Honk if you'd love an electrical lesson

Q: My horn quit working on my '99 Civic; I was hoping to fix it myself. Can you offer pointers?

A: Fixing your horn will be a great way to learn and apply basic electrical tests.

Think of your battery as an elevated water tank; the wires are pipes; switches and relays are valves; and the horn (the load, or purpose of the circuit) is a water wheel you wish to spin. The tank supplies water under pressure (volts), and the circuit directs the flow of water (current, measured in amps).

The most common electrical problem is high resistance (measured in ohms), the equivalent of a clogged pipe, caused by loose or dirty electrical connections.

Open circuits are next; think of them as broken pipes. They're caused by a blown fuse, disconnection, bad switch or a burnt-out load.

A short circuit, the least likely, is like a misdirected pipe — electricity goes places it shouldn't.

The first step in diagnosis is to be sure what works and what doesn't. If an additional circuit is also not working, look for things they have in common, such as a fuse, shared wiring, or a ground connection. If just the horn is faulty, we can rule out things shared with other circuits.

The next step is to look at a wiring diagram, and create a testing plan. Many carmakers, particularly Honda, display each circuit individually and provide location photos and other really helpful information. Public libraries often subscribe to ALLDATA or Mitchell On-demand, making these pro-level information systems available to everyone.

Next: tools. An inexpensive test light can be used on simple circuits and those that don't work at all. Computer controlled circuits and those that work poorly (typically caused by high resistance) require use of a digital multimeter. This tool is gentle to the circuits being tested and is accurate enough to spot even tiny problems. A basic DMM can be purchased for about $20.

To be continued next week.

E-mail Brad Bergholdt at under-the-hood@earthlink.net.