DC voltage and fuel control repair COLUMN BY KEVIN MARKEL hy is this car running pig rich?” The question has come up numerous times in my training classes. I have seen the problem in my shop as well as many OEM and independent shops across the country over the years. Not surprisingly, the vehicle was avail- able for my inspection. I would listen to the tech as to what the customer com- plaint was and the work that had been performed. More commonly than not, the vehicle had no DTCs (trouble codes) or at the very least, we would retrieve an O2 code or system rich code. Naturally the oxygen sensor had already been replaced, in some instances along with several of the following combi- nations: the MAP (manifold absolute pressure) sensor, ECT (engine coolant sensor), ACT (air charge temp.) sensor, fuel pressure regulator, fuel injector and possibly the ECM/PCM (engine control module/powertrain control module). So, in what our experience has shown before, how did the repair get so far off the mark? You might have gathered from the headline of this article, the vehicle had an alternator problem. How can that be? We are and have been for a number of years dealing with electronic fuel injection. One wire at the fuel injector is the control side from the PCM/ECM. Most com- monly the computer grounds the fuel injector to activate the injector. That has not always been the case, some manufac- tures used power control. Fortunately ground control has prevailed. The remaining wire at the injector is supplied power from several possible sources such as a relay, fused circuit and the computer itself. The computer has an operational range of fuel control based on several sensor inputs that dictate load and or operating condition. What is typically not known is that control is coupled to voltage. So, if you have a runaway volt- age regulator, the vehicle will run rich, because the voltage is beyond the range the computer can correct for. Conversely, if charging system voltage is too low the vehicle will run lean. At one point in engine control, the computer could not correct if charging system voltage dropped into the 11s. The upper end limit is typically in the 15.5 to 16.0 volt range. With the concern for leaving the customer stranded and to improve reliability, the operational cor- rection range in voltage is far greater, whereas before with decaying charging system voltage, the resultant would have been a lean condition. Today the drop in voltage will be lean initially and the PCM will force maximum pulse width to get as much fuel through the injector. This is despite a lack of voltage to keep the vehicle running to the point where charging system voltage is so low that the ignition system can no longer be pow- ered. This is usually around 9.6 volts approximately. So, keep in mind the drivability prob- lem you or your customer has could be as simple as the charging system. Check the battery and the alternator. Kevin Markel is owner on Pro Auto Tech, Ft. Meyers, Fla. Please e-mail technical questions to polly@erenewspaper.com to be addressed in future issues. ETRIE|