Other Articles - August - 2018

A Common Sense Approach to Charging System Diagnosis

In the previous article, we discussed the issues and operations related to the charging systems in common use today. Charging system problems may cause various vehicle issues, such as transmission shift and pressure control problems, DTCs, and driveability-related complaints.

Testing the charging system is fairly straightforward if you understand how the system operates. A voltmeter, scan tool, and a charging system load tester are the common tools used by most shops to perform the diagnosis required to isolate the issue.

You can typically divide charging system issues into three categories:

  • Overcharging
  • Undercharging or no output
  • Transient voltage issues (EMI)

Isolating the cause of the condition will require you to conduct some tests. Prior to conducting any tests, scan and record any DTCs. Charging system testing may cause DTCs to set on some applications, so cover your bases before you start your diagnosis.

Prior to testing any charging system, always test the battery and clean the connections. The battery must be fully charged and in good condition for the charging system to function properly.


Charging system voltage is simple to test. With a voltmeter connected to the battery, load the charging system by applying an electrical load such as the headlights, blower motor, or windshield wipers. The charging voltage should fall within the manufacturer’s specification; generally in the 13- to 15-volt range.

Always compare your measured charging system voltage with the voltage displayed on your scan tool. They should be very close. If not, the problem may not be the alternator itself but rather the controller or its wiring.


Issues with the charging system may be related to voltage drop in the output circuit or in the alternator ground. To test the circuit’s voltage drop, connect a voltmeter between the alternator output terminal and the battery B+ terminal. Connect a second voltmeter between the alternator case and the battery B- terminal.

With engine running and a load applied to the charging system, the voltage drops shouldn’t exceed 0.2 volts (figures 1 & 2).


It’s quite possible to have proper charging system voltage but have low or no output amperage from the system. This is typically due to wiring and resistance issues with the system, so voltage drop testing and output current testing will help you isolate problems in the system.

You can output test the system with a conventional carbon pile load tester, or one of the newer electronic load testers.


Many charging systems today are capable of setting DTCs. Like other systems, check the basics first then follow the code chart to aid in diagnosing the system.


Electrical interference can cause a lot of problems in today’s vehicles. If you suspect the alternator is creating electrical interference, disconnect and insulate all the wiring connectors from the alternator.

If the interference is no longer present, it may be caused by an alternator problem.


Since the PCM or another module typically controls the charging system on today’s vehicles, scan data may provide you with the reason the charging system isn’t functioning correctly. The scan data is specific to the manufacturer and may also vary based on the vehicle (figures 3 & 4).


Ford’s Smart Charge system used on 1999-and-later applications is designed to default to 13.7 volts when issues with the GEN MON or the GEN COM are present and engine RPM exceeds 2500.

Perform power and ground general tests as previous discussed, after activating the charging system default mode.


The GEN COM signal represents the commanded voltage set point for the alternator. The signal comes from the alternator regulator and the PCM modulates the signal to ground at a frequency of 126-128 HZ. The higher the duty cycle, the higher the alternator voltage set point.

To diagnose the system:

  • Key off.
  • Disconnect the 3-pin GEN COM connector.
  • Measure the voltage at the center pin at the alternator.

The voltage should measure 7.5 volts or greater. If it’s less than 7.5 volts and the power and ground measurements were correct, the alternator may require replacement.

If the voltage was correct, reconnect the connector and backprobe the GEN COM terminal. With the engine running, the voltage should typically measure between 4-9 volts but it may vary somewhat, depending on application.

Add load to the electrical system by turning the lights or wipers on. The voltage should increase. In addition, the duty cycle command on your scan tool will change. If the voltage doesn’t change, inspect the wiring for an open. If the circuit isn’t the issue, you’ll probably need to install a new PCM.

If the voltage is low, inspect the harness connector pins for an open, or the circuit for a short-to-ground or an open. If the system tests as described, the problem may be with the GEN MON circuit rather than the GEN COM circuit, even though you may have DTCs related to the GEN COM circuit.


The GEN MON circuit provides load information to the PCM. The alternator regulator modulates the signal to ground at a typical frequency of 126-128 HZ. The regulator varies the duty cycle for the circuit, which the PCM monitors. The higher the duty cycle, the higher the load.

To diagnose this circuit:

  • Key off.
  • Disconnect the alternator connector.
  • Key on.
  • Measure the voltage at the harness side GEN MON (LI) pin.

The voltage should be at or near battery voltage. If it’s low, inspect the harness for an open or short-to-ground. If harness is okay, you may need to replace the PCM.

If you don’t find any problems, reconnect the alternator connector and backprobe the GEN MON circuit. Monitor the duty cycle/voltage for the circuit, which should vary based on load and battery state of charge.

As load increases, the voltage/duty cycle should also increase. If the circuit appears to be operating correctly, the alternator will likely require replacement.


The sense circuit is used to tell the regulator the actual battery voltage. Fluctuations, overcharge, or undercharge may be caused by issues with the battery sense circuit.

Issues with the sense circuit are common with Ford applications. Sense voltage should match the battery voltage and excessive voltage drop must not exist in the circuit (Figure 5).



Terminal L is the control circuit for the GM system. The PCM sends a 5-volt, variable duty cycle signal to the terminal to control the alternator voltage set point.

To test the circuit:

  • Key off.
  • Backprobe terminal L.
  • Monitor the voltage.

The voltage should be 0-1.5 volts. If the voltage is correct, make sure you check and correct the alternator power and ground circuits for voltage drop. If the terminal L voltage is greater than 1.5 volts but less than 3.5 volts, you may have a PCM issue. If the voltage is greater than 5 volts, inspect the circuit for a short to power.


Terminal F is an indication of the status of the field circuit, which indicates the alternator load to the PCM. The signal is duty-cycle controlled and is used to adjust the engine idle speed and alternator voltage set point.

Monitor the duty cycle command with your scan tool or by backprobing the circuit with the engine running. The value should be between 5-95% depending on the battery state of charge and load.

Key on, engine off, disconnect the connector. The duty cycle should measure 0%. If the duty cycle/voltage is high, inspect for a short-to-voltage in the wiring.

Disconnect the connector and connect a test light between the harness connector terminal F and battery positive (B+). Monitor the pin F value on your scan tool; it should now read 95-100%. If the duty cycle is low, inspect for a short to ground or an open in the circuit.

If the values are correct for both terminal F and terminal L you will likely need to have the alternator tested or replaced (Figure 6).


The Chrysler systems vary depending on application. Most applications house the voltage regulator for the system in the PCM. 2008-and-later applications typically use a singlefield wire rather than two as used on other applications. The alternator uses two wires in the connector: one voltage sense wire and one field control wire. Other applications typically use two field circuits to control the alternator.

On older applications, two types of field controls are used: type A or type B.


With type A systems, the PCM or the automatic shutdown relay controls the power to the field through one of the terminals. The ground for the field is controlled by the PCM using the other field terminal.


On type B systems, one field terminal is grounded and the other field terminal controls the power to the field via the PCM. The PCM pulse width modulates the field power (B+) to control the output of the system.


Diagnosis of the Chrysler systems got much easier with the advent of the single wire field control systems. Like the other systems, start with the basics. Good alternator case ground and no voltage drop across the alternator output circuit is critical to the proper operation of this system.

One thing you’ll also need to check with many Chrysler systems is the alternator pulley. Many of the applications use a “decouple” pulley. This pulley design sometimes fails, leading to alternator drive issues, so make sure you check it.


Backprobe terminal 1 of the alternator. With engine speed above 1500 RPM, monitor the voltage/duty cycle of the field circuit. The value will vary based on charging system load and battery state of charge.

Increase the load by turning the headlights or wipers on. If the voltage/ duty cycle doesn’t vary with load and the voltage is high, inspect the harness for a short to voltage. If it’s less than 0.5 volts, the field circuit is shorted to ground or open. If the circuit checks good, the issue is likely caused by a faulty PCM.


Like other alternators, the single-wire Chrysler uses a sense circuit to monitor battery voltage. The sense wire connects directly to the rectifier bridge within the alternator to make the voltage measurement extremely accurate.

Many of the Chrysler PCM-controlled alternator applications receive battery sense voltage from the alternator sense circuit and the TIPM smart fusebox. Both inputs are used to calculate the field command.

Charging system issues with Chrysler applications are common. Just remember to check the alternator case ground and for proper voltage drop across the output terminal before you condemn the alternator. I’ve seen a lot of alternators that have been replaced when there was no problem with the alternator itself. Just unbolting, cleaning, and reattaching the alternator may be enough to repair your problem (Figure 7).

As you can see, alternator diagnosis is not that difficult. Just use some common sense and try to keep to the basics, as a lot of alternators are replaced when they really don’t need to be.

Until next time, remember: “Perseverance is the hard work you do after you get tired of the hard work you already did.”