Vehicle Computer Ignition Control: Wiring, Modes and Delay Timers

Vehicle computer ignition control is the power management feature that watches a vehicle’s ignition signal and starts, holds and shuts down the computer with configurable delays, so it boots after the engine starts, stays on through short stops and closes down cleanly before the battery is drained. It is also called ignition power control, ignition sensing or ignition management, and it is standard on most computers built for vehicles.

One of the most common uses of a rugged PC is in-vehicle computing, where the computer runs from the vehicle’s battery and power is limited. Deployments range from buses and trains to autonomous mobile robots for last-mile delivery. The roles of rugged in-vehicle computers vary widely, but one function is common to them all: controlled, configurable and automatic start-up and shutdown.

Take a delivery vehicle. Its computer keeps the vehicle connected to a logistics centre over 4G or 5G, so it should not power down every time the driver switches off for a drop. Programmed ignition settings let the computer stay on with an uninterrupted connection, start in step with the other systems in the vehicle, and shut down properly, closing every application, once the working day ends. Our short explainer on what ignition control is gives the definition; this guide covers how it works, how to wire and configure it, and how to choose the delays.

The most common implementation senses the vehicle’s ignition signal. In a car this might be the ACC (accessory) position of the key or another start signal, all of which relate to switching the vehicle’s electrical power. A low-power microcontroller (MCU), separate from the main processor of the industrial PC, monitors that signal and manages power to the rest of the system. In its monitoring state the MCU itself draws only milliwatts, and complete systems typically draw a few tens of milliamps or less with the ignition off.

The MCU’s main job is to control an orderly power-on and power-off delay around the computer’s own start-up and shutdown. The basic sequence runs like this:

The operating system and applications keep their own procedures for booting, running and shutting down, distinct from the power controller. The MCU simply decides when those procedures start, which is why the operating system must be set to respond to the shutdown request, covered below.

Ignition control only works if the computer is wired to permanent power. If the main supply comes from a switched circuit, the computer loses power the moment the key turns, and no delay can help; repeated hard power cycles like that eventually corrupt the file system. The usual connection is three wires on a terminal block or a locking circular connector:

Some computers bring power and ignition in on one locking M12 connector instead, as on the Sintrones IBOX-601-G2 series. Many computers have a PC/Car or ignition mode switch next to the power input. In PC mode the ignition input is ignored and the computer behaves like a desktop; in car or ignition mode it follows the IGN line. Check this switch first if a newly wired computer does not respond to the key.

12 V, 24 V and engine cranking

Vehicle supplies are noisy. Cars and vans run on 12 V systems, trucks and buses on 24 V, and the voltage dips sharply when the starter motor cranks the engine, then spikes when large loads switch off. Wide-range inputs, commonly 9–36 V or 9–60 V DC, let one computer run on both 12 V and 24 V vehicles and ride through most of these events. The power-on delay helps too, because the computer does not try to boot during the cranking dip.

Vehicle electrical standards describe the events a computer must survive. ISO 16750-2 covers supply voltage ranges, cranking profiles and load dump, and ISO 7637-2 defines the electrical transients on supply lines; military ground vehicles use MIL-STD-1275, and rail vehicles follow EN 50155. For road vehicles in Europe, computers fitted to the vehicle usually need E-Mark approval, explained in our guide to E-Mark certification.

Rugged vehicle PCs with ignition power control offer several modes with different power-on and power-off delays, alongside the standard ATX and AT power modes.

The ignition modes are numbered, and as the mode number rises the delays typically get longer. On many computers a power-on delay can be set from about 10 seconds to three minutes, and a power-off delay from about 10 seconds to two hours. Some ranges offer 16 modes; Vecow, for example, lists 16-mode software ignition control on computers such as the EAC-6100 Jetson Orin NX series. Ranges differ by manufacturer, so check the manual for the exact pairs on your model.

Choosing the delays

The right delays depend on what the computer does after the engine stops. The figures below are starting points, not rules; test them on a real duty cycle.

A fleet telematics gateway reading the CAN bus needs only enough off delay to send its final trip report. Our case studies show the pattern in practice, from a connected ambulance fleet to bin detection on a refuse collection vehicle.

Ignition power control includes protective features that govern start-up and shutdown when things do not go to plan.

The figures vary by model. As one example, the Neousys MezIO-V20 ignition module draws 12 mA in standby with the ignition off, shuts the system down if the input stays below 11 V on a 12 V supply or 18 V on a 24 V supply for 60 seconds, and cuts power 10 minutes after the off delay if a soft shutdown fails. Low-voltage cut-off settings in the region of 11 V for 12 V systems and 22 V for 24 V systems are common elsewhere, and vehicle computers such as the Axiomtek UST210-83K-FL series handle start-up, shutdown and low battery through their smart ignition input.

Ignition settings vary between rugged PCs. Some models use a rotary selector switch on the rear panel to choose the mode and its delays; others use DIP switches; and on many current computers the settings can also be changed in the BIOS or through a software utility. DC power and the ignition signal connect through the standard power connector. Always refer to the manual for your computer for the wiring and configuration settings.

Setting up the operating system

When the off delay ends, the ignition controller asks the operating system to shut down, usually by emulating a press of the power button. The computer will only close down cleanly if the operating system is set to respond:

• Windows: set the power button action to “Shut down” in the power options, and make sure no application blocks shutdown with an unsaved-changes prompt
• Linux: set the power key handling in systemd-logind or acpid to power off, and check that the service runs on headless systems
• Applications: flush recordings and databases on a shutdown signal, and use a journaling or read-only root file system where possible

A watchdog timer complements ignition control by restarting a computer that hangs while the vehicle is running, and the hard-off timer covers a hang during shutdown. Our engineers set the ignition mode, the delays and the operating system power settings to your requirements before delivery, then test the start and shutdown sequence.

Ignition control decides when the computer runs. Other features handle what happens when power is lost unexpectedly, or when the computer needs to be woken remotely.

A supercapacitor backup between the vehicle supply and the computer rides through deep dips and a battery disconnect; our comparison of supercapacitor and battery UPS options explains the trade-offs, and some industrial UPS modules have ignition control built in. For computers parked at a depot, our guide to Wake on LAN covers remote power-on, and out-of-band management lets you control power without the operating system running.