www.acuravigorclub.com	Timely Topics Archive
A Monthly Article for Vigor Enthusiasts (1/04)

Automotive Electricity 102

Here's a chance to build on and apply what we discussed last month—the three aspects of electrical circuits and components: voltage, current, and resistance. Our objective with these two articles is to enable you to ...

To this, let's add, "... without harming the car or yourself."  Before we get out the meters and begin to make these tests, let's let's take a minute to stop and think—if I measure the voltage here, and it says 12V... what does that tell me? Before you measure a voltage, current, or resistance, you have to know what results you expect to see. Otherwise, you won't be able to tell whether the circuit or component is good. You have to know ahead of time what to expect in a "good" circuit. If you don't find that, you'll have to make further checks to determine why. Fortunately, the behavior of electrical circuits is usually easy to predict.

Series Circuit Behavior

We saw last month how electricity acts, in a way, similar to an air compressor. We saw how this behavior is described by Ohm's Law as E = I x R, or Voltage (in Volts) = Current (in Amperes) x Resistance (in Ohms). Now, let's see what this means within a circuit.

• First, the current (rate of flow) in any circuit is the same throughout the circuit.
• However, the voltage (pressure) in a circuit will be reduced by each load along the circuit. That is, each load produces a voltage drop as it "dissipates energy" and does its thing.
• Each load is a circuit resistance. It may be a small light bulb, a starter motor, a speedometer, a stereo system... but it's gonna be something that dissipates energy. The load is the reason for the circuit.
• On the other hand, a piece of wire, a switch, a fuse, a set of relay contacts, a connector assembly... these do not dissipate energy. Their job is to allow electrical continuity. Ideally, they provide no resistance and should produce no voltage drop.

One last thing. A circuit will "drop" all the voltage available.  Let's see how this works.

	Simple Series Circuit

The circuit at right has two resistances in series. (Switches and fuses are omitted for simplicity.) Lets say that R1 is an LED indicator and R2 is a relay coil. When current flows through the coil, energizing the relay, current will also flow through the LED since the components are connected in series. The LED will light up and let us know that the circuit is activated (eg., the cruise control light, or the rear window defroster light).

To make this explanation less unbearable, let's use some simple numbers. The supply voltage is 12V, the Vigor's nominal battery voltage.

The total resistance of this circuit is 12W—that is, R1 (3W) plus R2 (9W) equals 12W.

Ohm's Law tells us that E = I x R, or this case, I = E ¸ R. Therefore, this circuit will draw one amp of current (I  = 12V ¸ 12W = 1A). Since the current in a circuit is the same throughout the circuit, one amp of current will flow through R1 and one amp of current will flow through R2.

If R1 has 9W of resistance, and has one amp of current passing through it, then R1 will drop 9 volts. (E = 1A x 9W = 9V). If we start with a 12V supply and drop 9V, that only leaves 3V for R2. Hmmm.

Fortunately, Ohm's Law tells us that R2 will drop 3 volts. (E = 1A x 3W = 3V).

Together, R1 and R2 will drop 12V, which is all the voltage available in the circuit. And the universe remains at peace.

Parallel Circuit Behavior

Even though they follow Ohm's Law, parallel circuits behave quite differently. For starters, they present a different resistance to current flow. In a series circuit, as we just saw, you simply add up the resistances to get the total. R_tot = R1 + R2 + R3 + ...

But the total resistance of a 2-component parallel circuit is determined by the product over the sum.

In the below example, if the same two resistances (R1 and R2) were connected in parallel instead of in series, their total resistance would be 2.25W instead of 12W! That's quite a difference!

			Rtot = R1 x R2 =   3W x 9W = 27W   = 2.25W R1 + R2   3W + 9W 12W
Simple Parallel Circuit

That means that the total current flow through the circuit (I = E ¸ R) will be 5.33 amps—( I = 12V ¸ 2.25W = 5.33A)—instead of 1A in the series circuit! Over five times the current!

NOTE: If there are three or more resistances, the following formula is used:

One last thing. Since the 12V supply is connected to the top of both resistances... and ground is connected to the bottom side of both resistances... they will both drop the full 12V. (This applies to circuits with three or more resistances, too.) But the current will split among the paths in accordance with Ohm's Law (I = E ¸ R). That is, R1 will draw (12V ¸ 9W = ) 1.33 amps. Likewise, R2 will draw (12V ¸ 3W = ) 4 amps. And if we add the 1.33 amps to the 4 amps, we get a total current flow of... 5.33 amps! And the universe still remains at peace.

Summing Up

Let's summarize what we've seen so far. In a series circuit, the total resistance is the sum of all the resistances. The same amount of current flows through each resistance. Each resistance drops voltage individually.

In a parallel circuit, the total resistance is not the sum of the resistances. Instead, we have to use algebra. The current splits up and divides itself through the parallel legs. The voltage drop is across the entire parallel circuit.

And that's all there is to it!

Voltage Checks

If you stop and think about voltage (electrical pressure), every circuit will be in one of five conditions:

Four Voltage Conditions

(1) Everything shown is red is hot all the time. (2) Everything shown in orange is ground (zero volts) all the time.

Okay, these two are easy to see. But look at the second circuit, where there's a switch between the power and the load. NOTE: This is the schematic representation of a switch that Honda uses in their documentation. (3) The yellow line on the right side of the switch identifies it as power only when the switch is ON. If the switch is closed, your voltmeter will read the battery voltage. If the switch is open, the voltmeter will read zero volts. But is that the same as ground? No. There's still the load resistance between that point and ground. It's important to understand that zero volts doesn't necessarily mean ground.

In the third circuit, the switch has been moved to the "ground side" of the resistance. In this case, (4) the green segment will be ground only when the switch is ON.

The fifth possible condition is shown in the last circuit. (5) The blue are will be a varying voltage. This is a variable resistor that changes voltage levels according to the physical position of the "wiper arm.". It may be a throttle position sensor, a dimmer switch, a stereo volume or tone control...

	Measuring Voltage

Test Connections for Voltage Checks

To measure voltage, you must connect your voltmeter in parallel with the load you're measuring.

To connect the test leads to your meter, follow the instructions that came with it.

Then, connect the red test lead to the positive side of the load and connect the black test lead to the negative side. In most cases, you'll want to measure "voltage to ground," so you'll simply connect the black test lead to any chassis ground point.

If you want to measure the voltage of the battery, connect the test leads the same way—red test lead to the battery positive terminal and black to the negative.

In fact, since all voltage is supplied by the battery, let's go there. Connect the red test lead to the positive terminal and the black test lead to the negative terminal. Your meter will indicate battery voltage.

Move the black test lead to any good ground—a mounting nut on the shock tower is usually a handy ground, but so is almost any bolt that passes through the chassis. Notice that your meter still indicates battery voltage. This is because, electrically, the negative battery terminal is the same as any other chassis or engine ground.

Most other voltage checks will require that you disconnect an electrical connector to  measure the voltage to a component. For example, if you want to check a light bulb circuit, you'll have to disconnect the connector to that bulb and measure the voltage at the connector. When you're doing this, be very careful with your test leads! Make sure they don't touch each another... or anything else! A short circuit could result, causing components to burn up, fuses to blow, wires to overheat...

In future articles, we'll see how to use voltage checks to verify (or condemn) the sensors used to control engine management, ABS operation, etc.

Resistance Checks

	Measuring Resistance

Resistance checks are made with the ohmmeter in paralll with the resistance, just like voltage checks. However, when using an ohmmeter, make sure the circuit does not have power! Otherwise, you'll be in the market for a new ohmmeter. Also, when making resistance checks, the test leads are interchangeable

There's one other thing to keep in mind when making resistance checks—parallel circuits. In the example of a parallel circuit above, we saw that two resistances in parallel will yield a total resistance much different from either of the actual resistors. Using that example, if you tried to measure the resistance of R1 (9W) or R2 (3W), your ohmmeter would indicate 2.25W! To prevent this, you must remove the component from the circuit before measuring its resistance.

The Ignition Switch

Now we'll make some resistance checks. First, let's check the schematic of the ignition switch.

Ignition Switch Schematic

The switch itself is a multi-layered affair, making various interconnections at different levels. There's a "legend" to a switch like this In the case of a Vigor ignition switch, it's...

This means when the switch is in O position, there are no connections anywhere in the switch.

In the I position, battery voltage from the WHT wire is connected to the BLK/WHT wire on the ACC terminal.

When the switch is in the II position, battery voltage is applied to three wires: to the WHT/BLK wire on the ACC terminal, the BLK/YEL wire on the IG2 terminal, and the YEL wire on the IG2 terminal.

Position III is the START position, and battery voltage is applied to the BLK/YEL wire (IG1) and the BLK/WHT wire that goes to the starter, from the switch's START terminal.

Continue

 - Top -