Expert advice: what the ECU does and why it’s so vital for our car

All this is computer controlled. The ECU is normally positioned at the top and back of the engine bay (Picture iStock/jaym-z)

Modern cars are more like computers on wheels and central to that is the ECU. If the engine is the heart of the car, the Electronic Control Unit or ECU is its brain. Your car may develop a fault that you think is mechanical but actually the real culprit could be the electronics, caused by a malfunctioning ECU or one of its sensors.

The ECU is now such a crucial and integral part of our cars that I think it’s worth understanding exactly what it does.

What’s in a name?

You’ve probably heard people talking about Electronic Control Unit, Engine Control Unit, Engine Control Module (ECM) and sometimes even engine management system. These are all the same thing; it’s just some car makers prefer one name over another.

It’s important to understand that the ECU isn’t just one computer. It is a number of systems in a network; some new range-topping cars have up to 80 ECUs. Among other things, these monitor and control the engine, transmission (gears), brakes, electronics, body control and suspension. But to make life simpler, we’ll just refer to it as a single thing.

What does the ECU do?

No matter what you call it, what the ECU does is to take data from a network of sensors. These are placed all over the car and constantly relay a mass of information. Among other things they tell the ECU the air temperature, the temperature of various components inside the car, the angle of the car and its steering wheel, the speed of individual wheels, the position of the accelerator, the gear that’s being used, how much air the engine is taking in and the position of and pressure inside various parts of the engine.

How does it work?

The ECU processes and decodes all the information it receives from this sensor network in real time. It stores some of the data, such as faults, so that technicians can access it at a later date. Other information it will act on. The safety systems are a good example of this should the sensors think the car is out of control and crashing. The fuel injection is another.

The ECU knows how warm the engine is, how fast the car is travelling, and the position of the accelerator and where the crankshaft that controls the valves (and fuel use) is. It can therefore dictate and control the exact fuel-to-air ratio, second by second to maximise performance, or efficiency, or to compromise between the two.

What exactly is the ECU?

We suggest you don’t do this… Like most computers ECUs involve highly delicate circuitry (Picture iStock/Urcisxtrime)

Think of the ECU a bit like a personal computer. It has hardware, essentially a collection of electronic components on a printed circuit board, and software. This enables the ECU to be reprogrammed or updated if glitches are discovered or perhaps a better way to do things is discovered.

Where is the ECU?

The ECU is usually in the engine bay. Its precise location varies from car to car but it’s frequently above and behind the engine, beneath the windscreen wipers. But I’d advise you leave well alone. You probably wouldn’t go poking around in your PC and for similar reasons it’s best to leave your ECU to the professionals.

How expensive are new ECUs?

As we’ve seen, the ECU is an integral part of the engine so if you need to replace one, it’s not going to be cheap. In the unlikely event that you’ll need an entirely new one, it could cost anywhere up to £2000. Happily, most of the time, they can be reprogrammed if they start misbehaving.

There are services that offer to reprogramme ECUs for improved performance or economy. If you decide to have this done, make sure you go with a reputable company. The ECU is such an important component, if someone gets it wrong it could be very costly indeed.

What about the future?

As electronic technology has come on, so has the role of the ECU. And it’s only going to get more important. The self-driving technology that features on many cars now, such as cruise control that maintains a safe distance from the car in front and technology that keeps a car in lane, is all linked to the ECU.

How long has the ECU been around?

Car makers didn’t really start using ECUs to control engine functions until the 1970s. This was when microprocessors and integrated circuits came about. The Japanese started using these in engines in the early 70s. A digital computerised control for fuel injection was first used in 1980 by US company Cadillac. The last major car company to stop selling pre-fuel injection carburettor cars was America’s General Motors in 1991. That seems like an awfully long time ago now!

Nick Reid is head of automotive technology for Green Flag and a fellow of the Institute of the Motor Industry

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