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Wiring Diagram, Understanding System Can Help Diagnose Alternator ProblemPosted 9/4/2001By Brainard Janicki
Chrysler uses several variations to control the field to the alternator. Since engines have become fuel-injected, Chrysler has used a regulator built into the power module/logic module, or the SMEC/SBEC/JTEC. For simplification, let's use the term powertrain control module (PCM) for the controllers. On 1997 and newer Chrysler trucks and Dakota vehicles with JTEC, the field power is supplied by the PCM. This should be battery voltage. Current flows through one brush into the rotor and back out on the other brush to the controller. The field control wire terminates at the PCM to be grounded. There are some variations on the power source (or what feeds power to the alternator) and the wire colors after 1995, but the result is the same: The ignition switch, ASD relay or PCM powers up the field, flows through the alternator and ends at the PCM to be grounded. The system with an ASD relay will only have power at the field with the engine running so be sure to check your wiring diagram. The amount of the field is determined by the state of charge of the battery, and the PCM's ability to ground the field. A poor ground at the PCM can cause lack of field current, resulting in low or no charge. The loss of “battery sense voltage” at pin 22 of the PCM's connector C-1, or a grounded field control wire between the PCM, will cause an overcharge condition. If there is a loss of sense voltage at pin 22 of the C-1 connector, the PCM thinks the battery voltage is low, and commands the alternator to full field. If the field control wire is grounded, the result is also an overcharge. A grounded field brush will also produce an overcharge condition. A common complaint: “I've replaced the alternator and the power module, and it still won't charge or it overcharges.” Let's use a 1997 Dodge Dakota as an example. The wiring diagram shows a dark green/black trace wire from the PCM that powers up one of the brushes when the engine is running. A green wire on the other brush goes to cavity 10 of the C-2 connector at the PCM. This is the wire that is responsible for field current, and ultimately, output from the alternator. Determine first if the green wire is already grounded or is the PCM responsible for the overcharge. To clarify this point, I suggest cutting the wire at the PCM pin 10, connector C-2, to see what happens with the charging system. If there is still an overcharge condition, it is within the alternator or the wire is grounded between the PCM and the alternator. Repair the alternator or the grounded wire and the problem is fixed. If the overcharging problem goes away when the wire is cut, the problem is within the PCM or the battery feed to the PCM. Check pin 22 at connector C-1 at the PCM for correct battery voltage. Compare this to battery surface voltage. This is the battery feed to the PCM and it must be the same. The PCM uses this voltage to determine if the field is needed to keep battery voltage within the range of 13.6 to 14.2 volts. If voltage is low, the PCM tries to match the alternator charging rate with the target charging voltage the PCM has in memory, by grounding the field wire. As you can see, the diagnosis of this system can be simplified by using a wiring diagram and by having an understanding of the basic system operation. The same diagnostic routine can be used for an undercharge condition as well.
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Copyright (c) 1996-2008. Automotive Service Association. All rights reserved.
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