AC Output Overvoltage Shutdown Detected

Common Symptoms

• Generator starts normally but shuts itself down within seconds or minutes of coming online
• RDC2 controller display shows fault code 12 after shutdown
• Red fault indicator light is illuminated on the controller panel
• Connected appliances or transfer switch may show signs of power surge before shutdown
• Generator will not return to AUTO mode until the fault is manually cleared
• Repeated shutdown attempts all end with the same code 12 fault
• Generator exercise cycle completes a start but trips off before finishing

Probable Causes (Ranked by Likelihood)

• Faulty or out-of-calibration AVR (automatic voltage regulator) allowing voltage to climb above set limits Very Likely
• AVR sensing wire loose, corroded, or broken causing the regulator to lose feedback and push voltage too high Very Likely
• Failed or shorted capacitor on the exciter circuit driving voltage beyond the upper threshold Likely
• Engine overspeed condition causing generator output frequency and voltage to rise together above the limit Likely
• Faulty RDC2 voltage sensing circuit or damaged sense fuse inside the controller producing a false high-voltage reading Possible
• Loose or corroded AC output wiring connections creating a floating neutral or unbalanced phase that reads high on the controller Possible
• Stator winding fault or partial short that forces abnormal excitation and elevated output voltage Less Likely

Step-by-Step Diagnostic Procedure

1. Step 1. Safety first. Set the panel switch to OFF and disconnect the generator from the ATS or load before touching any wiring. Wait two minutes for capacitors to discharge.

2. Step 2. Clear the fault on the RDC2 controller display panel, then set the switch to MANUAL and let the generator run unloaded. Watch the display for the AC output voltage reading. Normal range for a 240V residential unit is 228V to 252V (plus or minus 5 percent). If voltage climbs above 252V before shutdown, the overvoltage is real, not a false code.

3. Step 3. Check engine RPM on the RDC2 display while the unit runs. A 60 Hz generator should idle at 3,600 RPM. If RPM reads 3,700 or higher, you have an overspeed condition that is directly causing the overvoltage. Do not continue running. This requires a governor adjustment by a qualified technician.

4. Step 4. Inspect all AVR sensing wires. Locate the AVR module mounted on or near the end bell of the generator. Look for any wires that are loose at the terminal, chafed against the frame, or corroded green at the connector. Reseat any loose connectors firmly.

5. Step 5. With the generator OFF, locate the main exciter capacitor. Inspect visually for bulging, cracks, or burn marks. A swollen capacitor must be replaced before further testing. Capacitor replacement is a moderate DIY task. Capacitor cost is typically $20 to $60.

6. Step 6. With the generator running in MANUAL and no load connected, use a true-RMS multimeter set to AC voltage and measure across the L1 and L2 output terminals on the transfer switch side. Compare your reading to what the RDC2 controller display panel shows. If your meter reads 240V but the controller reads 280V, the controller sensing circuit is likely at fault, and a technician with Kohler OnCue Plus monitor access or a factory harness test kit will need to diagnose the RDC2 board.

7. Step 7. Inspect the small sense fuse or fuse holder inside the RDC2 controller (refer to your model's wiring diagram, usually located on the back panel of the controller). A blown sense fuse can cause a false overvoltage reading. Replace with the identical rated fuse only.

8. Step 8. If all of the above checks pass and the fault persists, the AVR module itself is the most likely remaining culprit. AVR replacement is a moderate DIY task on most Kohler RES units. If you suspect a stator winding fault, that requires a winding resistance test with a megohmmeter, which is an advanced test best performed by a certified technician.

Frequently Asked Questions