This chapter covers some of the more complex installations and interfaces. If you opened the book and flipped straight to here, you may choose to go back to the beginning, as this chapter makes the assumption that you know a great deal.
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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Specifically, this chapter covers:
- Interfacing with power door lock and power window circuits.
- Interfacing with power sunroof and convertible top circuits.
- Upgrading headlights to higher powered units.
- Adding an auxiliary battery.
- Adding an accessory fuse panel.
This is a lot of ground to cover. I think a great place to start is learning how to interface with the power door lock and power window circuits that I described in Chapter 5.
Interfacing with Power Door Lock and Power Window Circuits
Why on earth would you ever want to do this? Simple—maybe you’d like to add a keyless entry system or a security system that has keyless entry to your vehicle. Or, maybe you like the idea of being able to remotely open or close your windows. Personally, I love both of these features and have enjoyed these conveniences on numerous vehicles that my wife and I have owned since 1992 or so.
Power Door Lock Circuit Interfaces
When you make such an interface with power door lock circuits, really what you’re doing is duplicating the efforts of the OEM switches and nothing more. Recall, the four most common circuits:
- Negative Pulse
- Positive Pulse
- Voltage Reversal
- Variable Voltage
For the sake of this explanation, I make the assumption that the key-less entry unit or security system, you are installing only has negative trigger outputs that have low current capability, say around 500 mA. Perfect to close the coil of a relay, but that’s about it. If the unit you’re installing has built in relays, it makes the installation easier. Either way, everything shown here applies.
CAUTION: Before you start, it is imperative to verify the kind of power door lock circuit you’re interfacing with. An improper interface could cause damage to the circuit, the unit you’re installing, or both. In addition, you should always verify any wiring you plan to interface to with your DMM to be sure that you’re connecting to what you think you’re connecting to.
Negative Pulse: To interface a key-less entry feature with this switch, all you have to do is tie directly to the switch wires so that the CPU sends negative pulses to them when you press the buttons on the remote. This triggers the coils of the OEM relays, which operates the actuators, as it exactly duplicates what the OEM switches do (Figure 8-1).
See why this is my favorite type of door lock circuit now?
Positive Pulse: Interfacing is just like the negative pulse circuit, but you need to send +12 VDC and not ground to the switch wires to trigger the coils of the OEM relays. This requires an additional pair of Bosch type relays (Figure 8-2).
Admittedly, many of the keyless entry systems and auto security systems available today have at the very least low-current outputs that are programmable as negative or positive pulsing. Check your installation manual to see whether the unit you’re installing offers this feature. If so, then the interface does not require the relays as it is very similar to the negative pulse interface once the outputs are programmed to be positive.
Pulsing Peculiarities: Many of the OEMs house the relays for the door lock actuators themselves within a module that is responsible for a host of vehicle functions, as is the case with the GEM module in my Mustang (see Fig 7-4). As I discussed in the last chapter, some modules have been designed to enter a sleep mode (or hibernation, as some call it) after the vehicle sits for a period of time in an effort to reduce the overall current draw from the battery. As a result, some modules require more than a simple pulse to unlock the doors after the module has entered hibernation. (The GEM module in the Mustang does not.) At this point, the module may require an extended pulse or even multiple pulses to unlock the doors after the vehicle sits for an extended period of time. This is to first “wake up” the module, and then to unlock the doors.
As such, most keyless entry and security systems have this provision built in. This is typically an installer programmable feature and varies by manufacturer.
Voltage Reversal Rest at Ground: Interfacing with this kind of circuit is a little trickier. The interface is most commonly done between the master and slave switches, and the wiring can typically be accessed in the kick panel areas of most vehicles. A pair of Bosch type relays is all that’s necessary to make the interface. I caution you to first determine which side the master switch is on before attempting this interface—the easiest way to do this is:
- Determine which of the wires is the LOCK wire with your DMM.
- Cut this wire in half and measure each side of the wire independently with your DMM.
- The half of the wire that has voltage on it when the LOCK switch is depressed connects to the master switch.
Look closely at Figure 8-3. This is another great example of the versatility of the S.P.D.T. relay. How would you do this otherwise? No amount of S.P.S.T. relays could pull this one off.
Variable Voltage: Recall, this kind of circuit (Figure 8-4) sends signals that vary in voltage level to a controller, which in turn operates the actuators. Interfacing with this circuit is also easy, you just need to know the value of the resistors used within the switch part of the circuit. In some cases you can use your DMM to measure the value of the resistors between the switch common (pole) and the input of the controller, while depressing the switch, of course; in other cases you cannot.
If you cannot make this determination with your DMM, you should contact the manufacturer’s technical support department to get this information. If they are unable to provide this to you, then you might pay a visit to the dealer that you purchased the system from to begin with.
Factory Wiring Diagrams
Where exactly do you find the wiring information for your vehicle?Your single best resource is a service manual that contained a wiring diagram of your vehicle, such as the diagram from Mitchell that I referred to in Chapter 7. Remember, most auto parts stores carry a number of consumer manuals from Chiltons, Haynes, etc., and these may also be available to check out at your local library. Mitchell has such information available via its website at www.mitchell1.com. For a nominal fee, you can access every bit of information they have on a certain vehicle, including factory TSBs, for a period of time.
Finally, there are any number of sites on the Internet that may contain this information, as well. Like anything else on the ’net, some of it is good information and some of it is not. In the days before the Internet, I used my DMM to determine the function of any wire in a vehicle that I tied into and the same holds true today. You’re simply asking for trouble if you neglect to do this. Here are some hints to make your job easier:
Hint 1: As you’re simply duplicating the function of the OEM switches, you can pull them out and see how they’re wired. You can assume the following:
If both switches have three wires, they’re more than likely wired in parallel.
- The gauge of wiring is 16-gauge or smaller; there is typically a pair of relays in the vehicle (or in a controller in the vehicle) that do the actual work—just like in my Mustang.
- The gauge of wiring is 14-gauge of larger, you may be dealing with an older GM dual coil system like I spoke of in Chapter 5.
- If one switch has four wires and the other has five wires, then you’re more than likely looking at a voltage reversal rest at ground system.
- If the wiring to the switches is not obvious because the switches are part of an elaborate switch panel that encompasses numerous other switches, then you are more than likely to need the advice of a professional.
Hint 2: While you have the switches out, you can surely note the color of their wiring and verify their operation with your DMM. Then, you can typically locate this wiring in the kick panel area for the interface—verifying this with your DMM, of course. In some vehicles, the harness comes through the jamb and into the kick panel area and then immediately goes up in to the darkness of the dashboard, as in my Mustang. In these cases, it may be easiest to do the interface within the boot between the door and body (unsnap the boot at the body and door and slide it partially into the door to make accessing the wiring easy), or at the switch itself by running your wiring through the boot in the jamb and into the door. I’ve seen a few cases where this was extremely difficult. If you’re attempting to connect to a negative or positive pulse circuit, you can always stick your head under the dash and listen for the click of the relays themselves to determine their location. The relays could be external or built into a controller. Either way, it may be easiest to make the interface there. This is exactly what I did in the Mustang, as the GEM module was easily found just above the driver-side kick panel.
Hint 3: For both negative- and positive-pulse (low-current) door lock systems, you can use an ATC fuse holder with a 1-amp ATC fuse in it to test the interface before you actually make it. After you’ve verified the wiring with your DMM, of course. This is how to do it:
- Clip the fuse holder assembly to either the plus or minus clip of a cigarette lighter adapter such as the one shown—negative for negative pulse and positive for positive pulse (obviously, right?).
- Touch the bare end of the other side of the fuse holder to the switch wire you’re going to interface with.
- The actuators should lock or unlock depending on which wire you’ve connected the fuse holder to.
You’re not likely to damage a door lock circuit with a 1-amp fuse because it blows immediately if you accidentally connect it to the wrong wire. If the locks work with the 1-amp fuse trick, they’ll surely work with your keyless entry system.
Hint 4: Assume that wiring information from the Internet or a buddy is a guess at best. Do not connect to any wiring without verifying it with your DMM first! Blue wire in the driver-side kick panel? There might be twelve of them that fit that description.
Power Window Circuit Interfaces
The main difference between interfacing with power door locks and power windows is that power windows operate individually of one another. The circuits can be very similar, but you will have two or four unique power-window circuits in most vehicles. Although the driver’s door can sometimes have the switches for all of the power windows, I’ve never made my interfaces there. For vehicles with express or venting features, this is certainly not the place to make the interface because these circuits are in between the switches and motors.
I know this is going to sound like a hassle, but the tried-and-true way to interface with power windows is to do so at the window motors them-selves. Yes, this means that you have to remove the door panels to access the wiring to each individual motor, and that you have to run wiring into the doors, but this is the way the pros do it.
Following are the interfaces for the circuits I outlined in Chapter 5. For these examples, I assume:
- You’re interfacing with a pair of windows.
- You have a pair of low-current auxiliary outputs from your CPU to work with.
- You’d like for one channel to roll both windows up and the other channel to roll both windows down.
- These low-current outputs remain (or can be programmed to remain) constant as long as you hold the button down on the key fob. (Many keyless entry systems and security systems allow the installer to program the auxiliary outputs to be pulsed, timed, or latched. When using them in this application, you need to program them to be pulsed. A one-second pulse is not sufficient because this would require several button presses to fully raise or lower a window. Rather, you want the output to be active as long as you hold down the button on the key fob.)
Voltage Reversal Rest at Ground: No different than the power door lock circuit of the same variety, a pair of Bosch-type relays is all that’s necessary to make the interface. Note that this means one pair per window.
Even though Figure 8-5 shows only two switches, the interface is identical for multi-switch systems as the interface is made at the window motors directly. As you can see, you need eight relays to make this kind of interface in a four-window vehicle.
Voltage Reversal Rest Open—Power Windows Only: Even though this circuit’s net result is the same as the Voltage Reversal Rest at Ground circuit, the interface required is radically different. Interfacing with this kind of circuit is a little trickier, and it requires two pair of Bosch-type relays per window, which is twice as many as a voltage reversal rest at ground window circuit (Figure 8-6).
As I mentioned in the Chapter 5, this is the way GM did the windows in the Gen III Camaro (1982–1992). At least in those vehicles, you can make the interface at the switches themselves versus having to go into the doors to access the motor wires as there are no modules between them and the motors and no master/slave switch arrangement for the passenger window.
Most auto security manufacturers sell window roll-up modules that do the up part automatically when you arm the system. The interface with them is much the same, except for a trigger wire between the security system and window roll-up module instead of the auxiliary output channel. Because they have on-board relays, external relays are not required. Some of these even include one-touch features for any window you connect to them. Over the years, I’ve installed a bunch of these from many different manufacturers. One of the best things about them is that they don’t require an auxiliary channel to do the up feature, just the down.
Interfacing with Power Sunroof and Convertible Top Circuits
Power Sunroof Circuit Interfaces
As I discussed in Chapter 5, power sunroof circuits can be more similar to power door lock circuits than to power window circuits. At this point, you know enough to interface with them with one exception—using a window roll-up module to do the job automatically.
As I said before, these mechanisms can be very fragile. Continuing to power the motor after the sunroof has reached the end of its travel can take its toll on the mechanism rather quickly. If you’re making an interface to the auxiliary outputs of a unit and intend to operate the sunroof manually (but remotely), you’re typically fine making this interface at the switch itself and duplicating its efforts. On the other hand, if you intend to use a window roll-up module to close the roof automatically, you’re well advised to make the interface at the sunroof motor itself and interface the module with the sunroof’s limit switch. This ensures that the module doesn’t power the motor, even for a brief time, if the sunroof is closed. Be advised that these interfaces can involve removing the headliner entirely to access the wiring. If you decide to undertake such an interface, shop around for a window roll-up module that has an input designed to interface with this limit switch.
Convertible Top Circuit Interfaces
Think of a convertible top as a very high-current power window. As the switch itself cannot support the current required of the convertible top motor, a pair of high-current relays is typically located near the convertible top. If you desire to operate the top remotely, you’re advised to use auxiliary channels of your unit and duplicate the efforts of the switch. Again, choosing a system with auxiliary outputs that can be programmed to be active as long as you hold down the button on the key fob is the way to go. I do not recommend window roll-up modules of any type for this application, even ones of the high-current variety.
Be advised, though, that the following are challenges that you need to be aware of:
- The latching mechanism: If the top has manual latches, and many do, then they have to be manually unlatched before it can be operated remotely.
- If the top comes down hard when closing it, and you have to help it down slowly with your hand to avoid it crashing down on the windshield frame, there will be no way to prevent this when closing the top remotely—you may choose to do open only.
Either way, this is a hit at the local burger stand!
Upgrading Headlights to Higher Powered Units
For many years now, this has been a popular upgrade for older vehicles. High-powered halogen or xenon headlights consume far more current than the stock head lightswitch, hi/low beam switch, and wiring were designed to accommodate. To avoid damaging the wiring or switches as discussed in Chapter 4, add a few relays as shown in Figure 8-7.
Notice that now the entire OEM circuit only powers the coils of the relays. This greatly lessens the load on this circuit, generally speaking. In addition, since this new circuit uses larger gauge wiring and connects directly to the battery, these lights have plenty of current going to them. Feel free to mount the relays under the hood, just be sure to insulate them properly and mount them (with screws!) so that their terminals point down to keep water out of them.
HID (high intensity discharge) kits are also quite popular, and many of the kits on the market have been designed to plug into the stock head-light harnesses. Ballasts install between the stock headlight plugs and the new HID lamps because the lights require a higher operating voltage and have unique “start-up” requirements. In some cases they can actually draw less current than the stock incandescent lamps they’re replacing.
Adding an Auxiliary Battery
Let’s say that you needed to add a single auxiliary battery to your vehicle for whatever reason. We’ve discussed how to determine when auxiliary battery(s) are necessary, now let’s discuss what’s required to install one. The two challenges are the physical installation and connecting the battery electrically.
Obviously, this chews up some real estate in any vehicle. In that regard, there are a few considerations:
- Mount batteries externally under the hood, in the bed of a truck, in the undercarriage, or inside the frame rails, which is safest. (When possible, they should be protected from the elements.)
- When mounting a second battery under the hood, said battery should be mounted in a suitable tray and properly tied down.
- When mounting a second battery in some other place, it needs to be solidly mounted in a suitable rack, tray, or enclosure. Any of these need to be solidly mounted to the vehicle.
- Batteries installed in an enclosure of any type need to be vented to the outside of the vehicle for safety.
- Batteries installed in the passenger compartment should only be installed in a suitable enclosure and need to be vented to the outside of the vehicle for safety—I prefer sealed batteries in these applications.
I’m not really a big fan of batteries installed in the passenger compartment of a vehicle, but in some cases this is the only place it can go. If you choose to do this, it’s probably a good idea to make the cabin of your vehicle non-smoking, so there’s absolutely no chance of igniting battery fumes.
Connecting the Battery Electrically
Now that you’ve mounted the battery itself, you have to connect it to the vehicle’s charging system. You have three choices when doing so:
- Wire the battery in parallel with the OEM battery.
- Wire the battery in parallel with the OEM battery via an isolating solenoid.
- Wire the battery via a battery isolator.
No matter the application you choose, the battery negatives must be connected to a point of very low resistance. Now, let’s look at each scenario so that you can pick the one that’s right for your application.
Wiring a Battery in Parallel with the OEM Battery: The main reason to do this is to have additional current available to the vehicle’s starter. If this is the case, there is no reason to isolate the two batteries and the installation is simple. This is the one type of installation where it’s mandatory to use the same kind and type of battery as the stock one, otherwise they can feed off of each other when the vehicle is parked, causing them to go dead. In addition, if the stock battery has in excess of 10,000 miles on it, you should consider installing a pair of fresh batteries, as you want them to be close in age.
This is a common upgrade for big trucks or work trucks, and in most cases the second battery can be installed under the hood. As the Big Three all make diesel trucks, they typically leave room under the hood for a second battery tray, which is sometimes available for purchase at the parts counter, making this job easy. Incidentally, if I have to put the battery somewhere else besides under the hood, I choose to isolate it via one of the other methods.
Figure 8-8 is the diagram to do this.
Use the same gauge of cable the OEM uses for the stock battery cables for this connection, unless it’s considerably longer. In that case, increase the gauge of the cable based on its length—remember, this battery will be used to supplement starting the vehicle. Note that there are no fuses between the batteries as they are unnecessary in an under-hood installation. I used to use them, but after years of noticing the OEM wiring from one to the other with no fuses, I don’t. The main reason they do not use them is because the fuse(s) limit current, which is contradictory to why you’re adding the battery to begin with.
Since you’re adding this battery to supplement starting of the vehicle, its negative cable needs to be connected to the engine block in the same manner as the OEM battery. (If the batteries were mounted right next to one another, it is acceptable to tie their negatives together directly.) In addition, you should connect a smaller gauge strap from the negative terminal to the chassis in the same manner as the OEM battery.
It goes without saying that you need to properly anchor the positive cable connecting the two batteries together and keep it out of the way of moving parts as well as the hood latch mechanism or hinges for obvious reasons. A short in the middle of this cable could ruin your day.
The next two cases are where you’ve added a high-current accessory to the vehicle and upgraded the charging system accordingly (refer to Chapter 6). Now you’d like to be able to also use this accessory for an extended period of time with the engine off and the ignition key in the ACCY position without danger of depleting the starting battery. You know, play the stereo with the windows down at the park for a few hours and then get in and start the engine with a fresh battery—how convenient!
Wire the Battery in Parallel with the OEM battery via an Isolating Solenoid: With the key in the IGN/RUN position, both batteries are tied together in parallel, which will allow the alternator to charge them both while the engine is running. With the key off or in the ACCY position, the auxiliary battery is totally disconnected from the charging system as is the high-current accessory. In this case, the battery does not have to be the same type and kind as the starting battery. In many cases, a deep cycle battery may be a better choice for the auxiliary battery due to the nature of its use.
Regardless of where you install the auxiliary battery, this is also a simple installation Figure 8-9.
Solenoids for this use are commonly available in 80 and 200 amp sizes and your local car stereo or electrical supply store typically stocks a few sizes of them. The current capability of the solenoid required is deter-mined based on the amount of current the auxiliary accessory and battery requires. For a long time, hot rodders have used Ford starting solenoids for this purpose as well. I should note that OEM starting solenoids are not really intended to pass continuous current through their contacts, so I don’t recommend their use.
Notice that this installation requires protection to be safe. Two ANL fuses are each mounted within 18 inches of the auxiliary battery, as shown. A third is required near the solenoid to protect the run of wiring between the solenoid and the auxiliary battery should the battery be located somewhere other than the engine compartment. You could also use heavy-duty, manually resettable circuit breakers for this job. Either way you go, be sure and select the fuse/breaker size based on the length and gauge of wire between the batteries that you’re using, which is determined by the current requirements of the accessory.
Wire the Battery via a Battery Isolator: For many years, this was the most recognized way to add an auxiliary battery. In fact, this was the only way the military did it. As I said a few chapters ago, a battery isolator is really nothing more than a pair of very high-current diodes mounted in an aluminum heat sink. There are three studs on an isolator, and they are typically labeled A, B1, and B2.
For whatever reason, a lot of folks just don’t understand this interface. I think the confusion sets in because the natural process is to connect the stock battery to the B1 terminal, when in reality the stock charge lead from the alternator, as well as anything else that was tied to the alternator output stud, connects to this terminal as shown in Figure 8-10.
Recall, the big cable on the stock battery is for the starter motor and nothing else. As you can see, this is quite different from the other two methods and at no point are the batteries connected together—vehicle running or not. Remember, diodes require seven tenths of a volt to be “turned on,” so you have at least seven tenths of a volt delta between the A stud and either of the B studs. This is why many folks choose to use an isolating solenoid instead.
When choosing an isolator, keep in mind that all of the current the alternator is capable of making will pass through the isolator. Given this, choose an isolator that is able to pass 20 percent more current than the alternator can produce. If you have a 150-amp alternator, you need a 180-amp isolator. So the $29 unit at the local auto parts store won’t cut it! High-current isolators are quite large, and can be a challenge to physically mount under the hood of a vehicle.
Be aware that on some vehicles, especially those fitted with a one-wire alternator, the alternator does not work properly unless you use an isolator with an “exciter stud,” which is labeled with an E. This stud is typically tied to the IGN/RUN output of the ignition switch via a small circuit breaker. In addition, the alternator may require a slightly different harness with a sensing lead that can be tied to the B1 terminal on the isolator so that the alternator will work properly. Since this varies by vehicle, refer to the manufacturer of the isolator.
Either way, when you start the vehicle, you should use your DMM to verify 13.8 to 14.4 VDC present at the A stud and roughly seven tenths of a volt less present at each B stud.
To make the best use of an isolated auxiliary battery, there are a few things to consider. Let’s say that you did add a high-powered audio system and wanted to be able to play it with the key in the ACCY position for an extended period of time with minimal current draw. In addition, you’ve chosen to go the route of using an isolating solenoid for your auxiliary battery. Here’s how to do it:
- Connect the audio amplifier(s) directly to the auxiliary battery.
- Connect ancillary low-current accessories, the radio for example, to the auxiliary battery via a relay that is triggered when the ignition switch is in the ACCY position.
Turning the ignition switch to the ACCY position would allow the audio system to operate independently of the starting battery (Figure 8-11). In addition, the ancillary devices would not draw current from the starting battery, only the coil powering the relay, and that is minimal.
Incidentally, these same considerations hold true when using either an isolating solenoid or a battery isolator. Remember to turn off other accessories, including the dome light(s), to minimize draw on the starting battery. This ensures minimum current draw from the starting battery.
If your accessory requires two or more auxiliary batteries, then connecting the additional batteries to the first is really quite simple. Mount the batteries as close together as possible and connect them in parallel as shown below.
Determine the correct gauge of wiring based on the accessory’s cur-rent requirements and use it for these connections. Another alternative is to use brass bars for these connections, but that is a whole lot more work. No fuses should be used between these batteries, and only one connection to the vehicle’s chassis (preferably the frame) is required.
Adding an Accessory Fuse Panel
Let’s say that you plan to add a number of aftermarket electronic accessories and you need a nice, simple, and safe way to interface them to your vehicle’s charging system. In addition, you’d like the ability to add more accessories down the road as your budget permits. No problem, it’s time to add an accessory fuse panel.
A second fuse panel you say? How hard is that going to be? Believe it or not, this is one of the simplest and safest upgrades that you can make. A good example of when to do this is in my Olds. A single aftermarket ATC fuse panel provides a way to safely and easily power a number of high-current accessories that could not be tied to the aftermarket Pain-less Performance fuse panel that I chose to build the wiring harness around.
These are readily available from all the major parts houses and in some cases, your local auto parts store. This fuse holder has only one input, so you have to use relays with it for accessories that are powered off of the ACCY or IGN/RUN circuits. Here are the accessories that I have connected to this fuse panel, from top to bottom:
- Fan 1 circuit
- Fan 2 circuit
- Starter solenoid trigger relay
- Headlight relay
- MSD 6-BTM Ignition
- Cigarette Lighter
- Start buttons on firewall for
adjusting the valves (I typically only put the fuse in when I need these, otherwise I leave it out.)
As many of these circuits are only required when the vehicle is running, a number of relays are required in conjunction with this fuse panel to accommodate this. When installing such a panel, take into consideration the combined current requirements of each of the accessories it powers. Just because they fit, it doesn’t mean you can fill the slots with 40-amp fuses; this panel cannot pass that much current through it. In addition, this panel accommodates up to a single 6 AWG or a pair of 8 AWG wires for its input, I elected to use a pair of 8 AWG.
Other types of aftermarket fuse panels are also readily available, like the one that I used under the hood of my Mustang.
I got this one at my local auto parts store. As it easily comes apart, one could cut the common strip at any point so as to make some fuses hot all the time, and other fuses powered by the ACCY or IGN/RUN circuits. This is just fine for a number of low- to medium-current accessories.
That about covers it for installing aftermarket electronics of all kinds, or does it? Turn the page and find out!
Written by Tony Candela and Posted with Permission of CarTechBooks