Chrysler Full-Size Trucks 1967-1988 Repair Guide



The alternator charging system is a negative (-) ground system which consists of an alternator, a regulator, a charge indicator, a storage battery, the wiring connecting the components, and fuse link wire.

The alternator is belt-driven from the engine. Energy is supplied from the alternator/regulator system to the rotating field through two brushes and finally two slip rings. The slip rings are mounted on the rotor shaft and are connected to the field coil. This energy supplied to the rotating field from the battery is called excitation current and is used to initially energize the field to begin the generation of electricity. Once the alternator starts to generate electricity, the excitation current comes from its own output rather than the battery.

The alternator produces power in the form of alternating voltage. The alternating current is rectified by 6 diodes into direct current. The direct current is then used to charge the battery and power the rest of the electrical system.

When the ignition key is turned ON , current flows from the battery, through the charging system indicator light on the instrument panel, to the voltage regulator, and to the alternator. Since the alternator is not producing any current, the alternator warning light comes on. When the engine is started, the alternator begins to produce current and turns the alternator light off. As the alternator spins and produces current, the current is divided in two ways: part to the battery to charge the battery and power the electrical components of the vehicle, and part is returned to the alternator to enable it to increase its output. In this situation, the alternator is receiving current from the battery and from itself. A voltage regulator is wired into the current supply to the alternator to prevent it from receiving too much current which would cause it to put out too much current. Conversely, if the voltage regulator does not allow the alternator to receive enough current, the battery will not be charged fully and will eventually go dead.

The battery is connected to the alternator at all times, whether or not the ignition key is turned ON or not. If the battery were shorted to ground, the alternator would also be shorted. This would damage the alternator. To prevent this, a fuse link is installed in the wiring between the battery and the alternator. If the battery is shorted, the fuse link is melted, protecting the alternator.


To prevent damage to the alternator and regulator, the following precautions should be taken when working with the electrical system.

Never reverse the battery connections. This would cause BIG PROBLEMS!
Booster batteries for starting must be connected properly: positive-to-positive and negative-to-ground.
Disconnect the battery cables before using a fast charger; the charger has a tendency to force current through the diodes in the opposite direction for which they were designed. This burns out the diodes.
Never use a fast charger as a booster for starting the vehicle.
Never disconnect the voltage regulator while the engine is running.
Avoid long soldering times when replacing diodes or transistors. Prolonged heat is damaging to AC generators.
Do not use test lamps of more than 12 volts (V) for checking diode continuity.
Do not short across or ground any of the terminals on the AC generator.
The polarity of the battery, generator, and regulator must be matched and considered before making any electrical connections within the system.
Never operate the alternator on an open circuit. make sure that all connections within the circuit are clean and tight.
Disconnect the battery terminals when performing any service on the electrical system. This will eliminate the possibility of accidental reversal of polarity.


There are many possible ways in which the charging system can malfunction. Often the source of a problem is difficult to diagnose, requiring special equipment and a good deal of experience. This is usually not the case, however, when the charging system fails completely and causes the dash board warning light to come on or the battery to become dead. To troubleshoot a complete system failure only two pieces of equipment are needed: a test light, to determine that current is reaching a certain point; and a current indicator (ammeter), to determine the direction of the current flow and its measurement in amps.This test works under three assumptions:

The battery is known to be good and fully charged.
The alternator belt is in good condition and adjusted to the proper tension.
All connections in the system are clean and tight.

In order for the current indicator to give a valid reading, the truck must be equipped with battery cables which are of the same gauge size and quality as original equipment battery cables.

  1. Turn off all electrical components on the truck. Make sure the doors of the truck are closed. If the truck is equipped with a clock, disconnect the clock by removing the lead wire from the rear of the clock. Disconnect the positive battery cable from the battery and connect the ground wire on a test light to the disconnected positive battery cable. Touch the probe end of the test light to the positive battery post. The test light should not light. If the test light does light, there is a short or open circuit on the truck.
  3. Disconnect the voltage regulator wiring harness connector at the regulator. Turn the ignition key ON . Connect the wire on a test light to a good ground (engine bolt). Touch the probe end of a test light to the ignition wire connector into the voltage regulator wiring connector. This wire corresponds to the I terminal on the regulator. If the test light goes on, the charging system warning light circuit is complete. If the test light does not come on and the warning light on the instrument panel is on, either the resistor wire, which is parallel with the warning light, or the wiring to the voltage regulator, is defective. If the test light does not come on and the warning light is not on, either the bulb is defective or the power supply wire form the battery through the ignition switch to the bulb has an open circuit. Connect the wiring harness to the regulator.
  5. Examine the fuse link wire in the wiring harness from the starter relay to the alternator. If the insulation on the wire is cracked or split, the fuse link may be melted. Connect a test light to the fuse link by attaching the ground wire on the test light to an engine bolt and touching the probe end of the light to the bottom of the fuse link wire where it splices into the alternator output wire. If the bulb in the test light does not light, the fuse link is melted.
  7. Start the engine and place a current indicator on the positive battery cable. Turn off all electrical accessories and make sure the doors are closed. If the charging system is working properly, the gauge will show a draw of less than 5 amps. If the system is not working properly, the gauge will show a draw of more than 5 amps. A charge moves the needle toward the battery, a draw moves the needle away from the battery. Turn the engine OFF.
  9. Disconnect the wiring harness from the voltage regulator at the regulator connector. Connect a male spade terminal (solderless connector) to each end of a jumper wire. Insert one end of the wire into the wiring harness connector which corresponds to the A terminal on the regulator. Insert the other end of the wire into the wiring harness connector which corresponds to the F terminal on the regulator. Position the connector with the jumper wire installed so that it cannot contact any metal surface under the hood. Position a current indicator gauge on the positive battery cable. Have an assistant start the engine. Observe the reading on the current indicator. Have your assistant slowly raise the speed of the engine to about 2,000 rpm or until the current indicator needle stops moving, whichever comes first. Do not run the engine for more than a short period of time in this condition. If the wiring harness connector or jumper wire becomes excessively hot during this test, turn OFF the engine and check for a grounded wire in the regulator wiring harness. If the current indicator shows a charge of about three amps less than the output of the alternator, the alternator is working properly. If the previous tests showed a draw, the voltage regulator is defective. If the gauge does not show the proper charging rate, the alternator is defective.


See Figures 1, 2, 3 and 4

Though internal alternator repairs are possible, they require specialized tools and training. Therefore, it is advisable to replace a defective alternator, or have it repaired by a qualified shop.

  1. Disconnect the ground cable at the battery.
  3. Disconnect and label the alternator output (BATT) and field (FLD) leads, then disconnect the ground wire.

Click image to see an enlarged view

Fig. Fig. 1: Disconnect field (FLD) leads

Click image to see an enlarged view

Fig. Fig. 2: Disconnect output lead

  1. You may need to spray a lubricant on the existing nuts and bolts for easy removal. Sometime it may be necessary to allow the lubricant to sit for a few minutes.
  3. Loosen the alternator adjusting bolt and swing the alternator in toward the engine. Disengage the alternator drive belt.

Click image to see an enlarged view

Fig. Fig. 3: Adjusting bolt removal

  1. Remove the alternator mounting bolts and remove the alternator from the vehicle.

Click image to see an enlarged view

Fig. Fig. 4: Removal of mounting bolts

To install:
  1. Install the alternator using the old bolts. Tighten the bolts, but not all the way, you still need to install the belt.
  3. Install the old belt, or a replacement if worn. Always look at your belts before reinstalling an old one, you wouldn't want to do this job over again due to a junk belt.
  5. Be sure to connect all ground wires and leads securely.
  7. Adjust the belt tension. See belt tension adjustment.
  9. Connect the battery ground cable.


On some models it may be necessary to remove the lower splash shield to gain clearance when installing a new drive belt.

Belt tension should be checked with a gauge made for the purpose. If a gauge is not available, tension can be checked with moderate thumb pressure applied to the belt at its longest span midway between pulleys. If the belt has a free span less than 12 in. (304mm), it should deflect approximately 1 / 4 in. (6.3mm). If the span is longer than 12 in. (304mm), deflection can range between 1 / 4 in. (6.3mm) and 3 / 8 in. (9.5mm). To adjust or replace belts:

  1. Loosen the driven accessory's pivot and mounting bolts.
  3. Move the accessory toward or away from the engine until the tension is correct. You can use a wood hammer handle as a lever, but do not use anything metallic.
  5. Tighten the bolts and re-check the tension. If new belts- have been installed, run the engine for a few minutes, then re-check and readjust as necessary.

If the driven component has two drive belts, the belts should be replaced in pairs to maintain proper tension.

It is better to have belts too loose than too tight, because overtightened belts will lead to bearing failure in the alternator. However, loose belts place an extremely high impact load on the driven components due to the whipping action of the belt.