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
48 Mobile Electronics January 2020
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.
