2007 Dodge Cummins 5.9L P2509 Code: High Voltage Reading (16-17V) After Alternator Replacement
I have a 2007 Dodge Cummins 5.9L diesel engine that consistently reads 16-17 volts when running, even after installing a new alternator and replacing both batteries with high-quality units. This voltage level has already caused two batteries to fail completely. The vehicle previously displayed a P2509 diagnostic trouble code (DTC), which indicates the Powertrain Control Module (PCM) is having difficulty reading the battery voltage. According to forum advice, if the charging system output exceeds 15 volts, a faulty alternator regulator should be suspected. However, I've just replaced the alternator and still see this issue. Should I replace the alternator again? Or should I shift focus to diagnosing potential electrical connections or wiring problems? I'm looking for confirmation on whether P2509 is directly related to a faulty voltage sensor or regulator, and if there are known TSBs (Technical Service Bulletins) addressing this issue. Any insights would be greatly appreciated.
Simply put: the PCM/ECM controls charging and monitors voltage from one of the two battery terminals, but if the alternator is supplying power to the other battery first, it can cause a mismatch. A common issue is a poor or loose positive-to-positive battery connection — always check this thoroughly. Using an OBD2 scanner, you can view what the PCM actually senses as voltage and compare that reading to the actual voltage measured at each battery terminal. This helps determine if the PCM's input is accurate. The P2509 code specifically indicates a problem with the PCM’s ability to read battery voltage — refer to TSB 18-001-07 for known solutions and diagnostic procedures.