Free-running ECUs

Since the early 1990s most vehicle manufacturers incorporated some kind of coding / security element into the ECUs fitted to their vehicles as standard practice. The main purpose was to reduce the theft of vehicles by immobilising the engine via a security system.

A consequence of this practice was that anyone needing to replace their engine ECU could not simply make a purchase, fit to the car and drive away. The official procedure required the purchaser to have the ECU programmed as part of the immobiliser system, and to pay an additional charge for this process.

Free running ECUs were developed for 2 reasons:

The first reason was to avoid the necessity of having to pay the additional cost when purchasing an ECU – the free running version is fitted and runs without any programming. The design process involved in creating the free running ECU varies depending on the type of immobiliser system employed. Some free running ECUs simply plug in without any further modifications required to the vehicle. Other free running ECUs require the disconnecting of some element of the immobiliser system so that the ECU remains isolated from the vehicle immobiliser.

 

The second reason was to get around the problem of defects within the vehicle immobiliser system. Essentially, a free running ECU could be fitted to a vehicle with a perfectly functioning ECU that will not start due to an external fault with some element of the immobiliser system. The convenience and economics of the free running version has to outweigh the diagnosis time in identifying the fault within the immobiliser system, the cost of said replacement parts AND the programming charge as highlighted in the section above.

 

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