One of the more common complaints these days from jobbers and retailers when dealing with electronic powertrain control modules (ECMs/PCMs) is the relatively high number of defective returns, exchanges or warranty claims demanded by repair shops. Yet, of all electronic parts that come back defective, ECMs tend to be the ones most often misdiagnosed. This doesn’t come as much of a surprise when one considers the level of complexity of modern ECMs, as well as the specialized equipment and level of expertise required to diagnose them.

Two ECM functions that are responsible for a disproportionately higher rate of ECM misdiagnosis are the vehicle theft deterrent system (VTD) and the computer-controlled charging system. When a new ECM is installed in a vehicle and the result is a no-start, don’t be too quick to blame the new ECM when you know the old one started the vehicle.

The same happens when an ECM is installed in a vehicle to cure a charging system problem, only to find out that the alternator still doesn’t provide a proper charge current. Reducing these unnecessary returns and warranties requires an understanding of how the VTD and alternator circuits work.

OE Vehicle Theft Deterrent Systems
A large percentage of vehicles manufactured over the last five years are equipped with OE anti-theft or engine immobilizer systems. VTD systems first became mainstream on some of the higher-end car...