Tech Today

Working with PWM Ignition Systems

What do you do when a vehicle lacks the classic 12-volt ignition wiring? These strategies will help installers work with pulse width modulation ignition systems in newer cars.

WORDS BY DAVID MACKINNON

If you have tried to install a backup camera, radar detector or dash cam in a late-model BMW or a 2020 Corvette, then you have likely run into a situation where you’ve had to go looking for an ignition wire. The odds are pretty good that unless you’ve worked on the car before, the entire experience was frustrating, and it felt like you wasted a lot of time. Where did the ignition wire go? More and more vehicles have abandoned the classic switched 12-volt ignition wiring in favor of circuits that are connected to power and ground, and monitor one of the many data networks in the vehicle to decide if, when and how they should operate.

The beauty of this digitally controlled arrangement is that the vehicle manufacturer can change how things work even after the car is designed and built. For BMW owners, being told there’s a large software update available is often a part of regular maintenance. If BMW wanted to, they could change how fast the turn signals flash, how long the dome light stays on after the last door is closed. In my Hyundai Genesis sedan, if you sit

in the car for 10 minutes after turning off the ignition, you’ll hear the blend door in the HVAC system close as the car moves to a deeper state of “sleep.” That’s the car’s computer doing its thing!

In terms of practical applications, these data-controlled circuits are part of the retained accessory feature that lets you listen to music for two or three minutes after turning off the ignition. More importantly, the car’s body control module can turn off the dome lights after 10 or 20 minutes, even if you have left a door or hatch open.

What is Pulse-Width-Modulation?

Aside from a discussion about data-controlled components in the car, some circuits in modern vehicles are controlled by a pulse-width-modulated signal. One example is the reverse wire for the brake lights on many BMW vehicles. Early daytime running light circuits here in Canada also used PWM controllers to dim the headlights. Other vehicles use a combination of PWM signals as well as a certain amount of DC voltage to check headlight circuits for burnt-out bulbs.

Connecting devices to a PWM signal can result in unexpected or unwanted operation. If I recall correctly, BMWs used a relatively steady-state voltage for reverse lights when the vehicle was off but switched to a PWM signal when the engine was running. Some cameras and monitors could handle the change and perform reliably, and of course, some couldn’t. Using a PWM where a device is designed for a fixed 12-volt DC input can also cause longevity problems, depending on the design of the device.

Why Do We Need An Ignition Wire?

If we are attempting to install conventional aftermarket electronics like a new radio, or the aforementioned radar detector or dashcam, we need a way to tell these products when the car is in on so that they can wake up. For this, we need a 12-volt source that comes on with ignition.

There are dozens of easily accessible hardware available to create a reliable switched ignition source, so let’s take a look at them (in no particular order).

Wāvtech IRAD

From the west coast, we have the IRAD from Wāvtech. I talked about the magical little IRAD in my article about dealing with stop-start vehicles. The IRAD can provide both a remote output for accessories and control a secondary battery to keep the voltage up as the engine cranks. Wāvtech calls it their Application 3 – Stop-Start Relay/Solenoid Control Model. For our needs, in terms of providing a reliable ignition source for accessories, we are going to look at Application 1 - +12V Ignition and Remote Generation with Relays.

The IRAD can monitor any wire you want, even one that has a PWM signal on it. Once the effective voltage is greater than 0.5 volts, the unit triggers a full +12V output. The unit provides up to 750mA of output that is within one volt ofthe battery voltage. If you need more current, you can wire in a relay or two.

Adjustable turn-on delays can help eliminate turn-on pops in audio systems with signal processors.

The Signal-to-Power from NAV-TV includes voltage, data and video inputs to trigger the switched 12V output.

You (or your installer) can also program a battery voltage threshold, above which the unit triggers. You can set a delay (up to five seconds) to prevent the ignition output from activating so the vehicle can complete the starting process and allow things to stabilize. Finally, a remote output with the same voltage and current delivery capabilities can be triggered by the Ignition output. This output also has a programmable delay option for both turn-on and turn-off functionality. If you want the remote to stay on for four seconds after the ignition sensing goes away, that’s easy!

Nav-TV Signal-to-Power (S2P)

Now, from the East coast, we present the NAV-TV Signal-to-Power. This ingenious and compact interface can provide a switch +12V output (rated for 500mA of current) under one of the three conditions. First, the system can monitor a low-voltage trigger input wire. If it detects a voltage above four volts, it activates the red output wire.

The second option is the ability of the S2P to monitor the vehicle CAN network for data communication to trigger the output. The unit includes both CAN HI and CAN LO connections and is compatible with GM LAN application by connected the brown wire

Compact and simple to use, the S2P provides a multitude of integration options beyond just providing a reliable switched accessory output.

(CAN LO) to chassis ground. If you’ve ever wanted to replace the overhead monitor in a Chrysler or Dodge minivan, you’ll know that the factory units are controlled by the data network in the car. The S2P might just be the tool you need to get a replacement working reliably in that application.

Compact and simple to use, the S2P provides a multitude of integration options beyond just providing a reliable switched accessory output.

The third option, which is a bit of a detour from our discussion, allows the S2P to create an output when it sees a video signal on its composite video RCA connection. If you have a DVD player or game system in the car and want to trigger a monitor or FM transmitter, this is a perfect solution!

The IGN-3 includes a latched output that makes it perfect for use in vehicles with stop-start technology.

The TR-4 from PAC is a simple and easy way to generate a switched accessory signal.

PAC IGN Modules

PAC has three different modules designed to help installers handle low-voltage and PWM conditions. The first is the IGN-1. This device monitors the battery voltage and produces an ignition output (at the same voltage, with up to five amps of current) when it detects the voltage has increased. The IGN-3 from PAC can provide a latched +12V ignition output (with up to two amps of current) when it detects an increase of voltage from the battery. Installation is simple with a red constant power connection and a black ground wire being the only required inputs. The blue output wire goes live when the alternator starts charging and stays live for either 10 seconds if the brown wire loop extending from the module is intact, or 140 seconds if the loop has been cut. The latter is provided for use with stop-start systems.

The TR-4 module works in a similar fashion to the IGN-3. It monitors an input wire for voltages above 0.8V volts, then provides a +12V output. The device applies a one-second delay between detecting the trigger voltage and activation of the output so it can be used as a delay to turn on amplifiers and prevent system pops from other devices in the audio path.

Though overkill in this situation, if you are in a bind and happen to have one in stock, the infamous and flexible PAC TR-7 can, when configured for feature one, be used as a low-voltage trigger generating device. The instruction manual for the TR-7 notes that this can come in handy for Ford radios that use a 5V trigger. It can also be used to convert a negative trigger to a positive one using very little current (unlike a relay).

The TR7 has long been the Swiss Army Knife of creative installers. Knowing how and where to use this module can help save installers hours of frustration.

The IBVSTALE products from Install Bay are a great way to power low-current electronics and ensure reliability in situations where supply voltage may dip.

Metra L-IBVSTABL and IBVSTABL

Products like the IBVSTABL and L-IBVSTABLE from the Install Bay division of Metra can be helpful when dealing with unknown voltage levels. With that said, care must be taken to ensure the circuit you are feeding these DC to DC converters with can provide the current you need to drive the intended load.

Low-Voltage Relays

If you are dealing with a circuit that operates on 5 or 6V, like a backup camera or a USB port, then you can, carefully, use a micro-relay to switch 12 volts to feed an accessory. If you are going to ‘brew your own’ relay solution and choose a relay that is designed to be secured to a circuit board, make sure you take precautions to prevent vibration damage to the connecting pins. Once you’ve soldered the wires you need to the pins and applied heat-shrink tubing, it would be worth bundling the wires together in the middle of the relay and gluing them in place.

Relays can be found from suppliers like Omron, Panasonic and TE Connectivity from suppliers like Digi-Key Electronics and Mouser Electronics.

Understanding how to use a relay is one of the first things a mobile electronics installer should learn.

Examine the Schematics

If you are bound and determined to find a true accessory wire in the vehicle you are working on, then you should have reliable resources at your disposal. Obtaining a subscription to ALLDATA or ProDemand from Mitchell 1 will not only provide you with access to vehicle wiring schematics, but

Long considered the reference for automotive service information, ProDemand from Mitchell 1 can save installers countless hours each year.

also to component and connector locations. In most applications, you will also have vehicle disassembly and reassembly instructions at your disposal. This information can make it easier to find the wire you are looking for while preventing possible damage to trim panels.

Unlike the infamous brown binders that line the shelves of automotive service centers around the country, the above services are accessible online through a desktop computer or tablet. ProDemand (and recently ALLDATA) offer the ability to select a specific wire you want to follow through the schematic and highlight it so that it remains visible as you switch from page to page. You can hide any wires you don’t want to see to speed up access to accurate information and in turn, speed up the installation process.

Finally, if you are creative in your sales process, you can increase your customers’ trust in your abilities by showing them that you have access to the same information and resources as automotive technicians.

Can I Use a Capacitor as a Filter?

Ah, so you fancy yourself an electronics guy and think you can get away with filtering a PWM signal using a capacitor to smooth the voltage going to the

device you are adding. This is great logic and frankly, a good idea. There is only one problem. The circuit in the car was designed for the signal without a capacitor on it. If you add a cap, this will change the perceived voltage at the load or the end of the signal. You have absolutely no way of knowing if the circuit will continue to function normally or not. I wouldn’t recommend this as a solution.

If you are feeling brave and do try this, make sure you choose a capacitor with a working voltage that is at least twice what you’d expect to see in a vehicle. Choosing a 25- or 28-volt cap is a good starting point. Since you will likely be using a polarized electrolytic cap, pay close attention to the polarity when you wire it up. Even a small capacitor makes a really big bang if you wire it backward. Don’t ask how I know.

Test, Test and Test Again

Last and certainly not least, you will want to test the solution you have installed as much as is reasonably possible. If you are working on a project that requires an ignition generating solution, see if you can get it done early in the day. If you can leave the vehicle to rest, then confirm it still works, this will help alleviate any potential problems.

Capacitors store energy in a similar fashion to a battery, making them an ideal filtering solution in some applications.

It’s worth noting that the majority of these solutions are looking for the alternator to start changing. This condition means the product you have installed likely won’t work with the key in the accessory position. Don’t be shy about explaining how you made everything work to the vehicle owner. A client who appreciates your approach to providing a solution will be much more understanding of any limitations.