According to my experience, I estimate that replacing the alternator solves 95% of all charging system failures. If that’s true, what happens in the remaining 5% of charging system failures that results in customer comebacks? To explore charging system comebacks in greater detail, let’s consider a hypothetical case involving a remanufactured alternator that you’ve just installed on a 2008 Toyota Tundra. A few days ago, it left your shop with a happy owner and now it has returned with the battery warning light glaring in the owner’s face. Let’s begin by reviewing a few charging system basics.
Most Toyota charging systems integrate their voltage regulators into the alternator assembly. While charging voltage specifications vary according to application, the voltage regulator generally adjusts battery-charging voltage according to ambient air temperature. For example, at 70° F, the voltage regulator charges the battery at approximately 14.2 volts. As underhood or ambient temperatures increase, the charging voltage is reduced to about 13.8 volts or less to prevent boiling the water from the battery electrolyte. During cold weather, charging voltage can increase into the 15-volt range to compensate for reduced chemical activity in the battery in sub-freezing temperatures. In any case, consult your service data before assuming that an apparently low or high charging voltage is incorrect for the system in question.
According to Toyota, a dynamic battery state of charge (SOC) and state of health (SOH) test can be performed with the aid of a simple voltmeter. First, turn off all accessories such as exterior lighting, HVAC blower and high-amperage components like the rear-glass defogger. Next, disable the fuel pump and crank the engine with a voltmeter connected in parallel with the battery terminals. If the cranking voltage dips below 9.6 volts, recharge or replace the battery as required before proceeding with the alternator test. See Photo 1.
I’ve found that if the battery is discharged and the battery core is at room temperature, the following recharging tips can be indicative of a battery’s state of health.
1. Charging amperage will rapidly decrease and charging voltage will rapidly increase on a badly sulfated battery.
2. The initial charging amperage should be high on a good battery.
3. Initial charging amperage gradually decreases as the charging voltage gradually increases on a good battery. 4. A battery with a weak cell will not achieve a desired charging voltage of 14.2 volts at room temperature. 5. Remember that many “smart” battery chargers will increase voltage in excess of 15 volts as they enter a “desulfation mode.” The smart charger might also enter a 13.2-volt maintenance mode when desulfation and recharging are completed. See Photo 2.
I’ve also discovered during the past few years that it’s important to initially connect a scan tool to determine the charging system configuration and help analyze charging output. Depending on the diagnostic capability of the scan tool and the software written into the engine control module (ECM), the scan tool might display diagnostic trouble codes (DTCs) indicating problems within the charging system. The scan tool might also reveal intermittent alternator problems by displaying DTCs indicating extremely low or high voltages in the electrical system.
Some charging systems also contain bidirectional controls designed to adjust alternator output. Most scan tools also display an electrical system voltage parameter. Using an accurate voltmeter, compare the scan tool’s voltage parameter with the voltage measured at the battery terminal. If the voltages aren’t within a few tenths of each other, the charging system problem might lie within the ECM or ECM wiring.
The most common cause of intermittent alternator failure is when worn or sticking carbon brushes cause intermittent contact with the rotor slip rings, which is why I never rely on removing an alternator for testing on the part store’s test bench. Testing in-vehicle nearly always provides a definitive diagnosis. Failed alternator diodes, while rare, often reveal themselves as a whining noise in the alternator, accompanied by a loss of charging voltage. Defective alternator diodes can be detected by using alternator test equipment that measures the amount of alternating voltage (commonly known as “ripple”) in the charging circuit.
Last, but not least, measure the voltage drop between the alternator and battery by connecting a digital voltmeter in parallel between the alternator (B+) and the battery B+ terminal. (Note: Make sure to do this with the engine running and with a load applied by running exterior lighting and heater blower fan in the “Hi” position.) Using the same method, measure the voltage drop between the alternator case and battery B-, and also measure between battery B- and the body. Voltage drop in all of the above parallel connections shouldn’t exceed Toyota’s specified 0.200 volts. See Photo 3.
Toyota Testing Procedures
Toyota recommends testing the alternator by first inspecting the alternator wiring. Always wiggle-test the voltage regulator connector at the alternator when testing for an intermittent charging condition. Toyota also recommends inspecting the condition of the drive belt. Modern serpentine belts are made from EPDM rubber that doesn’t crack or peel like older belts. So, belt manufacturers usually supply free wear gauges to measure wear in the V-groove area. Newer vehicles might also incorporate a decoupler pulley that allows the alternator to “coast” during deceleration. If the decoupler pulley fails, the alternator won’t charge because it is “free-wheeling.”
Next, inspect the battery warning light to see if it illuminates as the ignition is turned on and if it turns off as the engine is started. If the battery warning light turns off with the engine running, accelerate the engine speed to 2,000 rpm. If the warning light doesn’t turn off, inspect the charging system as described in the sidebar on page 25, or in Toyota service information.
Toyota recommends that the alternator fuses be tested for both voltage and resistance. According to service data, the Tundra’s charging system is supplied by four fuses: the Alt-H fuse, Alt-S fuse, MET-fuse and LH-IG fuse. According to Toyota’s wiring schematic, the Alt-H fuse connects the alternator stator with the battery. Alt-S allows the voltage regulator to sense battery voltage. The MET fuse connects the instrument panel combination meter with the IGN-2 relay. And, the LH-IG fuse supplies ignition voltage to the voltage regulator.
Due to the availability of poor-quality fuses in the aftermarket, never rely on a visual inspection to confirm the electrical integrity of the fuse. According to Toyota, the specified resistance through all four fuses should be less than 1 ohm. Key-on, battery voltage should also be available on each pin of each fuse. If you’re looking at an intermittent loss of amperage on a replacement alternator, I recommend performing a pin drag test at the fuse box by cutting one pin from an old fuse, which allows the “drag” or the fuse pin to be tested against each female connector in the fuse box.
In our hypothetical alternator comeback, it would be very easy to understand why an average diagnostic tech might overlook the possibility of a poor connection at the fuse box — especially since our 2008 Toyota Tundra charging system incorporates four separate fuses. If an amateur mechanic crammed a fuse tap or wire under a fuse leg to power an accessory, it will, without a doubt, damage the fuse box connection, which, in turn, will cause an intermittent charging complaint.
Courtesy of ImportCar.