How to Troubleshoot a Delco 10Si Alternator

Present-day alternators come with an internal voltage regulator and rectifier, which essentially means the alternator does three things: produce electricity, change alternating current to direct current and regulate output voltage. The Delco 10Si alternator was one of the first alternators to combine these three functions in one unit, and it was produced for about 10 years before it was phased out and replaced by the Delco 12Si in 1983. If you think your Delco 10Si isn't operating correctly, then troubleshoot the problem by performing four basic checks.

Things You'll Need

  • Protective gloves
  • Flashlight
  • Pen and paper
  • Wrench
  • Flat-head screwdriver
  • Multimeter

Prepare

  • Put on protective gloves before you troubleshoot your Delco 10Si alternator. You're sure to get your hands dirty and you'll need to use tools to touch the terminals on the car battery.

  • Open the hood of your car and secure it so you can access the Delco alternator. Make sure the engine is cool before you begin working under the hood. Familiarize yourself with the layout of the engine.

  • Locate the car battery, alternator and alternator belt. The alternator is half-way down the side of the engine block. An alternator belt goes around the pulley on the front of the alternator.

  • Read the back of the alternator: you may need a flashlight. Look for a label that tells you the output voltage and maximum amperes of the alternator. The voltage typically will read 13 to 15 volts, which is the regular voltage alternators produce to charge the car battery and operate the electrical system. Amperes may vary, however, depending on the version of the Delco 10Si that's installed in your car; the alternator might produce 37, 42, 55, 63 or 70 amperes. Jot down the figures on paper as you will need to refer to this information.

Alternator Terminal Connections

  • Locate the terminals on the back of the Delco 10Si alternator. You will find a red cable connected to a terminal labeled "Batt," a black cable connected to a terminal labeled "Gnd" and a dual plastic plug in a socket with a red and a white wire extending from the plug.

  • Check that the red cable is tightly connected. Use a wrench to tighten the nut holding the cable in place, if it is loose.

  • Verify that the black cable is tightly connected; it is held in place using a screw. Use a flat-head screwdriver to tighten the screw, if it is loose.

  • Gently pull the plug that's in the plastic socket. If it pulls loose, it wasn't connected properly -- push it into place until you hear it click. If any of the three terminal connections that you just checked are loose, it could explain the problem you are having with your alternator.

Alternator Belt

  • Check the tension of the Delco 10Si alternator belt. If the tension isn't correct, the belt will slip and the alternator will not rotate at the right speed to produce enough electricity to power your electrical equipment and charge your battery.

  • Locate a central area of the alternator belt, such as halfway between the alternator and the top pulley on the engine. Pull the alternator belt away from the engine as far as you can with one hand. Using your other hand, place a ruler along the alternator belt you are pulling. Slide the alternator belt along the ruler towards the engine as far as you can.

  • Measure the distance between the two positions on your ruler. If the distance between the two points is greater than 2 inches, the belt needs tightening. This could be the reason why you are experiencing alternator problems. Have a professional mechanic tighten the belt, if you are unsure of how to do it yourself.

Alternator Voltage

  • Measure the Delco alternator output voltage and make sure it is in the range between 13 and 15 volts. Turn on the car's engine. Make sure you are not wearing any loose clothing when working under the hood of the car. Have your multimeter handy and set it to measure voltage.

  • Connect the two metal clips at the end of the wires from the multimeter onto the car's battery terminals. One wire is red, indicating positive and the other is black, meaning negative. The clip on the end of the red wire goes on the positive battery terminal -- it will be labeled "Pos" or "+." The clip on the black wire attaches to the negative battery terminal, marked "Neg" or "-."

  • Read the multimeter display. If the reading is between 13 and 15 volts, your Delco alternator and internal voltage regulator are operating correctly. It is not a problem if the voltage is 10 percent above "15" or 10 percent below "13" but if the voltage is lower than 11.5 volts or higher than 16.5 volts, consult with a professional. Low voltage means your battery may not charge properly, whereas high voltage could damage the electrical equipment in your car.

Alternator Amperes

  • Change the multimeter setting to measure amperes. Place the metal clips on the end of the multimeter wires onto the battery terminals as you did for measuring voltage.

  • Refer to your notes and the ampere rating you wrote down earlier. Read the amperes on the multimeter. The reading will be less than the amperes on your paper; in fact, it may be 20 to 30 percent less. This is because the alternator adjusts the ampere output to the energy consumption of the electrical equipment on the car at that particular moment. If nothing is turned, the alternator doesn't need to produce high amperes.

  • Turn on energy-consuming electrical equipment in your car such as lights, wipers and air conditioning. Repeat the above step and check the ampere reading on the multimeter. It should be close to the maximum rating shown in your notes if the Delco alternator is operating correctly. This is because the alternator ampere output has adjusted to meet the energy requirements to power the electrical equipment you have turned on. If the amperes are lower than the figure you wrote down by 15 percent or more, get the alternator checked.

  • Turn off the car engine. Close the hood of the car.