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CampWrench
ElectricalModerateAbout 60 min

RV generator will not start

The generator cranks without firing, or does nothing when the start switch is held.

Applies to: Onboard gasoline, diesel and LP generators in motorhomes and towables, including Cummins Onan and Generac units.

Quick answer

Two things catch most owners. First, onboard generators draw fuel from a pickup tube that sits above the main engine pickup, so the generator stops working at roughly a quarter tank even though the vehicle still drives. Second, the generator starts from the chassis or house battery, not from its own supply, so a weak battery gives a slow crank or a single click. If fuel and battery are good, check the oil level shutdown switch, then read the fault code the control board flashes.

What usually causes it

  • Fuel below the generator pickup

    Most common

    The generator's fuel pickup is deliberately set higher in the tank so a generator cannot strand you without drive fuel. In practice this means the generator dies at around a quarter tank.

  • Weak start battery

    Most common

    A slow lazy crank or a single solenoid click is a voltage problem. Generators are usually fed from the chassis battery in a motorhome and the house bank in a towable.

  • Low oil shutdown active

    Common

    Generators carry a low oil level switch that blocks starting outright. It will also trip if the RV is parked significantly out of level, because the sensor reads the sump angle rather than the true quantity.

  • Stale fuel or a varnished carburettor

    Common

    Petrol generators left through a storage season with untreated fuel gum up the carburettor jets. The engine will crank strongly and never fire, or fire briefly and die.

  • Clogged fuel or air filter

    Common

    A restricted filter starves the engine. Generators are usually serviced far less often than their hour meters justify.

  • Control board fault or failed start solenoid

    Less common

    Most units flash a diagnostic code at the start switch or on the panel. That code narrows the fault far faster than parts swapping.

Step-by-step diagnosis

Work in this order. Each step is cheaper or safer than the one after it, so stop as soon as the symptom clears.

  1. Add fuel before anything else

    If the tank is below a quarter, add fuel and retry. This resolves a large share of complaints, and it costs nothing to rule out. On an LP generator, confirm the tank valve is open and there is genuine capacity, not just gauge indication.

  2. Check the starting battery

    Measure the battery that actually feeds the generator, which is the chassis battery on most motorhomes. Look for 12.4V or higher at rest and a crank that stays above 10V. Clean and tighten the terminals, including the generator's own ground strap, which corrodes and is easy to overlook.

  3. Check the oil level and the RV's levelness

    Pull the dipstick and top up to the mark with the specified oil. Then confirm the RV is not parked on a strong slope. A generator on a slope can trip its low-oil shutdown with a technically adequate oil quantity.

  4. Read the fault code

    Hold the start or stop switch as your model's manual describes and count the flash pattern. Codes commonly cover overspeed, overcrank, low oil pressure, high temperature and service faults. Match the count to your manual's table before you touch a single component.

  5. Prime the fuel system

    On many units, holding the start switch in the prime position for 30 seconds runs the fuel pump without cranking. Generators that have run dry or sat through storage often need two or three prime-and-crank cycles before they catch.

  6. Service the air and fuel filters

    Replace the air filter if it is gray or oily, and the inline fuel filter if it has not been changed within the interval in your service schedule. Both are inexpensive and both cause hard starting.

  7. Check the spark plug on gasoline units

    Pull the plug and inspect it. Wet with fuel means it is getting fuel and not spark. Dry and clean means it is not getting fuel. Black and sooty means a rich mixture or a blocked air filter. Re-gap or replace to specification.

    Let the generator cool fully before removing a spark plug, and disconnect the start battery so it cannot crank while you work.

  8. Exercise it monthly once running

    Run the generator under at least half load for two hours a month. Under-loading and long idle periods cause more generator failures than heavy use does, particularly wet stacking on diesel units.

Safety notes

  • Never run a generator with the exhaust obstructed or near an open window, hatch or neighbouring RV. Carbon monoxide from generator exhaust is a leading cause of RV fatalities.
  • Test your carbon monoxide alarm before every trip and replace it at its expiry date.
  • Disconnect the start battery before working on the ignition system.

When to call a professional

Carburettor rebuilds, fuel pump replacement, control board faults and any repeating overspeed or overcrank code should go to an authorized service dealer. Generator fuel systems sit directly under living space and are not the place for improvisation.

Frequently asked questions

Why does my RV generator stop at a quarter tank?

By design. The generator's fuel pickup is set higher in the tank than the drive engine's, so the generator runs out first and leaves you enough fuel to drive to a station. It is not a fault.

Which battery starts an RV generator?

On most motorhomes the generator starts from the chassis battery. On towables and some coaches it starts from the house bank. Check your wiring diagram, because testing the wrong battery sends people down the wrong path for hours.

Can I run my rooftop air conditioner off the generator?

Usually yes, if the generator's output rating covers the air conditioner's starting surge. A 3600W class unit typically runs one air conditioner. Starting surge is far higher than running draw, so a soft-start device on the air conditioner helps considerably.

How often should a generator be serviced?

By hours, not by calendar. Follow the hour intervals in your service schedule for oil, filters and plugs, and run it monthly under load to keep the fuel system healthy.

Also described as

  • onan generator wont start
  • rv generator cranks but wont fire
  • generator clicks but wont crank rv
  • onan fault code flashing

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Want the background rather than the symptom? Understanding your RV electrical system explains how the whole system fits together.