One of the best ways to formulate technical stories is by paying attention to what enthusiasts are working on in their garages. Chances are, if they are dealing with a particular problem, they are not the only ones. Every so often, we receive emails from enthusiasts about a particular issue, and to the best of our ability, we try to help find a solution. We received this question from Dave in California, in response to a story about one-wire alternators we featured a while back. Dave’s question revolves around using an amp gauge in his Corvette, but the answer applies to most Chevrolet products, as well as many other brands of cars, trucks, and even heavy machinery.

Question: I am upgrading my C2 Corvette’s charging system. We installed a one-wire alternator, however the amp gauge no longer works. I have not messed with my amp gauge yet because I am not sure just what to do.
Dale
Sacramento, CA

Answer: There are several reasons why there is no simple answer to your question about using an ammeter (amp gauge) with a one-wire alternator. Automotive wiring systems have historically been one aspect of our cars that perplex many enthusiasts. When you add in the complexities of converting the alternator’s alternating current (AC) into direct current (DC) for the battery, things get kicked up another notch.

Another reason is due to the advancements that have been made throughout the years as the next generation of vehicles came on the scene. There’s no doubt that modern cars have much more complex wiring systems than our earlier muscle cars. On the one hand, advancements such as internally regulated alternators — which permit a one-wire charging system — can benefit our earlier cars, but they can also contribute to some new issues as well.

Amp Gauge Or Idiot Light

As its name implies, one of the benefits of a one-wire alternator is that the charging system only needs a single wire running directly to the battery. This is due to having an internal voltage regulator to adjust the one-wire alternator’s level of charging. Earlier GM charging systems like your midyear Corvette, had an additional two-prong connector on the back of the alternator which included an exciter wire from an external voltage regulator to tell the alternator to start charging, how much charge the system needed, and another terminal to feed the amp gauge or idiot light in the dash. These additional wires were no longer needed with a one-wire alternator. In your case, the ammeter in your dash used these wires to operate.

An amp gauge measures the magnitude and direction of current flow at any given time. To do that, it compares current flow to what is known as a “shunt wire.” This shunt wire has a known resistance, which is used in parallel to the gauge to calibrate the ammeter’s movement between Charge and Discharge. The ammeter senses the small difference in flow and direction of amps through the gauge to illustrate how much current is being produced, or lost. The idiot light in the dash works similarly but is a lot less accurate.

By changing the output of the alternator, you have rendered the OEM ammeter irrelevant, since it would no longer be accurate. Also, for the very same reason, it is not recommended to direct the increased amperage through under-dash wiring that was never designed to carry such loads. That is why many enthusiasts switch over to a volt meter in place of the factory ammeter in their vintage cars and trucks. And, that is why many are hesitant to recommend using an ammeter when upgrading to a one-wire alternator with more amperage output.

But, if you have either a “GEN,” “ALT,” or “BAT” idiot light in your dash, you can wire them to your one-wire alternator so you will have some indication if your charging system fails. We asked Tuff Stuff Performance’s Matt Oliver about the best way to wire up a one-wire alternator for a warning light in the dash, so you can be sure your charging system is working properly.

“Our 10si-series alternators for GM (Part Numbers 7127, 7139, and 7140) and Ford (Part Numbers 7068 and 7069) are the only ones that have the two-spade connectors in the back of the alternator to connect a warning light,” Mike says. “The connectors are located under a rubber cover in the back of the alternator.”

These two terminals can be found under the rubber plug on the backside of the one-wire alternator. Notice the terminal on the left is the “1” or “R” terminal.

Looking at the rear of the alternator, the lead to the dash light will hook up to the terminal on the left, which is usually listed as “1” or “R” on the housing of the alternator. The terminal on the right (as viewed from the rear of the alternator) is the field wire. This wire is used as a voltage-sensing wire on three-wire charging systems but is not used on a one-wire-style charging system. Mike insists, “We let everyone know not to hook anything up to the terminal on the right because it will damage the internal regulator.”

We mentioned that the indicator light in the dash works similar to an ammeter in that it still shows if there is current flow through the circuit. If there is a current flowing, the bulb will be illuminated. If there is no flow, the bulb will not be illuminated. Here’s how it works.

An ammeter is pretty simple. There are two posts on the backside to which wires attach. The gauge simply illustrates the fluctuations and direction of the current through the wires.

The circuit to the charge indicator light must be switched. Then, when you turn the key on, the indicator bulb receives 12 volts from the ignition switch. The “1” terminal on the back of the alternator is grounded, which provides a current flow to illuminate the lamp. Once the engine is up and running, the alternator provides 12 volts to the “1” terminal. Now, because the voltage is equalized between the ignition switch and the ”1” terminal, there is no current flow and the bulb is not illuminated. But, if the charging system fails or the engine stops, the ground is once again restored and the light illuminates again, indicating an issue.

Bringing A One-Wire Alternator’s Charging Up To Speed

Converting to a one-wire alternator typically brings an increase in available charging amperage, which is why it is recommended to increase the wire size of the charging system. One concession to a one-wire, internally regulated alternator is that they need to get up to speed to trigger the regulator to start charging.

Most one-wire alternators start charging when the engine speed reaches around 900 rpm. As a rule of thumb, alternators only put out 50 percent of their maximum amperage when at idle. That is why a higher-amperage Tuff Stuff alternator can provide more amperage at low-RPM operation. But of course, a car’s electrical system will only pull as many amps as it needs for proper operation. Besides having increased amperage at idle, one of the selling features of a Tuff Stuff one-wire alternator is the ability to start charging at the lowest RPM possible on the market today.

Connecting 12 volts from the indicator light circuit to the “1” terminal triggers the alternator to begin charging almost immediately once the engine is running. The light bulb also acts as a resistor to limit the amperage going into the alternator through the “1” terminal. In the case of Tuff Stuff’s one-wire alternators, the “1” terminal is already resisted internally, so this is not a concern. That means you can also do this by running 12-volt power to the “1” terminal. But there are a few things to keep in mind if you use this method.

Many auto parts stores will sell a jumper wire that would connect the “BAT” stud on the back of the alternator to the “1” terminal. The issue is that when the engine stops and the “1” terminal gets switched to ground — remember, that’s what turns the idiot light on in the dash — it will make a direct short to the battery. This will not only drain your battery but could burn out the internal voltage regulator in the alternator.

Tuff Stuff’s one-wire alternators are available in a variety of applications and can deliver up to 225 amps. They are also internally resisted on the “1” terminal so you can either use an indicator light or run a wire directly from a switched power source to trigger near-instant charging.

You can get power from the ignition switch’s “Accessory” circuit, so when you shut off the key, the wire from the “1” terminal is separated from the wiring that supplies the engine’s ignition. Since the alternator will be spinning for a brief moment after turning off the switch, any power still being generated is prevented from flowing up through the “1” terminal to the ignition coil, which would result in the engine not shutting off.

Upgrading our cars and trucks with newer technology is one of the best ways to improve the way they drive and increase the enjoyment we get from them. Understanding how to implement that technology is as important as the engineering that went into creating it. Thankfully, there are aftermarket companies, such as Tuff Stuff Performance, that help put all that technology at our fingertips. And, their knowledgeable staff is there to help bring it all together under the hood of our cars and trucks.