RV Power Systems: Onan Generators, Shore Power, Inverters and Tracing Electrical Faults

The 12 volt direct current system runs lights, fans, water pump, control boards, slide and jack motors and the ignition side of most appliances. It is fed by the house battery bank, which is charged by shore power through a converter, by the generator through the same converter, by the engine alternator while driving, and by solar if fitted.

The 120 volt alternating current system runs the air conditioners, the microwave, the domestic outlets and the heating element in a hybrid water heater. It is fed by shore power at a campsite, by the generator when running, or by an inverter drawing from the house batteries. A transfer switch decides which of those sources reaches the panel, and it decides automatically.

The charging system is what ties them together, and it is where most confusion starts. A converter turns 120 volt input into 12 volt output to charge batteries and run the house circuit. An inverter does the reverse. An inverter charger does both in one box. Knowing which of these your coach has determines what a symptom means.

Because the systems interlink, a fault in one presents as a fault in another. A failed converter shows up as batteries that go flat while plugged in. A failed transfer switch shows up as a generator that runs but powers nothing. A weak battery bank shows up as a slide that will not move. Chasing the symptom rather than the supply is the classic mistake.

Generators need four things and fail when any one is missing: fuel, air, spark or compression ignition, and a healthy start battery. On an Onan installation the start circuit runs from the house or chassis battery depending on the coach, and a bank too weak to crank is the single most common reason a generator will not start after storage.

Fuel supply on gasoline units is the next suspect and it is usually about the pickup tube. Most coaches use a generator pickup set higher in the fuel tank than the engine's, so the generator loses fuel before the vehicle does. A coach showing a quarter tank may be below the generator pickup entirely, which produces a unit that cranks and never fires and an owner who is certain there is fuel.

Stale fuel is the storage problem. Gasoline degrades over months, leaves varnish in the carburettor jets and gums the float. A generator that ran fine before a long layup and will not start afterwards has usually not broken; it has a fuelling system full of degraded fuel. Diesel units suffer less from this and more from air in the fuel line and blocked filters.

Then there are the protections. Low oil pressure shutdown, high temperature shutdown and overload protection will all prevent starting or cause an immediate stop after firing. These are doing their job. Checking the oil level and looking for a fault code on the control panel before assuming a failure saves a great deal of unnecessary work.

A generator that runs happily and delivers nothing to the coach is almost always a distribution problem rather than a generator problem. The three usual causes are a tripped breaker on the generator itself, a failed transfer switch, or a fault between the generator output and the main panel.

Start at the generator's own breaker, which sits on the unit rather than in the coach panel and is frequently overlooked. Reset it and see whether output returns. If it trips again immediately, there is a fault downstream and repeatedly resetting it is not a diagnostic method, it is a way to damage something.

The transfer switch is the next candidate and it is a genuine wear item. It carries the full load of the coach through mechanical contacts that arc every time they change over, and burnt or pitted contacts are common. A switch that has failed in the shore position will pass shore power perfectly and ignore the generator entirely, which produces exactly this symptom.

Beyond that, output faults come down to the generator's own control board, brushes or windings, which is where genuine generator repair begins. OCRV Center services generators in-shop at Yorba Linda, and because this work needs a technician on the unit before anyone can quote honestly, the systems estimate is $150 and it is credited against an authorised repair.

Battery complaints outnumber every other electrical complaint on an RV, and a large share of them are not battery faults. A bank that will not hold charge may be failing, or it may be being undercharged by a converter that has failed, drained by a parasitic load nobody knew about, or simply too small for how the coach is being used.

Flooded lead acid banks fail predictably through sulfation and water loss. They tolerate abuse poorly and they need their electrolyte level checked, which many owners never do. AGM batteries remove the maintenance and are less tolerant of overcharging. Lithium iron phosphate batteries behave differently again: high usable capacity, indifference to partial charging, and an internal management system that disconnects the pack to protect it.

That management system is why lithium diagnostics differ. A pack that has shut itself down looks dead at the terminals but is not faulty. It has protected itself against something, and the useful question is what: low voltage from over discharge, high voltage from a misconfigured charger, excess current from an oversized load, or a temperature outside its charging window. Reading the state tells you the real fault.

Parasitic drain is the quiet killer of stored coaches. Control boards, propane detectors, radio memory, antenna boosters and inverters in standby all draw current continuously, and a coach parked for two months with no charging source will flatten a healthy bank into damage. A battery disconnect switch, actually used, prevents more battery failures than any product.

Intermittent faults are the expensive ones because they resist the obvious approach. Replacing components until the symptom disappears works occasionally and costs a fortune reliably. Systematic tracing costs diagnostic time and finds the actual problem, which is usually a connection rather than a device.

The core technique is voltage drop testing under load. A connection that reads fine with a meter across it at rest can drop several volts when current flows, and that drop is where the energy is going. Testing at the source, at the connection and at the load, with the circuit working, localises the resistance to a specific joint far faster than continuity testing does.

Grounds deserve particular attention on RVs because there are so many of them and because they are often taken to painted or corroded surfaces. A poor ground produces symptoms that make no sense: a light that dims when a pump runs, a control board that resets when a motor starts, a fault that only appears when the coach is damp. Cleaning and remaking a ground fixes a surprising proportion of inexplicable faults.

Instrumented testing is where diagnostic time earns its cost. Clamp meters reading current draw across a cycle, data logging to catch a fault that appears once an hour, and thermal imaging to find a connection heating under load all find things that inspection cannot. OCRV Center charges diagnostic work at $285 per hour with a one hour minimum, credited against an authorised repair, precisely because this work is skilled time rather than parts.

Solar is popular and frequently oversold, because the panel is the visible part and the rest of the system does the actual work. A solar array charges the house bank through a controller. It does not power the coach directly, it cannot run an air conditioner in any realistic residential sized installation, and its output on a winter morning bears little relation to its rating.

The useful way to size a system is by consumption rather than by ambition. Add up what the coach actually uses in a day: the fridge on 12 volt, lights, fans, water pump, device charging, and any inverter loads. That number, in amp hours, is what the battery bank has to store and what the solar array has to replace. Working backwards from consumption produces a system that works; working forwards from a panel count usually does not.

Inverters convert battery power to 120 volt output and their limits are real. A large inverter running a microwave draws enormous current from the bank, and the cable between them has to be sized for it. Undersized inverter cable is one of the more common serious faults found on aftermarket installations, because it heats under load and the heat is inside a cavity where nobody sees it.

Installation quality is what separates systems that work from systems that cause problems. Roof penetrations need backing plates and proper bedding, not a bead of sealant. Cable runs need protection where they pass through metal. Every branch needs fusing sized to the cable rather than to the device. These are unglamorous details and they are the difference between an installation that lasts and one that becomes a fire risk.

The 12 volt system is broadly approachable for a careful owner. Voltages are low, and checking connections, cleaning terminals, testing battery voltage and replacing fuses are all reasonable tasks. The hazard on the 12 volt side is not shock but current: a battery bank can deliver hundreds of amps into a dropped spanner and start a fire in seconds. Remove rings, use insulated tools, disconnect the negative first.

The 120 volt side is different and should be treated as household wiring, because that is what it is. Shore power inlets, transfer switches, distribution panels and outlets carry lethal voltage, and an RV adds the complication that the vehicle may be energised from three different sources. Unless you are qualified for this work, it belongs with someone who is.

Generators combine both hazards with a hot exhaust, a rotating assembly and a fuel supply. Owner maintenance sensibly stops at checking oil, changing filters where accessible, keeping the compartment clean and running the unit under load monthly. Anything involving the control board, the windings or the fuel system is shop work.

One safety item is worth stating regardless of skill: carbon monoxide. Generator exhaust is lethal, exhaust systems corrode, and a leak under a coach can enter through a floor penetration. Working carbon monoxide detectors, tested rather than assumed, are not optional in a vehicle with a generator, and an exhaust inspection belongs in any annual service.

There is a point where an old electrical system should be replaced rather than repaired, and it usually arrives when the wiring itself has become the problem. Corroded conductors, brittle insulation, undocumented modifications by previous owners and a distribution panel full of connections that have been remade several times cannot be economically chased fault by fault.

The tell is repeat visits. If the same coach returns for a different electrical fault every few months, and each visit finds a different corroded joint, the individual repairs are treating a systemic condition. At that stage a planned rewire of the affected sections costs less over two years than the sequence of diagnostic visits it replaces.

Generators reach their own threshold. A unit with low compression, a worn control board and a corroded wiring harness can absorb its replacement cost in repairs without ever becoming dependable. Age and hours matter, but condition matters more, and an honest assessment of whether the core of the unit is sound should precede any major generator spend.

It is also worth questioning whether the system is needed at all. Owners who never dry camp gain very little from a large solar and lithium installation and would be better served by a reliable shore power system. Conversely, owners who dry camp constantly are often better off investing in storage and charging than in repairing a generator they use rarely. The right answer depends on use, not on specification.