ELECTRICAL PRIMER

Here's the long awaited (yeah right!) Electrical Primer. What's an electrical primer??? Well, if your electrical troubleshooting consists of checking the fuses or replacing a bulb when something doesn't work then you should benefit from this article. If you are familiar with using a meter and test light, know how a circuit works and are fairly confident when it comes to electrical troubleshooting, you probably know all this stuff.

Have you ever had a bulb go out only to find that when you put in a new one, it still didn't work? This article will attempt to explain the basics of electricity as it applies to automotive systems in a way that the novice, without any experience can determine the causes of basic wiring problems.
First the basics. All electrical systems must have three things: a Source (battery in the case of a VW), a Path (wires) to carry the voltage, and a Load (lights, starter motor etc.…) Put these three things together and you have what is referred to as a circuit. Sounds simple huh? It is after you separate all the other devices that are part of the system such as switches, relays etc.… One thing to note, circuits frequently share common parts, the battery is a good example, it's the Source for the entire car. Many circuits use the same fuse and are common up to that point so realizing where a circuit become distinct (separate) is essential in troubleshooting. Lets discuss some basic electrical concepts.

VOLTAGE

  • Voltage can be of two types, AC (Alternating Current) which is found in a wall socket and DC (Direct Current) which is found in portable electrical systems such as automobiles (this article will focus on DC).
  • Voltage has an amount, 12 volts, 10 volts 6 volts, etc…. The amount 6 or 12 volts is the basis for the car's electrical system and all connected devices must rated for that same amount.
  • Voltage "flow" is referred to as current and is measured in Amps.
  • Voltage is supplied from a "source" such as a battery and DC systems have a Polarity which must be observed.
  • Voltage is positive or negative, all modern automobiles use a positive system.
  • Negative is usually referred to as Ground and is typically the metal of the car's body.
  • Voltage runs circuits such as lights, radio etc. Each circuit must have a source (12 volts from the battery) and a ground, without both, the circuit will not come on or energize.

THE WATER TANK AND PIPES CONCEPT

I like to explain voltage and current using a water tank and pipes as an analogy. Think of the battery as a tank filled with fluid. The size of the tank determines the system's capacity (6 or 12 volt). The amount of water in the tank is akin to the amount of charge in the battery. The pipes (wires) connected to the tank carry the fluid (voltage) to the loads such as the lights, radio starter etc...

CURRENT

The size of the pipe (wire) is referred by a gauge number. The lower the number, the larger the wire. A wire's size determines how much current it can carry which is why the starter has such large cables going to it. The load's (starter, lights etc...) requirements determine the size of the wires that supply it. Back to the watertank, the direction of this flow determines if the tank is filling (battery charging) or emptying (draining). If your lights are on and the motor off, the flow is out of the battery which will eventually run it dead. Conversely, as the engine is running, the alternator (or generator) should be supplying more then is being used keeping the battery charged up.

RESISTANCE

In my water tank and pipes analogy, resistance would be the opposition to the flow of water such as when a drain pipe is clogged. The pipe itself may be large (capable of carrying large amounts of current) but if it's clogged the flow will be minimal. Lets apply that theory to a car's electrical system. Take your headlight circuit, the wires going to your bulbs are fairly large because they require a significant amount of current. Now lets say the wire is corroded or the connections are poor causing it not to receive the current it requires. There may still be 12 volts at the light but the current is being limited by a bad wire or poor connection, now instead of getting the normal 2 or 3 amps, there might only be ½ of an amp flowing through the circuit causing the light to be dim. Are you confused yet? Let me throw another one at you, say the wires and all the connections are good but the battery is down to 8 volts. This will also cause the light to be dim because it's not getting all the required voltage. Knowing the difference between the two is essential to fixing this type of problem.

A note about resistance: Excessive resistance in a circuit results in heat, when anything electrical heats up, it's using electricity(drawing large amounts of amperage) . Corroded battery cables usually get hot, this heat is using a portion of the available voltage which results in less getting to the circuit in question and being available for the car's other circuits.

SWITCHES, SHORTS AND OPENS

The switches are like faucets, they turn the flow on or off. Electrical Shorts are like leaks which result in some amount "flow" when it is not wanted. This flow can drain the tank (battery) or cause load (lights etc…) to operate when the circuit is turned off. An "Open is when a wire is broken or not connected resulting in no flow of current. This shows up as what ever is run by that circuit being inoperative.

Lets apply this concept to a simple circuit such as a flashlight.

Let's apply this concept to an automotive circuit such as the headlight circuit. Remember, this concept can be applied to any load device in your car. In the diagram, the two batteries are just like your car battery although they are connected directly to the bulb. In a VW, there's a large wire running from the battery connection at the starter to the headlight switch, from there it runs through the fuse panel and out to the headlight bulb. Think of this as the positive side of the circuit. If you pull the switch out to turn the lights on they should light up right? Wrong, the circuit hasn't been completed, there is another half that's equally important called the negative side (also referred to as the ground or return). At your headlight bulb, you'll notice another wire that is attached to the body by a screw. The battery's negative terminal is also connected to the body under the back seat. When the lights are turned on, current flows from the (+) terminal to the starter, through the wiring, headlight switch, fuse and out to the bulb. From there it passes through the bulb, into the body and back to the battery's (-) terminal through the negative cable, a complete circuit. It helps to simplify each circuit's wiring for troubleshooting otherwise it can get a bit overwhelming.

TEST EQUIPMENT

So now you have the basic (and I mean BASIC) concept of DC theory, it's useless unless we have a way to check out our circuits. We do this with test equipment. There are two pieces that any self respecting Shadetree mechanic can't do without. They are the Test Light and the Digital Volt Ohm Meter or DVM for short.

A TEST LIGHT is a pointed screwdriver shaped device with a pointed tip and a wire with an alligator clip coming off the handle end. Inside the handle is a light bulb that lights up when the tip is touched to anything that has a voltage providing the clip end is grounded. It's useful as quick way to test for the presence of voltage in a circuit. While this is good for basic troubleshooting, it doesn't tell you how much voltage is present.

A DVM is a hand held meter that provides a digital readout of the measurement when the test leads are connected to a circuit. By obtaining a value, it is possible to test the voltage level in a battery to determine it level of charge or the amount of voltage at a load such as your lights. This can be very helpful in determining why a light might be very dim. This is where a DVM capabilities shine, it's capable of making both those measurements so you can determine which scenario is culprit.


Lets discuss how to use a DVM for each of it's measurements:

ACV - AC Voltage - Used for household voltages (not applicable to this article)

DCV - DC Voltage - This is the required setting for all automotive voltage measurements All voltage measurements are performed in parallel with the circuit. That means you would touch one lead to the circuit, the other to ground. Lets check the voltage to the headlights. With the switch on, you touch the black lead to the body metal at a point where it is free of paint. The red lead is then touched to the wire supplying the headlight bulb. As long as the leads are connected you should see 11 ~12 volts on the display.

OHMS - This is used to measure resistance. In the Ohms setting, the meter provides a small voltage to the (+) lead and measures what returns on the (-) lead. Because of this, resistance measurements must be done with the circuit off and isolated from any power source, the meter is calibrated to it's internal battery and any voltage from the car's battery will throw off the reading and possibly burn up the meter. If the circuit is supplied by a switch, ensuring the switch is off usually enough. A circuit is isolated by disconnecting one of the ends. A reading of 0 indicates no resistance such as a straight piece of wire and is referred to as a short. For basic automotive resistance measurements for wiring, anything below 10 ohms is usually OK and considered a short between the two leads. An open is displayed as "O.L" indicating an open or very high resistance. Let's apply resistance measurement to a common problem, a suspect light switch. Set the meter to resistance, touch the leads together and ensure you get zero. This ensures the meter's battery is good. With the switch disconnected from the wiring, touch one of the leads to the switch's battery terminal (input) and the other to the headlight terminal (output). With the switch off, the reading should indicate "O.L" or open. Turn the switch on and it should go to zero. You have just ran a small current through the switch testing it's operation. Whereas a voltage measurement would prove the same conclusive results with the switch in the circuit and turned on, the use of a resistance measurement is essential in situations where there isn't power available or it's non-existant for some strange reason <grin>.

AUDIBLE CONTINUITY - This is just like the OHM setting except the meter emits a tone when the resistance is zero. I always use this setting when looking for a broken wire or tracing wires. Lets cover a typical continuity measurement. I have a wire under the hood that I know goes to the engine but I'm not sure which one it is at the other end since the previous owner has "done repairs" and the colors change between the front and back. Take the meter and set it to the audible setting, touch the two wires together to test for a beep. Disconnect the wire in question under the hood since resistance measurements must be isolated or disconnected at one end. Connect one of the meter leads (doesn't matter which) to that wire. On the other meter lead, attach a long wire that will reach to the engine compartment. Take the long wire and one by one I touch it to the various connections (except the big one on the alternator, it's live) in the engine compartment. When it touches the same wire the other meter lead is connected to, the meter will emit a beep. You'll have just tracked that wire through the harness into the engine compartment. Conversely, if your taillights don't work and you want to track down the problem, the same procedure is done. Disconnect the taillight wire under the hood and connect it to the meter, go to the back and touch the other lead to the taillight wire back there, if it beeps, the wire is not broken back to where you checked it. If it didn't beep, there is a break or disconnection between the front meter connection and my test point in the back of the car. Use the audible setting when you aren't concerned with a specific resistance value, just if a wire is broke (open circuit), shorted or for wire identification. Lets cover testing for a short, say the tail light circuit keeps blowing fuses, you know it's shorted but aren't sure where. Like the measurement above, disconnect the taillight wire at the fuse panel. Connect a meter lead to that wire. Touch the other lead to a bare spot on the body, if it beeps you have just confirmed that the wire you connected to is infact shorted to the body somewhere. If it doesn't beep, then the problem is probably "before" the point that you disconnected the wire. For discussion purposes, lets say it did beep, how will you track it back? Here's where the concept of troubleshooting comes into play. You must selectively isolate the circuit. What is the next point you could break the circuit without cutting a wire? That would be the connection in the engine compartment. So, leaving the wire under the hood disconnected take your meter and both leads to the engine compartment and disconnect the taillight junction. You'll have three wires, one from the front and one to each taillight. Touch one lead to ground and the other lead to each of the three wires one at a time, one of them will beep. Whichever it is will tell you where your short is at. You have just isolated the problem to a much smaller section of wire and it's an easy matter to examine the wires to look for a bare wire rubbing the body.

CURRENT - Current measurements, displayed in AMPS are useful to determine the health of your charging system. I don't recommend doing this measurement until you are comfortable with ALL the concepts explained in this article. I'd consider it a last resort for the novice since it's concept is probably the most in-depth and requires a good understanding of voltage vs. current. Also, if done improperly, there's a good possibility you could burn up your meter or short out your car's wiring. Amperage measurements a done in series, anotherwords you setup the test with the circuit running through the meter. An important note on AMP measurements: The starter draws in excess of 60 amps, the meter is probably only rated for 10 max (check your meter, unless it's rated for at least 10 AMPS, forget doing current measurements). This means you couldn't run the starter circuit or a main battery connection through your meter, if you do, the smoke will leak out of the meter..lol. This type of measurement requires a noninvasive type device known as a clamp-on meter. Lets use the alternator as an example for a current measurement You would place your meter in the AMPS setting and move the red lead to the 10A socket on the meter, if you forget to move the lead on the meter, you will burn it up with this test! Disconnect the large red lead from the top of the alternator (careful, it's live) and connect it to the negative (black) meter lead. Connect the positive (red) lead to the terminal on the alternator (internally regulated type). With this configuration, the current coming from or going to the alternator must run through the meter. Now start the car, as it's idling, the output should be low -1A to + 3A. Slowly rev it up and the reading should climb toward +10Amps. Stop before reaching 10 Amps or you'll smoke the meter. The fact that it climbed I proof that the system is working. Meter polarity isn't critical during current measurements, if you have the connections backwards, the reading will be negative when it should be positive and viseversa. Remember, since loads draw current the reading should be negative and since alternators/generators provide current, their measurements should be positive (with the engine above 1200 rpms) If the leads are backwards, simply switch them around.

REVIEW:

Purchase the basic electrical test equipment, a Test Light ($10) and a Digital Volt Meter ($20~30).

VOLTAGE - Familiarize yourself with doing basic tests such as measuring voltage. Testing at the back of the fuse box is a good place to start. Voltage is measured by touching the black lead to the body and the red lead to the connection under test. Use this basic concept for any circuit you want to test. Voltage tests are a good way to find open (broke/disconnected) wires.

RESISTANCE - This is used to test continuity of a wire or device. Continuity tells you if you have a "connection" between the two points. Remember to disconnect one end of the circuit. Use ether the OHM reading if you can see the meter during the measurement or use the Audible setting if you can't. Always touch the leads together before taking any resistance measurements, if the meter doesn't zero out (or beep), replace the battery. Use this measurement for finding shorted, broke or disconnected when power isn't available to use a test light or DVM volt test.

CURRENT - Checks the "Flow" into or out of a device. Good measurement for charging systems. Current tests are done in series with the circuit running through the meter. Check you meter's rating and put the leads in the proper meter jacks. Remember, a shorted circuit causing the battery to drain, a bad battery or bad battery cables appear to be a charging problem, always check those before doing a current measurement on the charging system.

That pretty much sums up my electrical primer. Once you feel comfortable with diving into your electrical system take a look at the Electrical Basics Tech Article, it will show you how to use the VW wiring diagram and circuit designators to track a circuit through your VW. If you have a specific question you can e-mail me at the site's webmaster link and I'll try to help.

Chris R.