I'm still pretty new to all this RV stuff and could really use some guidance. My Onan QG5500 generator was working great earlier today - had about 1500 watts running with the AC on a hot afternoon. I shut it down for about an hour, and when I fired it back up, it's only putting out around 400 watts now.
I checked the obvious stuff like circuit breakers and they all look fine. The generator is still running, but something is clearly wrong with the power output. I'm not sure what could cause such a sudden drop or even where I should start troubleshooting. Could this be an electrical issue? Any ideas on what might have failed and how I can figure out what's wrong?
Sudden power drop on QG5500 likely means lost phase, failed capacitor, or thermal protection activation. Check voltage on both legs and inspect for overheating. Test capacitors if voltage is low.
WARNING: Before performing any diagnostics or repairs, completely shut off the generator, disconnect all power sources, and allow the unit to cool completely. Generator repair involves electrical hazards that can cause serious injury or death. Always wear insulated gloves and safety glasses when working on electrical components.
Your ONAN QG5500 dropping significantly in power output while running indicates a significant power generation issue, most likely related to capacitor failure, voltage regulator problems, or rotor/stator winding issues. Since your circuit breakers are fine and it happened suddenly after running well on a hot day with A/C load, the heat stress probably triggered a component failure. The fact that it's still running but producing drastically reduced power points to electrical generation components rather than engine issues.
Heat-related capacitor failure is the most common culprit in this scenario. Your generator was working hard in hot conditions, and capacitors are particularly vulnerable to heat stress. The main run capacitor (correct part for your model - verify with dealer) could have failed, reducing the magnetic field strength needed for proper power generation.
Voltage regulator failure is another strong possibility. The Onan voltage regulator (correct part for your model - verify with dealer) controls the excitation current to the rotor windings. When it fails, you get exactly what you're experiencing - the generator runs but produces significantly less power. Hot weather operation can cause these to overheat and fail.
Rotor or stator winding problems can also cause this issue. Excessive heat can damage the copper windings, creating partial shorts or opens that reduce power output. Carbon brush wear or poor brush contact can also reduce excitation current, though this typically develops gradually rather than suddenly.
Less likely but possible common causes include loose connections at the main output terminals, faulty slip rings, or problems with the automatic voltage regulator sensing circuit. The sudden nature of your problem after heat stress makes capacitor or voltage regulator failure most probable.
Before starting diagnostics, ensure the generator is completely cool and shut off. You'll need a digital multimeter capable of reading AC and DC voltage, a basic tool set including screwdrivers and wrenches, and safety equipment including insulated gloves and safety glasses.
Locate your generator's service manual or download it from Cummins/Onan website using your model and spec numbers. You'll need to access the generator compartment and potentially remove the end cover to reach internal components. Have your phone ready to take photos of wire connections before disconnecting anything.
Purchase a capacitor tester if you don't have access to one, as standard multimeters can't properly test capacitor condition under load. If you suspect voltage regulator issues, you may want to have a replacement on hand (around $150-200) since they can't be easily field-tested without specialized equipment.
Ensure you have adequate lighting in your work area and consider having a helper available, as some components may be difficult to access single-handed. Review the electrical schematic in your service manual to familiarize yourself with component locations before beginning.
Start by checking output voltage at the generator's main AC output terminals with the unit running. You should see approximately 120V AC at the main output terminals (verify correct voltage specifications for your specific QG5500 configuration). If voltage is low (90-100V instead of 120V), this confirms a generation problem rather than a load issue.
Next, locate and inspect the main run capacitor, typically mounted on the side or end of the generator. Look for obvious signs of failure like bulging, leaking, or burned terminals. Test the capacitor with a capacitor tester - it should read close to its rated microfarad value. A failed capacitor will read significantly low or show no capacitance.
Check the voltage regulator next. With the generator running, measure DC voltage at the regulator output terminals - you should see the specified VDC per manufacturer specifications going to the rotor field windings (see service manual for exact voltage range). If this voltage is absent or significantly low, the regulator has likely failed. The voltage regulator is typically a small black box mounted near the main electrical panel.
Inspect all wire connections, particularly at the main output lugs, voltage regulator connections, and capacitor terminals. Heat cycling can cause connections to loosen or corrode. Clean and retighten all connections using torque to manufacturer specifications from your service manual.
If voltage output and regulator voltages check normal, you'll need to test rotor and stator windings using the procedures in your service manual. This typically involves ohm readings between specific terminals with the generator stopped. Significant deviations from specification indicate winding problems requiring professional repair or replacement.
Call a qualified Onan technician if your testing reveals rotor or stator winding problems, as these repairs require specialized tools and experience. Attempting to rewind or repair these components yourself can result in further damage and safety hazards.
Seek professional help if you're uncomfortable working with electrical components or don't have proper test equipment. Generator repairs involving internal electrical components carry shock and fire risks that require proper training and tools to address safely.
If you've replaced the obvious components (capacitor and voltage regulator) but still have the power output problem, internal mechanical issues like bearing problems or rotor position sensor failures may be involved. These require professional diagnosis and repair, typically costing $400-800 depending on the specific problem.
Consider professional service if your generator is still under warranty, as DIY repairs may void coverage. Also, if the unit is older than 10-12 years and requires major internal repairs, it may be more economical to replace the entire generator rather than invest in extensive repairs. Rotor position sensor problems, while less common, can also cause power output issues and typically require professional diagnostic equipment to identify properly.
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