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Automotive Electricity 102 (cont.)
Now, let's check the ignition switch. There's a harness from the ignition switch that plugs into a connector in the under-dash fuse box. You won't even have to take the cover off the fuse box—the connector is located above the cover. See photo at right. The connector has a small tab on one side. Push the tab in as you pull the connector out. With that connector disconnected, we've made sure of two things. (1) There is no power in the circuit. (2) The switch is disconnected from any parallel circuit that could cause an erroneous reading. With the connector in your hand, flip it around and take a look at the wire side of the harness connector. (Photo below.) Notice two things. First, you'll see the five colored wires we discussed above. Second, you'll see in the illustration below, glommed from the Service Manual, that Honda usually provides a connector legend as viewed from the wire side of the connector. When we make resistance checks, however, we'll probably do it from the pin side of the connector. So you'll have to mentally reverse the illustration.
First, let's measure between the BAT terminal and the IG1 terminal. In this case, the red test lead is connected to the BAT pin and the black test lead is connected to the IG1 pin. Remember, we're working with a mirror-image of the illustration. With the ignition switch in either the I or II position, there should be zero W of resistance. In any other position, there should be infinite resistance (or the maximum your meter is capable of displaying). Jiggle the switch around and see if the readings change. They shouldn't. If you read anything other than a few ohms, the switch should be replaced.
To continue testing, remove the black test lead and touch it to another connector pin. Using the legend above, you'll know what the expect on every pin—either open (infinite ohms) or shorted (zero ohms). Jiggle the key each time. Move it to and from the position you're testing—each time, you should see a smooth transition from the open position (VERY high resistance) to the closed position (NO resistance). Current Checks Checking current in a circuit can be tricky, and we'll be treating it in future articles as an "advanced topic." For our purposes here and now, we'll need to point out that current testing is generally done with the ammeter in series with the load. Usually, this means the circuit will have to be opened first. For this reason, the fusebox is a common place to make current checks. Just remove the fuse (which will open the circuit) and put the red test lead on the battery side of the fuse connector and the black test lead on the load side. This will measure the current through the entire circuit.
To measure current, you'll usually have to change the connections at your meter. The red test lead will probably go into a different jack on the meter's face. With some meters, the black lead will also have a different jack.
Since all the current in the circuit is going through the meter, you must make sure the circuit won't overload it. Most ammeters can only handle one amp, although many will handle up to 10A. Very rarely will a meter be able to handle more than that. Okay, but what about charging systems and starter motors? They're up into the 60 - 400A range! How do you measure charging or starting current? Easy. Not cheap, but easy. The answer is a current clamp. See illustration at right. A current clamp converts amps to millivolts. (Usually, 1A of current is converted to 1mV of voltage.) The current clamp is connected to the voltage inputs of your meter. Clamp the device around a wire and you can read the current through it. If the meter shows 95mV, that's 95A of current. Just what a Vig's alternator should put out! In future articles, we'll deal with current checks versus voltage checks when diagnosing starter and charging system problems. We'll see the value of current checks in fuel pump circuits. And we'll see what current checks tell us that voltage checks don't. But that's for another day.
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