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How to convert a Delco 10SI alternator to

"One Wire"


One of the most frequently modifications that are done to a Willys Jeep is a 12-volt conversion. While a 12-volt generator is an option, most people opt for an alternator and by far the most common is the "One Wire". However, many times the application is considered unacceptable because the alternator doesn't begin to charge until the engine reaches about 3,000 rpm. While this may not be considered a problem in a high revving short stroke engine, on the L134 and L226 3,000 is about the upper limit of where these want to operate. This 'problem' will be addressed later in the page.

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What to use

The one wire alternator has an internal regulator and thus only one wire is required to go to the vehicle electrical system. While other types are possible to use, by far the most common is a Delco model SI. These are found on early GM products and therefore are easily available at most salvage yards or auto parts stores. Be sure to get a model SI and not a CS. The CS is riveted together and considered to be 'disposable'. While the SI is bolted together and easily rebuilt by the average person.

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Why it requires 3,000 RPM to kick-in - How the alternator is designed to operate

The Delco alternator is designed to operate using more than one wire going to the vehicle electrical system. In fact there were originally at least 3 wires. One BAT, one field and one sensing. Actually the field wire goes to the "idiot light" and then to the 'RUN' position of the ignition switch. The sensing wire either goes to the 'BAT' side of the ignition switch or the 'RUN' side depending on the application. When the ignition is in the run position the sensing terminal is energized with battery voltage and the field terminal is energized with a current limited voltage through the lamp. When the alternator starts charging, the field voltage raises and the voltage differential across the lamp reduces and the lamp goes out. The charging voltage is 'sensed' through the sensing wire and the internal voltage regulator regulates the voltage to about 14 volts.

When used in the one wire configuration, the sensing wire is connected directly to the BAT terminal. The field terminal is not connected. If the engine is run at a high enough rpm the regulator will "leak" enough voltage to the field to energize it and the alternator starts. The trick is to get the regulator to "leak" at a lower rpm, and thereby "self energize". Replacing the regulator with one that "leaks" is covered in the next section.

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How to adapt Delco model SI to "One Wire".

Actually, adaptation is quite simple. Just change the regulator to a self energizing one, connect the sensing terminal of the regulator to the BAT terminal and you are done. The trick is finding a self energizing regulator. The one I use is manufactured by TRANSPRO and is catalog number D10SE12. D10-Delco model 10SI // SE-Self Energizing // 12-12 volt.

Because not everyone tears alternators apart, I thought I would guide you through the process with pictures.


Lets first get to know the alternator connections. Ground is not usually used. The alternator pivot bolt usually provides ground. The test hole is used to bypass the regulator for testing. See below for procedure.

These are the alternator connections to the outside world. Note: The case has BAT - 1 -2 cast into it.

Start by removing the alternator and marking the front and rear cases for reassembly.

Remove the (4) bolts that hold the front and rear together. Then separate the case. Be sure the stator stays with the rear case. Don't loose the brush springs.

Once the case is separated, remove the stator by disconnecting the stator connections and prying the stator from the rear case

Now, before we go much further, let's familiarize ourself with what is in here.

Find the (3) brush holder screws. Remove the (3) screws. Notice (2) are insulated. They MUST be reinstalled where they were. Otherwise the regulator will be damaged.

Next, remove the resistor (if present), diode trio, brush holder and regulator. The resistor may not be present in all applications.

Well here are all the parts, laying on the table. Now what??

Here is the new regulator. Doesn't look different than the original. The difference is inside.

Now for assembly. Install the new regulator and brush holder with an insulated screw. Don't tighten it yet.

Next install the Diode trio and resistor (if present) with the uninsulated screw and one insulated screw. Snug the screws and then tighten.

Install the brush springs and brushes into the brush holder. Hold the brushes back in the holder with a small drill or paper clip.

Now install the stator and reconnect the stator connections.

Next, assemble the front and rear cases. Match the marks previously made. Notice the drill holding the brushes back during assembly.

This is the original Delco regulator connection. Cut off the field (#1) wire and connect the sensing (#2) wire to the BAT terminal.

Another method is to connect a jumper wire from the sensing terminal (#2) to the BAT terminal.

This is the schematic diagram for a Delco SI alternator

click on image

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How to wire to your vehicle

All that is "required" is to run a heavy, at least 10 gauge wire from the BAT terminal to the positive (+) post of your battery. I would suggest you connect it through the AMP meter. Depending on the information you want, there are a couple of ways to wire the meter. The first, and preferred, is to measure the charge/discharge of the battery. The other is to measure alternator output. The alternator output also includes the current draw of the lights etc. of the vehicle. Alternator output is best measured by voltage not current. The charge/discharge method tells whether the battery is charging or discharging.

Below is a schematic showing the preferred wiring method.

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How to test an alternator and bypass the regulator

OK, so you have installed an alternator, wired it correctly and the thing doesn't charge. Now what?

First, check to see if the regulator is 'leaking sufficently'. This is done by connecting a test light between the BAT terminal and the 'Field #1 Terminal'. This should start the alternator charging. Actually this is the way it was designed to work. If the alternator starts charging, the regulator wasn't leaking enough. Follow the conversion above. If it doesn't charge, then continue below. (Read and understand the procedure fully prior to starting)

If it is suspected the alternator isn't charging the battery, the best indicator is voltage. When the engine is running, the voltage should be aproximately 14.7 volts DC on a fully charged battery. If the voltage is less than 14.7 VDC and the battery is fully charged, we can diagnose the problem by "full fielding" the alternator.

CAUTION: Full fielding the alternator can result in high voltage. This can result in damage to the vehicle and or personal injury. At higher engine speeds voltage can be LETHAL!

Pretest checks

  1. Battery connections are clean and tight. (both ends of both cables)
  2. Battery is fully charged. (if not charge the battery)
  3. Alternator wiring and connections are clean and tight. (everything from alternator to battery)
  4. Alternator drive belt is tight and in good condition. ( make sure the belt isn't bottoming in the pulleys)
  5. Alternator mounting bolts are tight. (These are the ground path for the alternator)
  6. Run engine and check for abnormal noises from the alternator.
  1. Perform the above pretest
  2. With the engine off, check the battery voltage. It should be about 12 volts. (if the engine has been recently run it may be higher, maybe 13 volts or so. )
  3. Start engine and run at idle. (a one wire alternator may require 3,000 rpm to start charging) Check the battery voltage. It should be about 14-15 volts.
  4. Now, locate the test hole as shown below. If there is no test hole you have a CS alternator, (sorry, take it to an alternator shop)
  5. With the engine idling and watching the volt meter, ground the tab located just inside the test hole with a screwdriver to the case. Take reading quickly.
  6. With the tab grounded the voltage reading should be 15 volts or higher. The faster the engine speed the higher voltage.
  7. If the voltage is low, the alternator is bad and needs to be replaced
  8. If the voltage is 15 or higher, either the regulator or field circuit is the problem. Either replace the regulator or have an alternator shop check/repair it.

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Where to Buy

Conversion kits and complete alternators are available from JC Whitney - (no recommendation intended)

Conversion kits - or enter "one wire alternator conversion kit" into the Google search box

Complete alternators - Your local automotive parts is also a good place to buy

5/8" x 2 1/4" (wide belt) pulley - The part numbers is  98-NAA5825, but enter  NAA5825 into the part number search box

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Delco SI schematics

Below are the schematics for the "as designed" and One Wire conversion.

Other than the external wiring, nothing else is changed. To get the 'One Wire' to start charging at a lower RPM the regulator is changed, as described earlier on this page.

Delco SI as designed

Delco SI 'One Wire'

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I hope this page has been helpful to you. If you have any questions please email me at cj2a@roadrunner.com I will try to answer any questions you may have.

Visitors since August 29, 2001

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Copyright Richard N. Meagley Sr.
Last revised: June 04, 2012.