So my Onan 5500 is being a real pain lately. Starts up fine, purrs like a kitten when nothing's plugged in, but the second I flip the breakers back on to actually use the thing, it bogs down and dies.
I've already done the usual suspects - fresh gas, new fuel filter, rebuilt the carb, new plugs. Interestingly, if I flip off the breakers, start the generator, and see the quick flashing light on the start button before it fully starts, then flip breakers back on, it'll run perfect for a few minutes. But then it'll croak as soon as I try to power anything. Sometimes it seems like it's struggling right before it shuts down, like it's not getting enough juice.
Anybody dealt with this quirky Onan 5500 behavior before? Is there something I'm missing that could cause it to handle no load fine but choke under even a small electrical load?
Shutting down under load after rebuild suggests carburetor still not delivering enough fuel or governor sticking. Watch governor arm movement during load test. Try gradual load increase to find threshold. Check fuel pump output if equipped.
Your Onan 5500 is experiencing an overload shutdown condition that's being triggered intermittently. The fact that it runs perfectly when the breakers are off, but shuts down when they're engaged, points to either a load issue on the RV's electrical system or a problem with the generator's overload protection circuit. Since you've already done excellent work on the fuel system (fresh gas, filter, carb rebuild, plugs), this is almost certainly an electrical issue rather than a fuel delivery problem.
The symptoms you describe - running great with breakers off, then shutting down when breakers are engaged - indicate the generator is detecting an overload or fault condition. When the breakers are off, there's no load on the generator, so it runs smoothly. The moment you flip the breakers back on, something in your RV's electrical system is either drawing too much current or creating a ground fault that triggers the shutdown.
Common culprits include a failing air conditioner compressor that's drawing excessive startup current, a water heater element that's partially shorted, or even something as simple as a extension cord that's been damaged and is creating a ground fault. The "quick flashing light on start button" you mentioned is the generator's way of indicating it's running but detecting an issue.
Another possibility is that the generator's overload protection circuit itself is becoming hypersensitive due to aging components. The control board can develop issues where it shuts down prematurely even under normal loads. Always verify the exact control board part number with an Onan dealer using your generator's model and specification numbers before ordering replacement parts.
Since this is intermittent, it could also be a loose connection somewhere in the transfer switch or main electrical panel that's creating resistance and heat, which the generator interprets as an overload condition.
For diagnosing this electrical issue, you'll need a digital multimeter capable of measuring AC voltage and current, a clamp-on ammeter to measure individual circuit loads, and basic electrical tools including wire strippers and electrical tape. A non-contact voltage tester is also helpful for safety when working around live circuits.
You'll also want to have some electrical cleaner contact spray and dielectric grease on hand for cleaning connections. If the control board needs replacement, having a basic soldering iron and electrical connectors will be useful, though many connections are plug-in style on the 5500 series.
Safety equipment is crucial - insulated gloves rated for electrical work, safety glasses, and make sure you have a good flashlight since you'll likely be working in tight spaces around the electrical compartments.
WARNING: Always disconnect all power sources and turn off the generator before performing any electrical work. Failure to do so can result in serious injury or death from electrical shock.
Start by systematically isolating the electrical load. With the generator running and breakers off, use your clamp-on ammeter to measure the no-load current - it should be very low according to manufacturer specifications. WARNING: When using clamp-on ammeter on live AC circuits, ensure proper PPE, maintain safe distances from other conductors, and never touch exposed terminals while measuring. WARNING: When using clamp-on ammeter on live AC circuits, ensure proper PPE, maintain safe distances from other conductors, and never touch exposed terminals while measuring. Turn on individual breakers one at a time while monitoring the current. When you find the circuit that causes the shutdown, you've found your problem area.
Check all connections at the transfer switch first. Look for any signs of burning, corrosion, or loose wire nuts. The transfer switch is often located near the main electrical panel and handles switching between shore power and generator power. Loose connections here can create resistance that shows up as an overload to the generator.
If individual circuits check out, the problem may be in the generator's control system. After properly shutting down the generator and waiting at least 5 minutes for capacitors to discharge, remove the control panel cover and inspect the control board for any obvious signs of damage - burnt components, swollen capacitors, or corrosion. The control board is typically mounted inside the generator compartment and has multiple wire harness connections.
Test the generator's voltage output under load using only the RV's existing circuits with breakers controlled individually - never back-feed or create artificial loads without proper equipment. It should maintain steady voltage of 120V ±5% line-to-neutral per leg when loaded. If voltage drops significantly under load, this could indicate internal generator problems. On brush-type models, this may be worn brushes or slip rings, while brushless models may have stator or rotor issues - which would require more extensive repair.
Clean all electrical connections with contact cleaner and apply dielectric grease to prevent future corrosion. Pay special attention to the connections at the generator output terminals and the transfer switch input terminals.
If you've isolated the problem to a specific high-current appliance like the air conditioner and the unit is drawing excessive current, you'll need an HVAC technician to diagnose the compressor or other components. Air conditioner repairs often require specialized tools and refrigerant handling that's best left to professionals.
Call an RV electrical specialist if the problem appears to be in the generator's internal components like the stator, rotor, or slip rings. These require disassembly of the generator and specialized testing equipment to diagnose properly. Also, if you're not comfortable working with live electrical circuits or don't have experience with multimeters, this is definitely a job for a professional.
If the control board needs replacement and you're not comfortable with electrical connections, have it done professionally. While it's not overly complex, incorrect wiring can damage the new board or create safety hazards. The labor cost is usually around 2-3 hours, and the control board itself runs about $200-300 depending on the exact model.
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