgrading a ’67 Chevelle with a 383ci small-block that has been converted to a CS-130 alternator. The car retains the original factory 10-DN external-regulator wiring. M&H Wire Fabricators can build a plug-in-replacement forward-lamp harness that integrates with the new alternator by simply plugging it in. This is the cleanest way to upgrade. As a less expensive alternative, Painless Wiring offers a replacement CS-130 pigtail connector that can easily be spliced into place.

alternator upgrade

A common charging system issue with older cars is excessive resistance between the alternator and the battery. A quick test with the engine idling is to turn on several electrical accessories like headlights and the heater fan. Then, test voltage at the rear of the alternator and compare that to the voltage at the battery. If the voltage at the battery is within 0.5-volt of the alternator voltage, then the charge wire is okay. If it drops more than 0.06-volt — like here at 13.97 volts — the charge wire is too small, or there is resistance at the connections.

All late-model alternators employ an electronic voltage regulator. If your car is like this Chevelle and has a voltmeter or factory ammeter gauge without a charging system warning light, a resistor must be wired into the warning light circuit. Essentially, the resistor takes the place of the load created by the warning light.

We were curious as to why this resistor is important, and according to Painless engineer Eric Cowden, “The resistor limits the amperage the exciter wire can supply. In factory applications, either a charge-indicator light or ECM provides this 1 amp or less, switched 12-volt source. Without this resistance, too much amperage reaches the regulator and causes it to burn up.”

This Painless Performance illustration shows how the CS-130 and CS-144 alternators should be wired. The “S” connection on the alternator (left image) is the larger pin on the far left while the “L” is second from the right. Note the drawing shows the external shape of the connector as rounded on one end while the internal shape is rectangular.

Cowden added this further explanation, “if you take watts divided by volts, you get amps. So, with this resistor, under normal charging circumstances, you are never putting more than 0.5 amp to the alternator.” Here’s the math that backs that up:

5 watts / 14 volts = 0.35 amps

Of course, simply wiring the Painless connector to a warning light accomplishes the same thing as the resistor, so one or the other is all that is needed. This is true for all later model alternators with internal voltage regulators and is why Painless includes the resistor in every pigtail conversion.

We make this point because it is possible to purchase a replacement alternator pigtail from almost any auto parts store. These are often less expensive, but not packaged with a resistor. If you are using a standard pigtail (perhaps pulled from a junkyard vehicle), you need to know which wire is the voltage-sensing wire and which one is the exciter wire that needs either a charge-indicator light or a resistor.

alternator upgrade

We’ve used these two-post terminal blocks, available from Jegs, for mounting both live battery power and switched power. This makes an excellent spot for the voltage-sensing wire connection when this terminal block is mounted near the battery.

To keep this story brief, we have not taken a deep dive into physically mounting these different alternators to various engines, as this can get somewhat complicated. To simplify this as much as possible, the newer CS130 and CS144 alternators are a great choice for Gen I small- and big-block Chevys, while the CS-130D is commonly used on factory accessory drives for LS engines.

With this review of the wiring harness differences, updating your charging system should not be very intimidating. It’s simple once you understand how the system works.

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About the author

Jeff Smith

Jeff Smith, a 35-year veteran of automotive journalism, comes to Power Automedia after serving as the senior technical editor at Car Craft magazine. An Iowa native, Smith served a variety of roles at Car Craft before moving to the senior editor role at Hot Rod and Chevy High Performance, and ultimately returning to Car Craft. An accomplished engine builder and technical expert, he will focus on the tech-heavy content that is the foundation of EngineLabs.
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