Auto Electrics
Part nine: Keyless entry systems.
There was a time when keyless entry systems (KES) were only fitted to high-end vehicles. As tends to happen with technology, KES have now become common on a much wider range of cars. There are many different brand names and acronyms for the systems but, by and large, they function in a similar way. The key can remain in your pocket or handbag and there is no need to press the unlock button on the key-fob to enter the car. Once inside the vehicle, the key is detected and the engine can be started with the push of a button. For drivers, the feature is a simple convenience, but the technology behind it is quite sophisticated.
Keyless entry systems utilise some of the following technology:
Transponders Devices for receiving and transmitting radio signals.
Multiple antennae For low frequency, ultra-high frequency and ultra-wide band communication.
Capacitive sensors Door handles use these to detect the drivers’ hands.
Fixed and rolling codes For authentication of vehicle keys.
Serial, LIN- and Can-bus communication between modules.
The following breakdown of the steps that take place before a door can be opened is a generic view of KES operation and is not specific to any brand or model.
Let’s start with a driver entering a vehicle that is locked and armed. The driver’s door’s exterior handle detects a change in capacitance as the driver’s hand enters the airspace around it. The capacitive sensor sends a signal to the KES module. The KES module sends a command to the low frequency (LF) antenna in the driver’s door.
The antenna sends an LF 125KHZ signal to the key. The key responds with the authorised UHF signal. The radio receiver detects the signal. The radio receiver converts the signal to serial communication and relays it to the KES module. The KES module validates the signal. The KES sends a ‘key valid – open doors’ message via Can-bus to the central gateway module. The gateway module relays the message via Can-bus to the door control modules. The door control modules open the locks and the driver opens the door.
All of the steps outlined opposite take place seamlessly in fewer than 500 milliseconds and the majority of drivers will be unaware of the complexity at work as they reach to open the door. The most common problems with smart keys relate to batteries. When troubleshooting, this is a good place to start. Some keys will need reprogramming after replacing the battery, but this process will vary from model to model and instructions can sometimes be found in the owners’ manual.