In this and the next chapter, you’re going to learn the basics of general automotive electronics. No matter what year vehicle you work on, everything in these two chapters is applicable.
Included here are the ignition switch, the wiring harness itself, controllers, and protection.
This Tech Tip is From the Full Book, AUTOMOTIVE WIRING AND ELECTRICAL SYSTEMS. For a comprehensive guide on this entire subject you can visit this link:
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The Ignition Switch
I like to think of the ignition switch as the gatekeeper. The ignition switch is simply the key to it all! The position of the switch determines the status and operation of most every accessory in the vehicle. These are the switch positions:
Note: The ACCESSORY position can be entered by turning the key either back or one click forward, depending on the make of vehicle.
As you would expect, the other side of the switch has many large-gauge wires connected to it. These are its input and outputs, and here are their functions:
- MAIN POWER Input
- ACCESSORY (ACCY) Output
- IGNITION/RUN (IGN/RUN) Output
- HEATER/AC Output
- START Output
Many vehicles on the road today have two or more outputs for each of the above due to the sheer number of circuits in these vehicles and their total current requirements. In addition, some vehicles may have multiple inputs for the switch as a result of these current requirements. To better keep things straight, let’s talk about what’s connected to what based on what position the ignition switch is in:
- OFF—self explanatory
- ACCY—ACCY circuits only
- IGNITION/RUN—IGNITION, ACCY, and HEATER/AC circuits
- START—IGNITION and START circuits only
There is a very simple reason why the switch works the way it does. When starting the vehicle, for example, all of the battery’s available current is directed to the IGNITION circuits and the STARTer motor.
Now, let’s say we were installing a tachometer. Would we want to connect its power feed to the ACCY or IGN/RUN circuit? Well, we should first ask when we want the tach to operate. I’m thinking that it would best serve us with the engine running and that we probably wouldn’t require it any other time. We connect it to the IGN/RUN circuit so that:
It didn’t switch on, off, then on again while we moved the ignition switch from the OFF to START and back to the IGN/RUN positions as we started the vehicle.
It wasn’t on needlessly consuming power, in the event that we were sitting parked and listening to the radio.
If you don’t have a wiring diagram of your ignition switch handy, the above chart outlines how to use your DMM to determine which wire is which on the back of the switch.
The Wiring Harness
Think of the wiring harness as the vehicle’s central nervous system. No matter how complex or simple it is, its function is to connect the various electronic components to their associated controllers so that everything works in harmony. As vehicle technology and the features within them has increased over the years, so has the complexity and size of the wiring harness. This has always been a concern of automakers because it adds weight and generally speaking, weight has a negative impact on fuel economy. Early on, automakers made the decision to halve that weight by using the chassis of the vehicle as a common point for all things negative. This is typically referred to as the return path.
This philosophy has presented its fair share of challenges for vehicle designers, engineers, and anyone adding aftermarket electronics! Here are just a few such challenges:
- High resistance return path: By using the chassis of the vehicle for all accessory grounds, each accessory has to “seek” the ground of the vehicle’s charging system through numerous metal panels that make up the chassis, which are typically joined by spot welds. As the vehicle ages and the elements take their toll on the metal, it puts additional stress on the wiring and/or connectors due to the increased resistance of the return path. This can cause any number of difficult-to-locate problems.
- Noise: By having all of the accessories seek ground through the chassis, a tremendous amount of “noise” can be present just about anywhere on the chassis. This noise is simply a byproduct of the many various accessories doing their job and can be in the form of pops, clicks, ticks, or even noise from the alternator. This noise can cause erratic operation of delicate or sensitive electronics if:
- It travels up its own ground wire due to poor filtering.
- It is picked up inductively by either its chassis or wiring—this is called induced noise.
Rest assured the engineers of the electronics on-board the modern vehicle have taken both of the above into consideration so they have high immunity to such noise. That may not be the case for the aftermarket CD player you’re adding, though. Over the years I’ve experienced my fair share of troubleshooting time with such problems, and they can be extremely difficult to resolve. There is simply no substitute for experience in these cases.
Since the beginning of auto manufacturing, automakers have gone out of their way to keep the wiring out of sight of the occupants. So, where is it all? The bulk of it has always been behind the dashboard. Runs from there to the following are common:
- Through the firewall to power the underhood accessories, such as the ignition system, headlights, parking lights, horn, etc.
- Down the kick panel(s), under the sill panels and carpet, to the rear of the vehicle to power accessories, such as brake lights, parking lights, back-up lights, fuel pump, etc.
- Through the doorjambs and into the doors to power accessories, such as power door locks, power mirrors, power windows, etc.
Again, as the number of on-board electronic features increases, so does the complexity of the wiring harness. If you’re working on a late model vehicle, assume some of the vehicles harness, and even control modules, could be just about anywhere!
Designation of Harnesses for Safety Equipment: Thankfully, the wiring for any on-board safety equipment (SRS systems, ABS brakes, etc.) is typically clearly called out. Some harnesses are yellow, others orange, and others red or green. In other cases, such as my wife’s Nissan truck, the harnesses are black, but the plug ends are bright yellow. This varies by vehicle, but assume these brightly colored harnesses and plugs are verboten!
CAUTION: Do not attempt to open, connect to, or otherwise service harnesses designated with brightly colored tape or split loom. To do so is to risk damaging the fragile and sensitive circuits that control the vehicle’s on-board safety components and/or computers. Leave this to the pros!
If your hot rod is a late model one, then you may want to consider purchasing a shop manual that has a diagram of the wiring harness. This certainly can’t hurt to have around. On the other hand, if you’re restoring a ’50 Merc, rest assured it has a relatively simple wiring harness and most of it is self explanatory.
If you have to troubleshoot a problem in a vehicle with a complex wiring harness, then you’re best advised to purchase a manual that shows an exploded view of this. You can buy a consumer manual at your local auto parts store from Haynes or Chilton that has the basics. You should also check with your local library because many of them have quite a selection of these on the shelf. A more advanced manual can sometimes be purchased directly from the manufacturer. When I was in the shop, we relied on manuals from Mitchell, and they were extremely accurate. Figure 4-1 is what you’re likely to see when looking at such information today.
Controllers: the Basics
OK, now that you know about the ignition switch and the wiring harness, you should understand their roles. As I said earlier, I consider the ignition switch to be the gatekeeper. It is the master control switch in the vehicle and ultimately allows the passage of power to all of the vehicle’s accessories—save for any that function with the key in the off position. A controller is considered anything that controls or governs the operation of an accessory—be it a simple switch or an on-board computer.
Switches are the simplest of all controllers. Most are manual, meaning that the operator of the vehicle has to operate them manually, such as a turn signal switch. Switches are typically used to control low- to medium-current accessories. Higher current switches are sometimes used for high-powered accessories, such as headlights or fog lights. When using a switch of any kind, it is important to match its current rating to that of the accessory it is controlling—a little bigger is never a bad idea.
Single Pole Single Throw: Commonly referred to as S.P.S.T., this is the simplest of all switches (Figure 4-2). As its name implies, it has a single pole (common) and a single electrical path that can be thrown open or closed. These switches are available in toggles, pushbutton, momentary, and many other styles.
In the on position, the switch allows current to flow from the pole to the accessory. In the off position, the switch interrupts the flow of current to the accessory. These switches are easily identified, as they have only two electrical terminals.
Single Pole Dual Throw: The single pole dual throw (Figure 4-3) is commonly referred to as S.P.D.T. This switch is simple in concept, but incredibly versatile as it can be used in a great many arrangements. This switch doesn’t have traditional on or off positions.
understand S.P.S.T. and S.P.D.T. switches, you should also understand the rest—including a D.P.D.T., for example. You can easily determine what kind of switch you’re dealing with by checking for continuity between terminals with your DMM—remember, to do so, remove the switch from the circuit. (If you’re buying one for a project, most switch manufacturers provide an electrical contact diagram on the back of the packaging or on the switch itself.)
Center OFF: Similar looking to a S.P.D.T., a Center OFF switch has three terminals, a C and two N.O. terminals. In the center position, the C is not connected to anything. In the up position, the C is connected to one of the N.O. terminals and in the down position, the C is connected to the other. A switch like this (Figure 4-4) is what automakers use to control the turn signals. Painless uses a Center OFF switch to control the headlights and parking lights in the panel I have in my Olds. By adding a simple diode to the switch, the up position allows the parking lights only to be on, and the down position allows both the headlights and the parking lights to be on—clever. (See the sidebar “Diodes” for an explanation.)
Rheostats: A rheostat (Figure 4-5) is a switch that allows one to vary how much voltage is applied to an accessory. In addition, a rheostat is typically designed to allow the passage of a considerable amount of current. The dimmer switch controller for your dash lights is one such example.
A rheostat is really nothing more than a variable resistor. It has a wiper that rides along a carbon contact surface that has a varying level of resistance, which is linear by design. This allows the switch to have any voltage between 0 VDC and 12 VDC available at its output, based on where the dial is manually set.
Current Ratings: Most switches have a specified current rating. This is the amount of current that the switch can safely pass through its contacts. This doesn’t mean that a 15-amp switch is the perfect choice for a 15-amp load. In fact, OEMs typically use a switch with a higher current rating than the load connected to it, so that the switch provides many years of service. Exceeding the current specification can cause the switch’s contacts to fail prematurely. In addition, this can be a fire hazard.
As I said earlier, this is my absolute favorite switch. Sometimes referred to as a “Bosch” relay, these are readily available with current ratings up to 40 amps. It’s no secret; the typical automotive relay has been shrouded in a veil of confusion for as long as I can remember. A relay (Figure 4-6) is simply an electromagnetic switch, so you already know what it does. The difference between a simple switch and a relay is how the switching is done. A switch is manual while a relay’s switching occurs when voltage is applied across its coil.
There are three main reasons automakers choose to use relays:
- Reliability: The current requirements of the accessory are in excess of 10 amps or so. Relays are often used in this application instead of a high-current switch for reliability.
- Use a low-current controller to operate a high-current accessory. This allows the high current required by the accessory to be routed through the relay’s contacts and the controller only has to power the coil. (Typical Bosch-style relays only require about 110 mA of current to power the coil. This makes relays friendly for the low-current outputs of the many ECMs, PCMs, and other controllers of the modern vehicle.)
- Serviceability: By locating the relays in a central spot, it is easy for a service technician to troubleshoot and diagnose a problem, then pull and replace a defective part. This is far less labor intensive than replacing a switch and it’s also a lot less likely to happen to begin with.
Unlocking the Mystery: OK, so why are relays typically regarded as black magic? Simple, if you don’t understand the different types of switches, you don’t have the foundation required to understand the relay. As a result, I’ve run into very few that actually understood what these little black boxes do and how they do it. Since you already know all about switches, the relay is simply an extension of that knowledge.
No different than switches, relays are also available in many different variations. Electrically speaking, they’re identical to switches of the same type with one exception—the switching is done electrically. This means that a relay really has two electrically isolated parts:
- The switch.
- The coil—when powered, this causes the switch to be thrown.
S.P.S.T. relays are the simplest, just like the switch of the same name. They can have four or five terminals. Even though they may look similar from the bottom, not all five terminal S.P.S.T. relays are identical.
S.P.D.T. relays are the most versatile and all have five terminals. I believe that this is where some of the confusion begins. After all, the five terminal S.P.S.T and the S.P.D.T. relays look identical, but electrically they aren’t even close. Diagram 4-15 illustrates the differences between the four most commonly used automotive relays.
How do you know for sure what kind of relay you have or need to purchase when looking at them in the store? Easy—the body of the relay itself typically has an electrical diagram embossed or stamped on it; this is called the legend, and it is similar to the diagrams in Figure 4-7, allowing you to determine which relay you need. All relays that I’ve ever seen have numbers embossed on their bottoms in the plastic next to the electrical terminals themselves. Here’s how to decode them:
- 85—Coil input
- 86—Coil input
- 87—Normally Open
- 87a—Normally Closed
Only S.P.D.T. relays have a terminal labeled 87a. Five-terminal S.P.S.T. relays typically have two 87 terminals as pictured.
The Bosch “silver can with green stripe” five-terminal S.P.S.T. relay has the following:
- 87b — Normally Open #2
The legends on the housings of this S.P.S.T. relay and their standard five terminal S.P.S.T. show that these relays are electrically different from one another—this is illustrated in Figure 4-7. This means that you can-not use one of these in a circuit that calls for the other.
To power the coil of a relay, voltage needs to be applied across terminals 85 and 86. This means that one terminal has to have +12VDC and the other side ground. Obviously, a coil of wire has no polarity, so it really doesn’t matter which side is connected to which. That being said, some relays have built in quenching diodes (see the sidebar on diodes for an explanation) so polarity must be observed with these. This should also be clearly labeled in the relay’s legend—remember, the stripe denotes the cathode of the diode and it is connected in reverse bias.
S.P.S.T. relays are simple. The coil is powered to establish an electrical connection between the common and normally open terminals. Note the electrical difference in the two five-terminal S.P.S.T. relays that I mentioned above—specifically, the Bosch silver can with green stripe relay allows the normally opens to be isolated from one another when the coil is at rest.
S.P.D.T. relays are just as simple as there are only two positions they can be in, electrically speaking:
- Coil un-powered—terminal 30 connected to terminal 87a (C to N.C.)
- Coil powered—terminal 30 connected to terminal 87 (C to N.O.)
See, I told you relays were simple. No different than an S.P.D.T. switch, S.P.D.T. relays can have numerous applications due to their versatility.(The next chapter explains how to use them in your own projects.)
Obviously, it’s important to understand the difference between the various types of relays. If you had to replace one that went bad over time for whatever reason—you’d better replace it with a new unit that was electrically identical, otherwise the circuit does not work correctly.
Current Ratings: No different than switches, relays have current ratings as well. The current rating of a relay is the amount of current that can safely pass through the contacts and has nothing to do with the current the coil requires. A 30-amp relay does not require 30 amps of current to power it up. Rather, that means that the relay can power an accessory requiring up to 30 amps of current. Again, the OEMs choose to use higher current relays than necessary so that they last a long time.
There are different grades of relays. Hella, Potter & Brumfield, Tyco, and Bosch are but a few of the many manufacturers that make quality relays. Of course you know that I have a personal preference, and that has been the Bosch-branded relays.
To date, I’ve never pulled a properly installed defective Bosch relay out of a vehicle. That’s a pretty impressive track record, especially considering that I’ve also used thousands of them over the years in projects of all types. (The current required to properly power the accessory determines which relay to choose and install. Using a 30-amp relay to power a 40-amp accessory causes its contacts to fail prematurely because they become pitted. In addition, a relay should always be mounted with its terminals pointing down to avoid the possibility of water entering its case.)
Relay Centers: Most vehicles have a central location for all of the relays used for the high current accessories. Depending on the vehicle, this can be under the dash or under the hood. My wife’s Nissan Frontier combines the underhood fuse panel and relay center as one.
In my Olds, I made my own relay center by locating all the relays on the firewall just above the transmission hump. This was done for serviceability because I know all the relays in the vehicle (except one) are in this location.
You should make a point to know if your vehicle has a relay center and more importantly where it is. The owner’s manual of your vehicle should provide this information.
Chapter 5 offers gives several scenarios of how to use relays in your own projects. By the time you’re done with this book, the relay will be second nature to you.
Solenoids and Switching
Now that you know all about relays, you know about switching solenoids. Solenoids are used to do all kinds of things—electrical and non-electrical. As usual, I’ll stick to the ones for electrical duty. An electrical automotive switching solenoid is really nothing more than a very high current relay.
Another example is the high-current aftermarket units shown. These are used for any number of things. One example is to disconnect an auxiliary battery from the vehicle’s charging system when the Ignition switch is in the off position. (More on this in Chapter 7.)
Sometimes the case of the solenoid itself can be the negative connection to the coil. If so, it needs to be solidly mounted to a clean metal surface. When voltage is applied to the trigger terminal, its contacts close, thereby making the high-current electrical connection between the battery and load terminals.
Flashers are typically found in the fuse panel and used in the turn signal and hazard circuits. Typically called LX flashers, they have two terminals only. One terminal connects to power, the other to the load.
As current flows through the flasher to the load, it has an element within it that is designed to break the electrical connection briefly and then restore it. Flashers are connected to the power input lead for the turn signal and hazard circuits. As the output of both to those circuits is typically directed to the same filaments of the same bulbs, one flasher is required for the turn signal switch power input and a second for the hazard switch power input.
The Fuse Panel: In a given auto-mobile, you have at least one central fuse panel. This panel is designed to protect the vehicle’s wiring harness from damage if any of the connected accessories attempts to pull more current than it is rated for or if the wiring between the fuse panel and accessory(s) is damaged. Most vehicles nowadays have one fuse panel under the dash and one under the hood. The underhood panel typically contains the fuses for the high current (up to 100A) accessories and the main power to the ignition switch. The under dash panel typically contains all of the fusing and breakers for the vehicles lower current accessories—such as the radio, cigarette lighter, power windows, etc. 3-, 5-, 7.5-, 10-, 15-, 20-, 25-, and 30-amp fuses are common.
As I discussed in Chapter 3, it is important to understand that every fuse has two connections within the panel. One side of the fuse is connected to the accessory; this is called the LOAD side. The other side of the fuse is connected to the source of power for the accessory; this is called the POWER side. The power side can be connected to the battery directly (such as a dome light circuit), the IGN/RUN circuit, the HEATER/AC circuit, or even the ACCY circuit. In some cases, you might run into fuses in the panel that are used to protect the output of a controller. Either way, the vehicle’s owner’s manual typically provides the legend to the panel.
I like to assume that the manufacturer of the vehicle has more knowledge of the wiring and accessories than I do. This means that I’m not going to replace a blown 20-amp fuse with a 30-amp fuse so it doesn’t blow again, because that may be exactly what happens. Worst case, this could cause a fire by exceeding the current capability of the wiring within that circuit. Most fuse panels have a spare fuse of each size in the panel itself or the fuse panel cover.
If you replace a blown fuse with the same size fuse and it blows again, then the fuse is doing its job! Obviously, this is an indication that there is a problem causing the fuse to blow. Chapter 7 explains how to find and solve this kind of problem easily.
Fuses: A fuse is a device that has a metal strip (or wire) with a known current limit. Exceed this limit and the fuse blows. All fuses have a current rating and typically the fuse can pass this current rating for an extended period of time before its metal strip burns in half. In fact, a fuse typically passes many times its current rating for short periods of time before its metal strip is burned in half.
Over the years, I’ve seen a bunch of different fuse types for automotive use, and these can vary widely between domestic, Japanese, and European vehicles. This book covers the main ones (glass and blade types) and all have the same intended purpose and that is to protect the circuit from damage. Keep in mind that even though two fuses may look similar and have the same current rating, they may react differently (slower/faster). These specifics can be determined via the three-digit alpha prefix that comes before the fuse’s current rating itself. (If you require that information, refer to the manufacturer of the fuse.)
Glass Fuses: Glass fuses used to be the norm and were available in a number of different physical sizes and values. If you own an older vehicle with such a fuse panel, you’d be well advised to have at least two of every fuse in the panel in your tool box so you can avoid searching for them if one blows. Although AGC fuses were the most common, your vehicle may require something different—double check both the rating and prefix to be sure and get the right ones.
These older style glass fuse panels can also have rust buildup on the contacts themselves. This causes resistance between the contact and fuse and can be a source of all kinds of problems. If you own a vehicle with this type of fuse panel and it has rusty contacts, you’re best advised to swap it to a newer ATC (also called ATO) blade-type panel. Fortunately, these are readily available from any number of manufacturers.
Painless Wiring and others offer replacement fuse panels designed to make the task of upgrading an older glass fuse panel to a newer and more expansive ATC fuse panel a snap. In addition, these fuse panels can be purchased with or without the associated wiring harness, should your vehicle require a harness upgrade as well.
Blade Fuses: These are widely used in today’s vehicles. In the 1980s, glass fuse panels gave way to ATC fuse panels, solving many problems in the process. These were used for 15 years or so, until the Mini ATC fuse and panel took over. Today’s vehicles typically contain Mini ATC, and MAXI fuse panels. Unlike the glass fuse, blade fuses of all types are problem free and readily available!MAXI fuses are used both by the OEMs and by the aftermarket, as they can be reliably built up to 100 amps in size.
Blade fuses of any kind and their corresponding fuse holders and fuse boxes have a proven track record of reliability. This is the main reason why all the OEMs as well as the after-market fuse panel and wiring harness companies use them exclusively.
High-Current Fuses: Used mainly by the aftermarket, the most common types are the ANL and Mini ANL fuses. These are available in sizes up to 500 amps (or even larger) for the highest current applications. In addition, the ANL fuse holder is designed to accommodate these incredibly high currents across it with minimal voltage drop because it securely holds the fuse in place.
As I said earlier, your local car stereo shop can be a great source for parts like this. In the past few years, ANL and Mini ANL fuses have become so common that many auto parts stores now stock them.
Fusible Links: Fusible links go back many years and are primarily used to protect high-current circuits. In the days of glass fuses, the highest rating available was about 30 amps. Fusible links were used to protect circuits with current flowing through them in excess of 30 amps. Currently, they’ve been replaced almost entirely with MAXI fuses, although my Mustang has both.
The fusible links pictured are on the stock alternator charge lead. Why would that be? Simple—if the alternator’s voltage regulator failed, this could allow voltage “run-away.” As voltage increases, so does current through the myriad of accessories in the vehicle that do not have over-voltage protection (like light bulbs) as well as the battery. If this occurs, the current from the alternator to the vehicle’s electrical system would exceed that of the fusible links and snap them, thereby protecting the accessories and battery from damage.
A fusible link has the same purpose of a fuse, but they are constructed quite differently. Typically, a fusible link is 4 wire sizes smaller than the wiring the circuit calls for—a circuit with 10 AWG wiring would call for an 18 AWG fusible link to adequately protect it. In addition, the insulation of a fusible link is non-flammable and this makes them ideal for underhood use. Due to the nature of how they fail, fusible links are only recommended for underhood use. These are readily available in many sizes at your local auto parts store.
Fuse cartridges are used in some vehicles and act similarly to fusible links. They’re smaller, more convenient, and much simpler to replace in the event of a failure.
Circuit Breakers: A circuit breaker is a device that is designed to open its contacts when the current flowing through it exceeds its rating. A simple bi-metal strip connects the contacts. As the current through this bi-metal strip exceeds its capacity, it changes shape and breaks the electrical connection between the two contacts.
Unlike a fuse, a circuit breaker does not need replacing in the event this occurs. There are two main types of circuit breakers:
- Auto resetting: These are the most commonly used by the OEMs. They are most commonly found in the interior fuse panel and can be used to protect circuits that have a very high current demand when first activated, such as power windows or power seats (or when they come to the end of their travel and the operator continues to hold the switch momentarily). In the event an auto resetting circuit breaker “trips,” it automatically resets after a period of rest, restoring operation to the circuit.
- Manual resetting: These are most commonly used in the aftermarket, and I’ve seen them with current ratings up to 200 amps. In the event this kind of circuit breaker trips, its arm must be manually pushed back into position to restore operation of the circuit. Some of these have a “valet” feature which allows you to manually open the circuit breaker to intentionally interrupt power to the circuit—handy if you’ve got something you don’t want people messing with in the event you had to leave your vehicle in someone’s care.
Either of the above examples is considered to be thermally activated circuit breakers. Over time, the auto resetting breakers can wear out with heavy use. Circuit breakers have two terminals, sometimes labeled as BAT (battery) and AUX or LOAD. If that is the case, then this must be observed for the breaker to work correctly.
Written by Tony Candela and Posted with Permission of CarTechBooks