You need to test the charging system. This is a very reliable and easy-to-diagnose system, but the alternator is just one part of it. Here's a copy / paste page on testing this system. The only difference in '91 is your system might only need the ignition switch in the "Run" position to measure voltages:
... This Chrysler charging system works exactly the same as all previous versions and the same testing procedures apply. The voltage regulator lives inside the Powertrain Control Module, (PCM), next to the battery. This allows the regulator to "know what the PCM knows" . Charging system output, and load on the engine, can be adjusted for such things as battery temperature, wide-open-throttle, engine overheating, etc. In most cases, this alternator can be repaired on the vehicle. These tests may apply to newer models although PCM pin numbers will need to be determined from the service manual.
Troubleshooting will be easier if you have a basic understanding of electrical theory and can read a digital volt-ohmmeter. In some instances, a less expensive test light will be recommended because it will complete an open circuit causing current to flow and voltage drops to occur. The result of these voltage drops is a dim test light.
If you do not understand electrical theory but would still like to diagnose this system, read the "Safety" section first, then follow the instructions for "Basic Diagnosis" . A digital voltmeter is preferred, but a test light should provide sufficient information.
If you followed the preliminary tests in the "Test Your Charging System" section and found battery voltage outside the range of 13.75 to 14.75, proceed with these steps:
No Change in Battery Voltage
Measure for full battery voltage between ground and the single large output terminal on the back of the alternator. If using a test light, it should be bright. If full battery voltage is present, continue to "Testing the Field Circuit" . The ground lead of the voltmeter or test light should be connected to the engine block or battery negative terminal, not the alternator housing.
No voltage indicates an open wire. Most of these vehicles did not have a fuse in this circuit, but many did have a large black "bullet" connector near the back of the battery. Check that the connector is not corroded or unplugged. If this wire is broken, a test light may light when connected between the alternator output terminal and the battery positive terminal and the engine is running. When connected this way, if the test light is bright before starting the engine, there could be shorted diodes inside the alternator. This would also cause the output wire to burn open or a fuse to blow. While diodes can be replaced, it is generally more economical to just replace the entire alternator. The output wire will also need to be repaired or replaced. These diodes are air-cooled, and they do short on occasion, but then typically overheat and burn open so the wiring is not damaged. Two of the six diodes must short for damage to occur to the output wire. Another symptom of a single defective diode is maximum output will be close to only one third of the alternator's rated output. See the "Full Field" test for more information.
Testing the Field Circuit
Start the engine and run it at idle for all of the following tests. DO NOT ALLOW LOOSE CLOTHING OR METER LEADS TO BE CAUGHT IN THE RADIATOR FAN OR BELTS.
Two small wires enter a black rectangular plastic block bolted onto the back of the alternator. Besides the mounting nut(s), two exposed tabs are bolted to the two field terminals. These are the two field terminals that you will be measuring voltages on.
By the 1991 model year the engine must be running to measure voltages in the field circuit. Some models in previous years also need the engine running. Voltage for the field circuit comes through the Automatic Shutdown (ASD) Relay which only turns on when the engine is rotating, (cranking or running). The ASD relay circuit will not cause charging problems. This relay also supplies voltage to the ignition coil(s) and injector(s). The engine will not run if the ASD relay doesn't turn on.
Begin by measuring the voltages on the two field terminals while the engine is running. At least one must have full battery voltage. If not, there is a break in the feed wire. The most likely cause is a corroded pin in the connector behind the battery.
If you find battery voltage on one field terminal, measure the voltage on the other terminal. 0 volts on the second terminal indicates an open brush. Continue to the section on "Brush Replacement" . A shorted voltage regulator will result in very low voltage on this terminal, but will cause the system to overcharge the battery which is not the symptom being diagnosed here. Also, due to other circuitry in the PCM, a shorted regulator will not totally ground the brush terminal. Rather, you will typically find 2 - 4 volts which for all practical purposes will still full-field the system and cause battery voltage to be much too high. This is quite rare.
During normal operation, you will find roughly 9 - 11 volts on the second brush terminal. The lower the voltage, (the greater the difference between the two terminals), the more output voltage and current will be generated to meet the needs of the electrical system. Varying this voltage is how the regulator maintains system voltage between 13.75 and 14.75 volts.
Should you find the same voltage, (battery voltage), on both terminals, there is an open circuit in the regulator circuitry or wiring. The most likely cause is a corroded pin in the connector behind the battery. You'll need to perform the full-field test to verify the alternator is ok. This test can also help narrow down the location of the problem. The regulator-side brush must be grounded, then monitor battery voltage to see if it goes up. The problem is, which terminal is the one right one to ground? See "Which terminal do I ground" in the FAQs section.
If the full-field test causes battery voltage to increase, the alternator is capable of producing output, and the brushes and battery wire are ok. To determine whether the wiring or regulator are the cause of the open circuit, locate the ten-pin plug near the rear of the battery. Most vehicles use dark blue and dark green wires for the field circuit. (Some vehicles use two dark green wires). The dark blue wire will have full battery voltage when the engine is running, and since your diagnosis found battery voltage on both alternator field terminals, expect to find the same battery voltage on the dark green wire. Measure the voltage on the dark green wire on both sides of the connector by carefully back-probing the wires using a paper clip as a probe. 12 volts on one side but not the other side indicates a corroded or stretched pin in the connector, or your back probe is not making good contact with the terminal. 0 volts on both sides means there is a break in the wire between the connector and alternator. If there is 12 volts on both sides, or if you can't find the connector, you can measure the same voltage at the PCM. This requires removing a snapped-on plastic cover from the 60-pin connector, and possibly sliding the battery out of the way. On early to mid 1990s front wheel drive cars and minivans, the regulator wire is dark green, and found in cavity # 20. The wires are in three rows of 20 pins in each row. Pin 20 is in the end of the top row toward the rear of the vehicle. Carefully back-probe this wire with a paper clip. Don't pierce the wire insulation. Corrosion will result in a broken wire in the near future. Battery voltage indicates an open regulator; 0 volts means there's a break in the wire between this pin and the alternator field terminal, most likely a corroded pin in the connector by the battery.
Overcharging is caused by a grounded wire between the field terminal and voltage regulator inside the PCM, or by a break in the voltage sensing wire for the PCM. Turn the ignition switch off, then use your ohm meter to measure resistance from either field terminal to ground. Any reading below 10 to 20 ohms suggests a grounded wire. Unplugging the connector behind the battery will allow you to narrow down which side of the connector the short is on. This is probably a good time to visit the professionals.
If resistance measurements are acceptable and voltage on the dark green wire is very low but not shorted to ground, the regulator may be trying to increase alternator output based on incorrect information. You'll need to refer to the proper service manual to determine which PCM pin is the system voltage sensing wire, and check for the presence of battery voltage. On older cars that have a Logic module in the passenger compartment, corroded pins in the firewall connector are a likely source of trouble. When this sensing wire is open, the regulator circuit might "see" 0 volts and interpret that as a severely discharged battery, or it could pick up some random voltage inside the computer and use that to determine how much output is needed from the alternator. Circuitry and wiring problems are typically not do-it-yourselfer repairs. Misguided attempts to locate these problems can result in making more work for the next person. To complicate matters, there is generally no wire labeled "voltage sense" ; rather, some other circuit that has battery voltage when the engine is running is used. A problem with one of these wires usually causes a no-start condition or some other problem, but not just a charging problem.
These charging systems are quite reliable and trouble-free, but a defective voltage regulator is not entirely unheard of. There are many advantages to planting the regulator inside the relatively expensive PCM, but the disadvantage is the need to replace the entire engine computer just because the regulator portion is bad, and the PCM costs more than the car is worth. Some people will cut the regulator wire going to the PCM (pin 20), and connect it to an externally-mounted regulator from a vehicle from the 1970s. This will work fine for the charging system, and these regulators do have temperature compensation built in just like the regulator in the PCM. However, since the original regulator turns on and off hundreds of times per second and is monitored for proper operation by the PCM, a fault code will be memorized, (" Field circuit not switching properly" ), and the "Check Engine" light may be turned on. The "Check Engine" light must turn on when a detected fault could adversely affect emissions. Incorrect charging voltages can affect fuel injectors, fuel pump pressure, and sensor readings. Therefore, even though an external regulator will run this charging system, the Check Engine light will always be on, so there will be no way to tell when a different problem arises.
The brushes on many Chrysler products can be replaced without removing the alternator from the engine. It is common for the nuts to rust, so it will be necessary to scratch them a little to find a shiny spot from which to make measurements. To double-check the need for replacement, measure between the two field terminals with your ohm meter. Normal resistance is around 4 ohms. Sometimes it's necessary to rotate the pulley a little for the brushes to make good contact. An infinite (open) reading is proof the brushes are worn and must be replaced.
Before starting brush replacement, the alternator output wire must not have battery voltage present. Normally this simply means disconnecting the negative battery cable, but this can lead to severe misery on 1996 and newer minivans. Once the battery goes dead or is disconnected, the heater / air conditioning / ventilation (HVAC) computer will "lose its mind" . Reprogramming it later can be extremely frustrating, and will not work if the air conditioning system doesn't work properly. See FAQs for the reprogramming procedure. Heater systems worked really well for many decades without computer controls. Not sure what the problem was that they found a solution for, but you should be aware of this problem before proceeding. For these vehicles, look around the battery for a "bullet" connector inline with a single heavy black wire. There may also be a high-amperage fuse bolted into the under-hood fuse box, called the Power Distribution Center, (PDC), labeled "Generator" . Either of these can be used to break the connection to the alternator output terminal without losing the memory voltage supply to the HVAC computer. Recheck for voltage on the output terminal with the test light or voltmeter. If it's gone now, it is safe to proceed.
The field terminal studs are very small and could twist off when removing the nuts. While these studs can be replaced, it might be easier to just replace the entire alternator. Once the wires are all removed, three nuts must be removed, then the stamped sheet-metal cover can be pried off. View the photo to see the three Philips screws holding the brush holder assembly in place. Remove them, and the assembly will slide off the slip rings. The new assembly is installed at an angle to make the brushes retract against spring pressure. A small drill bit can also be used to hold the brushes retracted while installing the assembly. This usually works best when the alternator is on a workbench.
When the new brushes are installed, measure between the two terminals with your ohm meter. There should be around 4 ohms. Rotate the pulley just a little if necessary. Once you have the normal 4 ohms, measure from either brush terminal to the metal alternator housing. Use the highest ohm meter range. Any reading other than infinite, (open) is unacceptable. When you obtain proper resistance readings, reinstall the rear cover, and double-check the resistance from either brush to the housing. If the reading is an open circuit, reconnect the wires and battery cable or fuse, and check for proper operation. Battery voltage with the engine running must be between 13.75 and 14.75 volts.
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Saturday, March 28th, 2009 AT 9:03 PM