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The power control module

If your car has ever broken down you'll know it can be both a humiliating and humbling experience, especially if a little prior warning could have rescued the situation or at least minimised its impact and allowed you to get back to a place of refuge for the problem to be fixed. Or worse still, if you're stuck beside the side of the track or special stage knowing that a minor issue could be cured and you could be on your way, and in with a chance of a good placing but for a want of a 30-second fix. At such times, it would appear that the world is against you.

Electrical gremlins can be behind many a DNF, and because they are often so apparently minor - a broken sensor or chaffed wire for instance - they are all the more frustrating. But some of them are avoidable. Better preparation is the watchword here, but for others perhaps the answer is a power control module (PCM).

A PCM is a solid-state module that replaces relays and circuit breakers, and as a kind of intelligent controller it can protect your electrical circuitry much as a fuse would. But a PCM goes much further than that. Unlike a fuse, a PCM can decide if the problem was just a spike in the current supply and switch the circuit back on again or, if the sensor or its circuitry was terminal, decide on some other course of action. A rally car for instance, using several hundred amps of forward lighting to find the way, can shut down just a sufficient number of the lights so that a failing generator can power the engine management. Without such a device maybe the generator would overload and fail all the sooner, leaving you with just the battery to get you home (or not).

But a PCM is even more than that. It can exchange data with all the other power modules around the car and can monitor the activity of the complete electrical system to even out the current supply and avoid unnecessary or unproductive current drain. Take for instance the initial cranking and firing of an engine. With the starter taking many hundreds of amps, the power to all other circuits (including the engine management) can be shut down until the engine has reached the required cranking speed. At that point the EMS can be switched back on again and the engine fired.

Sensibly integrated with rest of the electrical system, the installation of a PCM can even save weight, replacing maybe not miles of untidy wiring but a considerable amount. And if you've ever weighed an engine or vehicle wiring loom then you'll know that the weight is often recorded in kilogrammes not grammes. The provision of something more than just an intelligent circuit breaker can therefore often save you weight. Unlike a fuse, however, for which you need to have access, a PCM can be installed safely out of harm's way and in a place that is most beneficial to the vehicle and not the driver and/or his mechanic.

It may be fashionable to have the latest piece of electronic gizmo in the car but with a PCM it can only make sense.


Fig. 1 - The power control module

Written by John Coxon

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