The alternator functions as the heart of the automobile's electrical system. It not only cycles electricity back to the car battery for recharging purposes, it must also maintain enough output to assist the battery in powering all the vehicle's accessories. Fuses protect the alternator against overload and shorts that could otherwise damage the electrical system. Many electrical system failures occur when a fuse shorts or otherwise fails. Fortunately, troubleshooting the alternator fuses is a simple process.
Place the vehicle in park or neutral with the emergency brake set. Open the bonnet and locate the alternator. On many vehicles, the alternator is located just below or near the power steering pump. It looks like a round canister made of aluminium or magnesium with vents in its housing. On other models, such as older Chevrolets, the alternator sits high on a bracket in plain view. Inspect the wires on the back of the alternator and note the largest wire that might be marked "Bat', which stands for Battery.
Trace the large alternator wire along its path toward the battery. Look for a small tubelike object that has plastic screw-on connectors on each end. Be sure to lift up the wiring harness and examine any tape windings on the loom. Look underneath them to find the main fusible link. Unscrew or unsnap the link from two connector points at each end, usually male and female plug connectors.
Examine the condition of the fuse, whether it has a glass tube design or a plastic container with a small piece of metal inside. Any break in the metal filament will denote a blown fusible link that must be replaced. Replace the fuse with a fuse of the same amperage rating. Look for any additional aftermarket in-line fuses in the loom. Many times an additional fusible link will be located on the positive (red) battery wire a few inches away from the positive battery post.
Refer to your owner's manual for the location of the main fuse block. The fuse block contains all of the fuses and relays for accessories that receive electricity. Check inside the glove box, the plastic front seat kick panels, or under the bonnet, where the fuse block will be mounted in plain sight. Remove the cover to the fuse block.
Examine the fuse diagram schematic on the inside of the fuse block cover. Look for "Alt" or "Bat" fuses or relays. "Alt" stands for alternator, and "Bat" stands for battery. Relays will be square plastic boxes with multiple connector spades. Carefully remove each fuse and examine the condition of the small metal filament. Fuses will be glass tube types or push-in spade types. Replace any fuse that has a burnt or missing filament.
Pull out any relay marked "Bat" or "Alt." You can replace the relay with an identical relay in the fuse block by simply plugging another duplicate into the slot. Use a test light and stick the probe into the slots on the fuse block while the engine runs. This will tell you if the fuse block has a voltage signal. Remove the suspect relay while the engine idles. Look at the dashboard while pulling the relay. If a dashboard indicator light marked "Alt", "Bat" or "Charging" comes on, this indicates a good relay. No light would indicate a shorted relay.