Tech Tips: Generators

The generator is a key component in the electrical system of any vehicle. Its basic purpose is to provide current for the electrical accessories on the vehicle and to charge the battery. Without a generator, the vehicle could be started and run, but it would stop as soon as the battery charge was depleted. Our subject vehicle is a 2011 Jeep® Grand Cherokee and its electrical system.

Generator Basics
The generator is the heart of the charging system which includes the decoupler pulley (if equipped), electronic voltage regulator (EVR) which is located in the PCM, ignition switch, battery, generator lamp, check gauges lamp, voltmeter and the wiring harness and its connectors.

Figure 1.

The generator is belt-driven by the engine using a serpentine type drive belt (see Figure 1). As the energized rotor begins to rotate within the generator, the spinning magnetic field induces a current into the windings of the stator coil. Once the generator begins producing sufficient current, it also energizes the rotor. The stator winding connections deliver the induced alternating current to the positive and negative diodes for rectification. From the diodes, the rectified current is delivered to the electrical system of the vehicle through the generator battery terminal.

The charging system is turned on and off by the PCM and the ignition switch with the engine running. The field circuit will not be energized until the engine is running and the ignition switch is on. This voltage is connected through the PCM and supplied to one of the generator field terminals (Gen. Source B+) at the back of the generator. The generator is internally grounded and regulates the field using pin-1 of the field connector (high side driver). The engine drives the generator either through a serpentine belt and pulley or a decoupler pulley arrangement.

Refer to Figure 2. The PCM (1) receives a voltage input (5) from the generator (4) and also a battery input from the Totally Integrated Power Module (TIPM) (2). It compares the voltages to the desired voltage programmed in the EVR software and, if there is a difference, sends a signal to the generator EVR circuit to increase or decrease output. The amount of DC current produced by the generator is controlled by the EVR circuitry within the PCM.

Figure 2.

All OBD-sensed systems, including the EVR circuitry, are monitored by the PCM. Each monitored circuit is assigned a Diagnostic Trouble Code (DTC). The PCM will store a DTC in the electronic memory for certain failures that are detected.

One of the parameters monitored by the Check Gauges Lamp is the charging system voltage.

If an extreme condition exists, the lamp will illuminate. This is a reminder for the driver to check the instrument panel for additional information about the charging system.

Generator Problems
When problems arise with the generator, it might present a difficult situation for the driver. For example, if generator output is low (a very common problem), the generator might not be charging the battery sufficiently. This can result in the battery running out of charge. If the vehicle is being driven, it might come to a stop. More often, the vehicle won’t start after it is shut off, perhaps after sitting for a night.

A bad battery and a faulty generator often display similar characteristics. The problem is determining which of these two components is bad. It’s the age-old dilemma of the generator not charging the battery, or the battery too old to effectively be charged. Before jumping to any premature conclusions, do a visual inspection of the charging system to rule out any obvious problems.

Check the serpentine drive belt to be sure that it is not loose, worn, damaged of misaligned. Be sure the mounting bolts are tight. Also, check the alignment of the drive belt. Any of these conditions will have a negative impact on the performance of the generator. Also, inspect the condition of the battery. Don’t forget to look at the battery terminals and connections. These should all be clean and tight.

If everything checks out from the visual inspection and the battery is fully charged, check to see if any DTC’s have been stored. As far as generators are concerned, you’re usually looking for one of two codes: P2503 or P2504. If the generator output is low, code P2503 will be present. Let’s look at this DTC in some detail.

The test procedures for this code will allow you to determine if the generator is serviceable. First, check the Fused B+ Circuit High Resistance. With the ignition on, but the engine not running, measure the voltage between the generator B+ output terminal and the battery + post. Start the engine. Is the voltage above 0.4 volt? If yes, repair the excess resistance in the battery positive circuit. If no, check the generator operation.

Turn the ignition off. Disconnect the generator field harness. Using a 12-volt test light connected to ground, probe the generator field control circuit (K125) in the generator field harness connector.

Note: Be sure the DTC is cleared before performing this test.

Turn the ignition on, but don’t start the engine. Using the appropriate scan tool, actuate the generator field driver. The test light should illuminate. If the light does illuminate, the generator must be replaced.

Generator Replacement
Generator replacement is rather straightforward on our subject vehicle, a 2011 Jeep® Grand Cherokee equipped with the 3.6L V6 engine. Disconnect the negative terminal cable from the battery. Unplug the field circuit from the generator, then remove the B+ terminal nut and wire.

Remove the serpentine belt in the following manner: rotate the belt tensioner until it contacts its stop position; remove the belt, then slowly rotate the tensioner into the free arm position.

Remove the lower front generator-mounting retainer. Remove the three mounting retainers, then remove the generator from the vehicle.

Install the new generator into place in the vehicle. Install the three mounting retainers and tighten to 18 ft-lbs. Install the ground wire on the mounting stud, then install the lower front ground wire mounting nut. Tighten the nut to 18 ft-lbs.

Figure 3.

Install the serpentine belt. The belt must be routed correctly as shown in Figure 3. Failure to do so will cause the water pump to rotate in the wrong direction. This situation could cause the engine to overheat. Route the belt around all pulleys except the idler pulley (1). Rotate the tensioner arm (2) until it contacts its stop position. Route the belt (3) around the idler pulley (1) and slowly let the tensioner (2) rotate into the belt. Be sure the belt is seated on all the pulleys. With the drive belt installed, inspect the belt wear indicator. The gap between the tang and the housing stop must not exceed 0.94 inches. If it does, replace the belt.

Install the B+ terminal to the generator stud and tighten to 150 in-lbs. Plug in the field circuit to the generator. To finish the job, connect the negative battery cable.