Mustang MonthlyHow To Tech Qa
One Wire Alternators and Vintage Mustangs
Beyond the Basics
I have a 1965 Mustang fastback with a 289, Edelbrock Performer intake, a Holley four-barrel carb, and a mild cam. I want to run a one-wire alternator, but my friend says they don’t charge at low rpm. Should I stick with an original or go with the high-output one-wire?
Ron (last name withheld)
Tallahassee, Florida Your friend is basing his opinion on old information. I have installed original-style alternators, internally regulated with remote sensing, and the one-wire type, and I have not had any of the problems that have been reported by others. However, I do have a preference, and I need to explain the differences in the alternators to explain that preference.
The one-wire alternator is completely self-contained, which means that it starts itself up when it senses the alternator spinning, and monitors the voltage output through its own internal circuits. Your original alternator was started by the power lead coming from the ignition switch to the external voltage regulator. Ford started using internally regulated alternators during 1986, but they still were turned on through the key switch.
Some of the early one-wire systems had problems with low-rpm power output, and some of the information out there is outdated. The self-exciting alternators start to work when the rpm of the engine gets up around 1,200rpm, then the alternator will stay on until the engine has stopped turning. Once it is on, the alternator will generate voltage, even less than 1,200rpm.
One of the misnomers about these units charging low is when the pulley is swapped for an aftermarket performance pulley. Most alternators rotate between 2.5:1 and 3:1 ratios, and when you put a bigger pulley on the alternator, it will turn slower and the output is affected. This is a problem with all alternators, not just the one-wire versions. A one-wire will charge as well as a standard alternator at low rpm.
One thing all manufacturers still have is an idiot light on the dash. Most one-wire alternators are an upgrade of the GM/Delco style unit, and you can get a conversion harness to run a charge light on the dash. With most of my performance builds, we use a voltmeter rather than an ammeter, so the voltage loss can be seen, but there is still a good reason to have the light to warn you if something happens. Most aftermarket one-wire setups will have a provision for a light, but you can’t use it with an ammeter setup.
The one thing I like about most three-wire setups is remote voltage sensing. One-wire alternators sense the voltage right at the point where it is being generated—at the alternator. The farther the voltage runs through wiring, the more voltage drop you can have. Think back to your college dorm where you swore they had the thermostat down in the boiler room—and you are on the 4th floor. Not much heat was getting to your room. Well it is the same thing for electricity. By sensing the voltage away from the alternator, you can get a better idea of how much of a voltage drop is occurring, and you can adjust the output of the alternator. Ford still senses the voltage to their internal regulated alternator through a remote line. The result of not doing this can be dim lights, weak ignition, and electric fans that don’t run at full speed. This is why Ford mounted your voltage regulator on the driver’s side of the engine compartment and not near the alternator.
So, I prefer the remote voltage sensing in a three-wire or original-style alternator, even though I have used the one-wire setup with no problems. They are good pieces. A few notes for installing alternators:
1. Don’t rely on the mounting bolt on a one-wire setup to provide your ground. Most aftermarket cases have a point on the back where you can run a ground wire, and I recommend you run a ground wire to the engine block. Chrome and powdercoating can create resistance to current flow.
2. With a remote battery location (like in the trunk), definitely consider an alternator that has remote sensing. A lot of voltage drop occurs through that long positive battery cable. Some one-wire alternators can work like a three-wire if you wish.
3. All the manufacturers still use a light on the dash to notify you of a charging system problem. Consider adding one. Most one-wire units can run an idiot light. Don’t rely on the voltmeter.
4. Voltage drop occurs with corrosion and poor connections. When upgrading, go through your charging system and clean up the contacts. Poor grounds are the biggest reason for alternator performance issues.