The MET fuse connects the instrument panel combination meter with the IGN-2 relay. Alt-S allows the voltage regulator to sense battery voltage.
![toyota diagnostic system toyota diagnostic system](https://www.fueleconomysolutions.com.au/assets/thumbL/iLink400TOYOTA.jpg)
According to Toyota’s wiring schematic, the Alt-H fuse connects the alternator stator with the battery. 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. Toyota recommends that the alternator fuses be tested for both voltage and resistance. Photo 4: This inductive ammeter, which is clamped on the alternator’s B+ output wire, indicates 33.57 amperes charging output. Voltage drop in all of the above parallel connections shouldn’t exceed Toyota’s specified 0.200 volts. (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. 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. 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. Failed alternator diodes, while rare, often reveal themselves as a whining noise in the alternator, accompanied by a loss of charging voltage. Testing in-vehicle nearly always provides a definitive diagnosis. 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. If the voltages aren’t within a few tenths of each other, the charging system problem might lie within the ECM or ECM wiring. Using an accurate voltmeter, compare the scan tool’s voltage parameter with the voltage measured at the battery terminal. Most scan tools also display an electrical system voltage parameter. Some charging systems also contain bidirectional controls designed to adjust alternator output. The scan tool might also reveal intermittent alternator problems by displaying DTCs indicating extremely low or high voltages in the electrical system.
#Toyota diagnostic system software
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. 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. Some AGM batteries might produce as much as 12.9 OCV.
![toyota diagnostic system toyota diagnostic system](https://www.stahlcar.nz/image/cache/cache/1-1000/328/main/4178-5ad5f7352401e-0-1-1000x1000.jpg)
Photo 2: A fully charged wet cell battery with the surface charge removed should produce about 12.69 open-circuit volts (OCV).