This chapter covers some fairly lengthy topics. Specifically, it gives the run-down on power door lock circuits, power window circuits, power sunroof and convertible top circuits, and an in-depth look at the charging system.
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Before I get into the specifics of power door lock, power window, power sunroof, and convertible top circuits, I should mention that this chapter focuses on traditional analog circuits. No different than the evolution of the ignition switch that I discussed previously, many of these vehicle circuits have undergone similar changes with advancing technologies. Digital BUS systems bring about the opportunity to allow controllers to interface digitally between one another. Again, as these systems are the exception, rather than the norm, this chapter focuses on the norm.
Power Door Lock Circuits
When it comes to power door lock circuits, you need to understand that there are multiple types that all accomplish the same job. These can vary by manufacturer and can even vary among vehicles made by the same manufacturer. At the end of the day, the operation of the circuit is the same. Press the unlock switch and the doors unlock; press the lock switch and the doors lock.
Most of the door lock actuators are of the two-wire variety, no matter the manufacturer, OEM, or aftermarket supplier. Apply power to one wire and ground to the other and the actuator moves one way; reverse this and the actuator moves the other way. This is commonly referred to as “voltage reversal” and is the way things have been done in vehicles for a very long time. What varies is the way the switching is done to get voltage and ground to the motors. Here are four of the most common switching methods:
- Negative Pulse
- Positive Pulse (with and without relays)
- Voltage Reversal Rest at Ground
- Variable Voltage
Consider for a moment that the number of doors a vehicle has can be the determining factor as to what type of door lock circuit is most suitable. For two-door vehicles, it’s safe to assume that a door lock switch is present in each door. Some four-door vehicles have door lock switches in the front doors only, while others include them in all four doors. In most cases, four-door vehicles are not wired via the voltage reversal rest at ground scheme because this would add unnecessary complexity, weight, and expense.
In the diagrams to follow, I’ve done my best to simplify each over-all circuit for clarity. In addition, the diagrams simply show the door lock actuators wired in parallel—where exactly this occurs is vehicle specific.
This type of door lock circuit is my favorite because they’re very simple and easy to interface to, as out-lined in Chapter 8. This circuit (Figure 5-1) is very common in Japanese vehicles.
Note that a pair of parallel-connected switches (S.P.D.T. Center Off) are used to switch low-current ground to power the coil of either of the relays, which in turn powers the actuators. In addition, both relays rest at ground, so when a switch is depressed, it:
- Excites the coil of one of the relays, which causes
- the voltage reversal circuit to the actuators to be completed, which causes
- the actuators to move in the corresponding direction.
Four-door vehicles have two additional motors wired in parallel in the same fashion.
Positive Pulse—With Relays
Very similar to the negative pulse circuit, the switches just switch low-current +12VDC to power the coil of either of two relays, which in turn power the actuators. This is what Ford uses in my Mustang (Figure 5-2).
From the relays to the actuators, the circuit and operation is the same as the negative-pulse circuit. Four-door vehicles have two additional motors wired in parallel in the same fashion.
Voltage Reversal Rest at Ground
This circuit does not use relays (Figure 5-3). Instead, a pair of D.P.D.T. switches is wired in series and the overall circuit rests at ground. Note that one of the switches has four wires, and the other has five wires. The four-wire switch is the MASTER and the five-wire switch is the SLAVE. This over-all circuit is easy as both actuator wires rest at ground as a result of the switches resting at ground. When one switch is depressed, it:
- Sends +12VDC to one of the lock/unlock wires, which causes the actuators to move in the corresponding direction.
In this circuit, the switches excite the actuators directly. As a result, they are of a higher current variety than either of the pulsing-type circuits and therefore require a larger gauge of wiring, which is typically 14- or even 12-gauge, depending on the vehicle. This large-gauge wiring and number of wires on the switches are a telltale sign of a voltage-reversal switching configuration.
This circuit was very common in two-door domestic cars and trucks for many years, as illustrated in the diagram at the beginning of Chapter 4. Keep in mind the master switch can be on either side of the vehicle. The master switch is often located on the same side of vehicle as the fuse box. A good example of this was the all-new 1997 Ford F150. Ford laid out the wiring harness in those trucks exactly backward and the fuse box was on the passenger side of the dash. The master lock switch is in the passenger door in these trucks.
Variable voltage (analog) configurations (Figure 5-4) use a single wire to send multiple signals to a controller and, as the name implies, these signals vary in voltage. The first vehicle I remember working on that used this type of door-lock circuit was the Ford Probe, which was actually built by Mazda. Today this is commonplace, especially in Chrysler vehicles for the last 10 years or so. A variable-voltage system requires some kind of controller between the switch and actuators as shown. This controller decodes a voltage level as a command and takes the appropriate action.
Positive Pulse—No Relays
In the early days of power door locks, GM built vehicles with three wire actuators. These actuators had two coils—one for lock and the other for unlock. Mounting the actuator to the metal door interior completed the circuit. At the end of the day, this is just a positive pulse circuit (Figure 5-5) that requires 5 to 10 amps per actuator to operate them, so the switches and wiring have to be up to the task. I remember the first time I came across this. I was looking at wiring diagrams in a Mitchell manual and I couldn’t believe what I was looking at.
There are a few other types of door lock circuits that are not covered in this book—like the one wire lock/unlock circuit used in Mercedes Benz vehicles for many years. In addition, some of the European and British vehicles have “closure” wires—a single wire that when ground (or in some cases voltage) is applied to it, causes the door locks to be locked as well as all of the vehicles windows/sunroof to be closed. As the specifics of these circuits can vary by vehicle, get your hands on the wiring diagrams if you have to work on them.
Power Window Circuits
When it comes to power window circuits, there are also a couple types that accomplish the same job. These can vary by manufacturer and can even vary among vehicles made by the same manufacturer. At the end of the day, the operation of the circuit is the same: press up to raise the window and press down to lower the window.
The main difference between power window and power door lock circuits is that the windows are able to be operated independently of one another. Electrically, the overall circuits have many commonalities even though the power window circuits appear much more complex on paper.
Most of the power window motors are also of the two-wire variety, no matter what the manufacturer OEM or aftermarket and “voltage reversal” is the nature of the circuit. Like power door locks, this is also the way things have been done in vehicles for a very long time. Here are the most common switching methods:
- Voltage Reversal Rest at Ground.
- Voltage Reversal Rest at 12 volts Switched.
- Voltage Reversal Rest Open.
Before I go into the differences, we should consider that the location of the switches typically determines the kind of circuit employed.
Here are some common scenarios for two-door sedan with front power windows only:
- Switches in the center console—one switch per motor.
- Switches in the doors—one switch for the driver-side motor and two for the passenger motor.
Here are some common scenarios for two-door sedan/convertible with front and rear power windows:
- Switches in the doors and rear panels—one switch for the driver-side motor and two for all other motors.
- Four-door sedan with front and rear power windows.
- Switches in all four doors—one switch for the driver-side motor and two for all other motors.
In all of these circuits, the switches operate the motors directly. As a result, they are of the higher current variety. In addition, the wiring in these power window circuits is a minimum of 14-gauge, and in some cases 12-gauge.
Voltage Reversal Rest at Ground (two-door with switches in console)
This is the simplest power window circuit of all (Figure 5-6). A pair of D.P.D.T. switches is all that’s required because the switches are centrally located. As shown, both switches rest at ground.
This circuit functions similarly to the voltage-reversal rest at ground power door lock circuit. That is, with the exception of being able to control the windows individually.
Voltage Reversal Rest at Ground (two-/four-door with switches in doors)
- A single D.P.D.T. switch is used for the driver-side window and a pair of D.P.D.T. switches is used for each additional window—typically one in the driver’s door at the main switch panel, and a second in each door.
- Paired switches are wired in series.
- All switches rest at ground.
Notice that in the case of the paired switches (Figure 5-7), one of the switches has four wires and the other has five. The four-wire switch is the MASTER and the five-wire switch is the SLAVE. This overall circuit is easy as both motor wires rest at ground as a result of the switches resting at ground. When one switch is depressed, it:
- Sends +12VDC to one of the up/down wires, which causes the window motor to move in the corresponding direction.
Voltage Reversal Rest at 12 Volt Switched
This circuit (Figure 5-8) is very similar to the voltage reversal rest at ground circuit, except, as the name implies, the switches rest at 12 volts when the ignition switch is on. Which on position varies by vehicle, but rest assured the windows work with the ignition switch in the IGN/RUN position.
The specifics of this circuit’s operation are the same as the voltage reversal rest at ground circuit, but as the switches rest at 12 volts when the ignition switch is on, depressing a switch with the ignition switch in the IGN/RUN position:
- Sends ground to one of the up/down wires, which causes the window motor to move in the corresponding direction.
Early Mercedes Benz vehicles used this circuit. I have also seen this used in some Chevrolet Corvettes.
Voltage Reversal Rest Open (two-door with switches in console)
Over the years, I’ve only seen this circuit in a handful of cars—Gen III Chevrolet Camaros being one of those vehicles. To be honest, I’m not sure why GM chose this type of circuit because it really doesn’t save on wiring. As each of the switches rest open, so do both motor wires. When a switch is depressed, it sends both power and ground to the motors—weird.
The net result of this switching is identical to the voltage-reversal rest at ground example I gave for the two-door circuit with switches in the center console.
No different than power door lock circuits that employ this scheme, this can also be used for power windows. Although this is relatively uncommon, there are some late model vehicles that use a variable voltage scheme for the window and/or sunroof motors as well. How do you tell? Measure voltage at the switch wire(s) with your DMM.
Early GM vehicles also employed three wire window motors. Just like their three-wire door lock actuators, these motors have two coils—one for up and the other for down. The motors sourced ground via their mounting to the metal regulator, which is in turn mounted to the interior of the door. This is considered a positive pulse circuit that obviously requires high current to operate the motors.
Power Sunroof and Convertible Top Circuits
These circuits are really more similar to power door lock circuits as they have a single switch and single motor. As such, they can be switched via any of the following varieties:
- Negative Pulse
- Positive Pulse
- Voltage Reversal
- Variable Voltage
I’ve seen examples of each of them over the years. Circuit layouts for them are similar to power door lock circuits, so working on them is the same. Either of them consists of a two-wire motor that operates in a voltage-reversal fashion. Obviously, a convertible top requires much higher current to operate than a window or sunroof, so a pair of high-current relays mounted nearby is quite common.
Finally, many sunroofs have limit switches that are designed to limit their travel forward or rearward so that the mechanism isn’t damaged. As their mechanisms are typically in a very confined space, this often determines what kind of motor and/or drive is used. As a result, some of these mechanisms can be quite fragile necessitating the limit switches. Remember, many sunroofs are a dealer-added feature at the time of the sale, so the OEM wiring may not apply to your particular unit.
The Charging System
The charging system consists of the battery, alternator, and return path. I’ve given many lectures on this topic over the years, and I’ve found that it is a topic of unbelievable controversy. Quite simply, there is a bunch of misinformation out there in regards to charging systems—more than any other topic I can think of in this book.
Let’s get the obvious out of the way for any vehicle with a properly functioning charging system. When the vehicle is running, the alternator is the source of power for all electrical components. Furthermore, the battery is a load. How do I know this?
In Chapter 1 you learned that voltage causes current to flow. Ohms Law also implies that voltage flows from the place with the least resistance. And, guess what? In any circuit, that’s always the place with the highest voltage potential. In a vehicle with a properly functioning charging system, that is the output stud of the alternator. Secondly, a typical battery requires between 7 to 10 amps of current to keep a surface charge on it when the vehicle is running so that it’s ready to restart the vehicle on a moments notice. Where does this current come from? You guessed it, the alternator. OK, now that you agree, let’s move on.
The battery’s primary role is to start the vehicle. A good battery can do this on a very cold winter day—quite a feat indeed with a high-compression, dual quad tunnel-rammed setup that needs to be started at least five times to keep it running.
A typical lead acid battery has a chemical reaction that occurs when current is pulled from the battery. This chemical reaction occurs between the electrolyte (the mixture of water and sulfuric acid), and the lead plates that make up the individual cells of the battery itself. In addition, said battery has six 2.1-volt cells connected internally in a series arrangement. This nets a standard nominal voltage of between 12.6 and 12.7 VDC. When charge is flowing into the battery, such as from a spinning alternator, this chemical reaction is reversed, allowing the charge to be stored within the battery for future use. This chemical reaction can be enhanced by increasing the temperature of the battery, as is the case when it is located under the hood of the vehicle.
By nature of its design, the battery has an incredibly high amount of capacitance. This capacitance, coupled with the output of the alternator, has a filtering effect. A fresh battery does an incredibly good job of filtering any non-DC voltage present (i.e., ripple) at the output of an alternator. Unfortunately, this filtering benefit can be lessened as the distance between the battery and alternator increases. Worst case example are vehicles that have a trunk-mounted battery, as many of the German cars do. (Same net effect you can have when relocating your battery to the trunk—just goes to show you, there is no such thing as a free lunch.)
As the battery ages, its ability to release and store charge, as well as its ability to filter ripple, is lessened. This is why sometimes replacing an old, worn-out battery can solve all kinds of problems.
As I said above, the alternator is the primary source of power when the vehicle is running. An engine’s spinning alternator generates AC voltage. This AC voltage is then rectified into DC voltage. Any residual AC voltage present is considered ripple (or noise) and much of that is filtered off by the vehicle’s battery.
There are two main types of alternators—one-wire and three-wire. Either can be internally or externally regulated. Most cars on the road come with a three-wire alternator while most hot rodders choose a one-wire because they’re easier to install…or are they?
Three-Wire: A three-wire alternator has a distinct advantage over a one-wire alternator because one of its connections is made to a common power point in the vehicle to sample voltage. Consider this similar to the probe of your DMM as this wire can give the alternator more information about the actual operating voltage of the accessories in your vehicle. In addition, it can help the alternator to react more quickly in response to voltage dips caused by current draw of the accessories. Figure 5-10 is a simple diagram of a typical three-wire alternator installation.
Three-wire alternators typically have three connection points:
- Charge Stud
- Ignition Sense
- Battery Sense
The charge stud is typically connected to the battery (+), or nearby power distribution center, via heavy-gauge wiring. Some OEMs elect to protect this connection using fusible links, as is the stock charge lead on my Mustang that was pictured earlier. As mentioned previously, these links can protect the vehicle’s battery and accessories from damage in the event of a failed voltage regulator causing a voltage runaway.
The ignition sense input is used to tell the alternator when the vehicle is running. This connects to the IGN/RUN output of the ignition switch. It is common for a ballast resistor to be used between this and the ignition sense input of the alternator, as the sensing circuit can be damaged by too much voltage. In other cases, an instrument-cluster-mounted “idiot light” may be wired in series between the ignition switch and this input. This can sometimes negate the use of the resistor.
The battery sense lead is the “probe-like” lead. It is connected to a point of commonality for power, such as a power distribution center. This gives the alternator feedback in real time to the power requirements of the vehicle’s accessories.
One-Wire: A one-wire alternator has a distinct advantage over a three-wire in that it has fewer electrical connections. The alternator monitors voltage at its output stud, which isn’t nearly as accurate as monitoring voltage the way a three-wire unit does. In addition, as a one-wire alternator does not have an Ignition input, you have to get it to some RPM to “turn it on”—typically over 1,200 rpm. Once it is turned on, it can charge even at lower RPM. Figure 5-11 is a simple diagram of a typical one-wire alternator installation.
Three-wire alternators are typically preferred over one-wire alternators because they can vary their output based on the actual needs of the vehicle. This is why the OEMs all use them. Finally, a one-wire alternator can drain a stored vehicle’s battery over a month or two as it has a slight bleed-off between the output stud and its internal sensing due to its design.
The Return Path
The return path is the path of commonality between the accessory ground wires and the charging system ground. Remember that by nature of design, all of the accessories are grounded to the vehicle’s chassis. For clarification, it’s easiest to separate this into two separate parts—the return path for the starter motor and the return path for all other accessories.
Look at the battery under the hood of your vehicle. Notice that it typically has 4-gauge leads connected to both terminals. In addition, it may have an 8- or 10-gauge lead from both terminals. The 4-gauge leads are for the starter motor, and the smaller leads are for the accessories.
Return Path Starter: In Chapter 2, in the section on how to measure voltage drops, I described the return path for the starter motor in great detail.
Return Path Accessories: In most domestic vehicles, the small lead from the negative connects to the chassis of the vehicle and is the return path from the charging system negative to all accessory grounds in the vehicle. This can be different for import vehicles, as shown in the beginning of Chapter 1 in the Nissan Frontier. It is important to understand the difference between the two return paths.
Now, let’s consider the return path of the accessories themselves to better understand it. Let’s consider the return path for the tail lamps, brake lamps, and back-up lamps:
- From lamp negatives to chassis of the vehicle somewhere in the trunk (all typically tied together to a single grounding point).
- Through numerous different pieces of metal, typically spot welded and bolted together to the front clip.
- From the front clip (fender, inner fender, or front support) to the battery negative via the small lead.
As you can see, this certainly isn’t ideal. In many cases, grounding problems can arise during the manufacturing process that can have all kinds of ill side effects. When this happens, a TSB (technical service bulletin) is typically issued to the dealer service departments so that the technicians can quickly repair these problems. Even if the vehicle was put together correctly, over time, this grounding scheme can lead to problems.
What if you’re going to be adding all kinds of new electronics to your vehicle? How do you ensure that they function properly for years to come and that you don’t place too much of a load on the stock return path? Simple—you upgrade the return path accordingly. The next chapter explains how to do this.
Tying it All Together
OK, now you know all the basics we’ve covered so far:
- The Ignition Switch
- The Wiring Harness
- Power Door Lock Circuits
- Power Window Circuits
- Power Sunroof and Convertible Top Circuits
- The Charging System
What is the path of current flow to the accessories? For this example, let’s assume we’re talking about the radio in my Mustang:
- The ignition switch is turned into the ACCY position (so the vehicle is not running).
- This allows current to flow from the battery through the 40 amp MAXI fuse in the underhood fuse panel that supplies the power to the ignition switch itself.
- This current then continues through the ignition switch via its ACCY output lead to the interior fuse panel.
- This current then continues through fuse # 32 and on to the radio’s power lead.
- The current travels through the power switch of the radio to the power supply of the radio .
- This current then exits the radio’s power supply via the radio’s ground lead, which is tied to the chassis of the vehicle.
- The current travels through the chassis of the vehicle, returning to the battery (-) by way of the jumper between the front support and battery, thereby completing the circuit path.
OK, it can be argued that current travels in the opposite direction from what I outlined, from negative to positive as free electrons are negatively charged. This is scientific theory versus electronic theory. Either way, the net result is the same.
And now you know. Regardless of how complex a vehicle is, the circuit paths are similar to what I out-lined above. This is clearly outlined in the electrical diagrams available to you, which I spoke of in the last chapter.
Now, you have the knowledge you need to do anything and everything else in this book and you should have a feeling like you did when you first solved the Rubik’s Cube (what—you haven’t solve this?). The first five chapters in this book unlocked many a mystery to the topics I covered.
Written by Tony Candela and Posted with Permission of CarTechBooks