Control units (ecu) for your car

The brains of the car: control units

The control unit is the central switch, regulating and storage unit in a car. It permanently processes the information supplied by the car's sensors. According to these data, it gives orders to the separate actors or interpretes these as error. By doing so, the control unit can considerably simplify the search for damage

Construction and position

The control unit is mostly found at an accessible location in the engine compartment, generally placed on the left side of the vehicle in close proximity to the bulkhead. It consists of a small computer placed in a shock-free and waterproof housing. You can find it by following its connections, often thick cable strands leading to it.

Function of the engine control

In the old days, no electronics were needed for the operation of a self-priming engine. Old diesel cars didn't need power, not even for the ignition. The technology was a lot simpler, although a lot more inefficient and producing dirtier exhaust. The development of exhaust cleaning and the improvement of fuel efficiency made it necessary to closely monitor ignition, fuel mix/exhaust mix and fuel supply. Furthermore, unburnt fuel landing in the catalytic converter had to be prevented. The engine couldn't operate either too hot or too cold. In order to reliably accomplish these tasks under continuously changing external conditions, the electronic engine control became indispensable. The engine control unit controls the following values:

• amount of air intake
• air intake temperature
• engine temperature
• oil temperature
• coolant temperature
• engine rpm
• position of camshaft and crankshaft
• ignition
• the level of coolant, engine oil, power steering oil and fuel
• internal pressure of the fuel lines
• residual oxygen in the exhaust
• level of the soot particulate filter (in diesel cars).

Nowadays control units have multiple parameters for monitoring and controlling. If the control unit in a VW Passat indicates an engine oil check, a sensor on the bonnet lock checks if it was opened. This is only an example of the complexity of the modern control units in a car. Control systems have become so sophisticated, that several procedures require the use of additional control units.

Working with the control unit

The control unit functions in the background. It is only noticed in case of an error. It then issues a general error message. In order to interpret the error message correctly, the control unit needs to be read. In the past, the required devices for reading the control unit were very expensive. Nowadays these devices are available rather cheaply or even as a wireless emitter and accessory app for your mobile phone. In any case it is important that the error memory is deleted after repair of the failure. Furthermore, when installing new components like e.g. injectors, it is necessary to program these into the control. This is not easy and requires some expertise.

Defects of the control unit

A control unit is an electronic system which can fail just like any other electronic component. Mostly, burnt elements or loose connections are responsible for the error. The problem is, that control units for older cars are often no longer available or very expensive. It is always useful prior to replacing the control unit to inspect the connection points closely. These often are slightly corroded, which might interfere with data transmission. With a piece of sand paper an expensive repair can thus be avoided. Several service providers are specialised in repair of defective control units. With a targeted replacement of separate elements, e.g. dried up or swollen condensers a defective control unit can be cheaply repaired. The car manufacturers generally advise against these repairs and always recommend the replacement with a new control unit.

Costs of a new control unit

A new control unit can cost over 1,000 EUR (± £900). Its installation is very easy, although it must be programmed afterwards. This is a case for a professional, who asks ca 100 EUR (± £90) for this. The use of used control units is an option. These are available at 50 EUR (± £45) and have to be programmed as well. No guarantee for lasting function of a used control unit applies and therefore this solution contains a certain risk.