A wheel loader was operating normally until the engine reached high idle. At exactly 1,800 RPM, the dashboard would go black, the transmission would drop into neutral, and the engine would stumble. Two seconds later, the dash would light back up, the transmission would engage, and the machine would run fine until the next time the operator hit high idle. The shop replaced the ECU and the transmission controller, but the ghost remained.
We hooked up a digital multimeter to the battery terminals, set it to AC voltage, and reved the engine. At low idle, the AC reading was 0.1 volts. At 1,800 RPM, the AC reading spiked to 4.5 volts. The alternator had a failed diode in its rectifier bridge.
Instead of producing pure DC current, the alternator was injecting high-frequency AC "ripple" into the 24-volt electrical system. At low RPM, the ripple was small enough for the ECU's internal capacitors to absorb. At high RPM, the alternator output increased, and the 4.5-volt AC spike rode on top of the 24-volt DC signal. The ECU saw the 28.5-volt peak as an overvoltage condition and instantly shut down to protect its microprocessors, rebooting when the voltage dropped. A $150 alternator rebuild cured the$5,000 ECU-replacement phantom.
