87 4runner 15A engine fuse / alternator problem

[tim7802]

When I turn on the ignition switch, the 15A engine fuse (fuse#8) blows in about 5 seconds. ( none of the gauges work, turn signals don't work---) This happens without the engine running. Without the engine running, the voltage across the battery terminals is about 12.6V. When I start the engine, the voltage does not increase - it stays at about 12.6V. When the lights and fan are turned on, the voltage drops. This seems to indicate that the alternator is not working. If the engine fuse is blown, will that prevent the alternator from working? Could there be a short in the alternator causing the fuse to blow?

[tim7802]

When I disconnected the backup switch, the engine fuse quit blowing and the alternator changed normally.

This is a good site for information on how the alternator works http://www.autoshop101.com/forms/h8.pdf

[fourwd1]

Did you figure out what in the b/u light circuit is causing the problem?

[tim7802]

I don't know for sure yet. I cleaned the connector to the back-up light switch with electric parts cleaner. When I connected the back-up switch and turned on the ignition switch, the 15A engine fuse blew again. I didn't see anything in the service manual about how to test the backup switch, so next weekend I am going to buy a new one an try it.