Chrysler Charging System, 1970 - 1989
Chrysler was the first manufacturer to introduce the AC generator in 1960 and copyrighted the term "alternator". During the first ten years, the system used a mechanical voltage regulator in a "B"-type field circuit. Beginning with the 1970 models, an electronic voltage regulator was used in an "A" field circuit. Depending on the electrical options on the vehicle, it could have come with a 35, 55, or 78 amp alternator. Cars with air conditioning had a minimum of the 55 amp unit. That system was used through the 1989 model year on rear-wheel-drive cars and is discussed here. A version of this system was also used on some front-wheel-drive cars and minivans through the mid 1980s. See the special notes at the bottom of this page for additional related details.
This charging system is arguably the world's least complicated and most easily-diagnosed and repaired system. All dark blue wires under the hood have full battery voltage when the ignition switch is turned on. One of those wires feeds one of the alternator field terminals and one is the system voltage sensing wire for the voltage regulator as well as the power wire for its circuitry. A dark green wire connects the second field brush to the control circuit of the regulator. The last circuit that must not be overlooked is the ground for the voltage regulator. That is simply the case being bolted to the body sheet metal. The voltage regulator has temperature compensation built into the circuitry. Storing electrical energy in a battery is a chemical process, and chemical reactions slow down when the temperature goes down. At cold temperatures the regulator will bump up charging system voltage a little to help that chemical reaction take place. That helps the battery become fully charged. In hot weather, the charging voltage is lowered to prevent boiling the water out of the battery's electrolyte.
Current flow for the input circuit begins at the battery positive post, and goes through the bulkhead connector at the firewall, to the ignition switch, back through the bulkhead connector, then to all the dark blue wires under the hood. One of those wires goes to a field terminal on the back of the alternator case. Current travels through the field winding by way of the two brushes. From there current follows the dark green wire to the voltage regulator, then to ground and back to the battery negative post. The blue and green wires can be connected to either field terminals. Switching them will have no effect on system operation.
Voltage induced into the stator windings causes current to flow through the diodes, then out to the car's circuitry and back to the battery to recharge it after starting the engine.
System Diagnosis and Repair
Once this charging system has been identified as not working, much of the diagnosis can be done from taking a few voltage measurements at the alternator. The output wire must have full battery voltage at all times. The dark blue field wire must have full battery voltage when the ignition switch is on. The dark green field wire should have less than battery voltage but not 0 volts. You can typically expect to find 4 - 11 volts when the engine is running. When voltage is missing at the dark blue wire, there is no need to go all the way back to the battery positive post. There is a splice in the harness where another dark blue wire feeds the ignition coil. A break before the splice would result in a no-start condition due to an inoperative ignition coil so you would not be diagnosing a charging system problem. If there is voltage to the ignition coil but not to the alternator field terminal, follow the dark blue wire back to the splice to find the break.
We should stop here to point out one clinker that can lead to incorrect voltage readings. Be aware that both brushes are insulated from ground with the plastic holders and with fiber washers under the screw heads, so voltage readings must be taken on the terminal itself, not on the screws.
Once battery voltage has been verified on the dark blue wire, three possible conditions will be found on the dark green wire; 0 volts, full battery voltage, or something in between. No voltage on that wire is probably the most common problem and is usually caused by worn brushes. A verification test is to measure continuity between the two field terminals. Normal resistance is around 4 ohms. Due to carbon particles breaking off the brushes it may be necessary to rotate the pulley just a little to help the brushes make better contact. Those particles will usually cause a very high resistance reading while worn brushes will cause an infinite reading. This condition almost always starts out as an intermittent problem. Replacing the brushes is very easy and can often be done without removing the alternator from the engine. A screw holds each brush in place. One brush pulls straight out the back of the housing and one pulls to the side.
The same symptoms can be caused by rusty brush terminals or loose wire connectors. You will often find sparks occurring at those connections when you wiggle them, and the alternator will start working if the engine is running.
No voltage on the dark green wire can also be caused by that wire being grounded but that would result in a severe overcharge condition since the system would be full-fielded. To find the cause, unplug the dark green wire. If the overcharging stops, either that wire is grounded on the engine or the voltage regulator is shorted. Neither of those conditions are common. To determine which is the cause of the overcharging, reconnect the wire at the alternator, then disconnect the voltage regulator's plug. The overcharging will continue if the dark green wire is grounded, and it will stop if the regulator is shorted.
In very rare instances there will still be 0 volts on the field terminal and the overcharging will continue even after the dark green wire has been disconnected. Inspect the washer under the head of the screw holding that brush in place. Metal replacement washers were supplied separately years ago with replacement alternators. Removing the fiber washer and installing the metal one grounded that brush so the alternator could be used on 1960 - 1969 model cars with a "B"-type field circuit and mechanical voltage regulator. To solve this, you'll need to find and install a fiber washer.
If you find the same full battery voltage on both field terminals, an open circuit exists between the field winding and ground. The green wire could be open, but the most likely suspect is a defective voltage regulator. Also look for a poor ground where the regulator is bolted to the body sheet metal. Measure the voltage on the regulator's housing. It should be 0 volts. Any voltage indicates a less than perfect connection that must be cleaned and tightened. To verify your suspicions of an open regulator, remove the connector and use a jumper wire to ground the green wire. The system will be full-fielded if everything else is working. The alternator can also be full-fielded by grounding the green wire at the brush terminal on the back of the unit.
The final scenario has to do with a problem in the output circuit. It is rare for these diodes to short, but when one does, you will find the voltage on the dark green field wire will be in the normal range of around 4 – 11 volts. Under a load test, you will only be able to get one third of the rated current. Earlier versions used diodes that were pressed into an aluminum heat sink and the rear housing. The stator wires are soldered to those diodes so replacing them is a little involved. Later versions have air-cooled diodes mounted to metal brackets that are bolted in place. This type generally burns open after shorting, and that can be seen quite easily. They are visible through the rear housing.
Special Notes for 1978 - Mid 1980s FWD Cars and Minivans
The early front-wheel-drive vehicles with the 2.2L engines used the same external voltage regulator and the same wire colors. Troubleshooting procedures are the same with two additional things to be aware of. First of all, most of these alternators were mounted on rubber bushings to isolate them from engine vibration. That means there is no solid mechanical mounting bracket to provide the ground for the output circuit. A braided strap or a large diameter cable is bolted to the alternator case and the engine block. Common problems are the strap corrodes apart and the cable is accidentally left off after other service work is performed. The symptom is no output but the voltages on the two field terminals are normal. For a verification test, measure the voltage between the engine block / negative battery post and the alternator housing. You should find 0 volts. When the ground is missing, you'll find full battery voltage on the housing.
The second problem pertains to the two field terminals. On many models, the two wires go through a black plastic block so you can't tell which wire goes to which terminal. When there is a break in the circuit going to the regulator, both field terminals will have exactly the same voltage. A full-field test will verify the alternator is okay. To identify the terminals for further testing, see "Which Terminal Do I Ground" in the Troubleshooting Tricks.
Interpreting the Test Results (Chrysler Charging System, 1970 - 1989)