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| Joined: | 03-12-2005 |
| Location: | Kingman, Arizona USA |
| Posts: | 532 |
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According to the shop manual, the steady state power consumption of the various lights in the headlights circuit is:
Headlights 50W x2 = 100W (high beams)
Side markers 6W x4 = 24W
Tail lights 21W x2 = 42W
Instruments 2.2W x8 = 17.6W (6 gauges, heater console, hazard switch)
License plate 6W x2 = 12W
The total is 195.6 watts. Since watts equals volts times current, at the miminum system voltage of 12.0 volts the total current is 16.3 amps. That means that a fuse rated for a steady state load of 20 amps is just about right. Therefore, if your fuse blew when you switched on the lights, either there was a (perhaps momentary) short, or the fuse failed due to internal metal fatigue. And fuse fatigue failures are extremely rare unless the fuse is being used in a circuit with a history of shorts, near-shorts, or overloads.
Normally the only time when it makes a difference whether power for a car's electrical bits is provided by the alternator, the battery, or both, is when the alternator regulator has failed and the alternator tries to compete with the nearest electrical utility. This is the sort of rare event that causes wiring harnesses to melt, so blowing fuses would then be a good thing.