This guide covers Cummins Onan sets, the built-in generator on many motorhomes and some trailers, whose operator manuals write down most of the ways a set fails, with a blink code for each one. This guide is that manual’s own list, in its words, so that when the set quits at a Quartzsite boondocking spot in January or a Lake Havasu campground in July you know whether you are looking at a five-minute fix, a call to a mobile tech, or a limit the machine was never going to beat.
Read the blink code first
Onan RV sets report faults through the light on the start-stop switch. The KYD manual, which covers the QG 3600 and 4000 LP sets: “Three blinks indicates a service fault.” “Press Stop once to cause the two-digit, second-level fault code to blink.” Two blinks is low oil pressure. “Four blinks indicates that cranking exceeded 15 to 20 seconds without the engine starting.” The larger HGJAB sets say 30 seconds.
The two-digit codes are where the manual tells you who fixes what. Code 36, “Engine stopped without command by controller,” gets a seven-step owner list that starts with fuel. Code 13, undervoltage, and code 15, underfrequency, get one line: “Reduce the number of connected appliances, especially when air conditioners and battery chargers are running.” Code 32, low cranking speed, is the battery: “Clean and tighten the positive (+) and negative (-) battery cable connections at the battery and at the genset,” then “Recharge or replace the battery,” and a note that “High oil viscosity can slow down cranking speed.”
Codes 12, 14, 19, 27, 41, 45, 48 and 51 all get the same instruction: “See an authorized Onan dealer.” Those are the voltage regulator, governor, sensing and processor faults. There is nothing an owner is meant to do about them beyond writing the number down.
What the manual says to check before you call
Oil. The first-level low oil code is code 2, and the fix is on the dipstick in either direction: “Check engine oil level and add oil as necessary,” or, if you overfilled it, “Drain excess oil (above dipstick Full mark.)” The maintenance table in the A035D009 manual puts the oil level check at “Every Day or Every 8 Hours” and the oil and filter change at “Every 50 Hours,” with the KYD manual adding “Change oil more often in hot and dusty environments.” Arizona is both.
Fuel. When the generator and the vehicle engine share a tank, the manual is direct about the trap: “The genset fuel pickup tube is probably higher up in the fuel tank than the vehicle engine pickup.” Its suggestion is to learn where that is: “Marking the genset empty point on the fuel gauge will make it easier to tell when to stop the genset before running it out of fuel.” On propane sets there is a temperature version of the same problem: “On cold days the LPG container may have to be kept at least half full to provide the rate of vaporization necessary to keep up with the genset fuel demand.” A Show Low morning in March qualifies.
Priming. After running dry, the overcrank fix is to “Prime the engine fuel system by holding the control switch at Stop/Prime for 30 seconds.” Hold it longer than three minutes and you get code 57, overprime.
Load. If the set starts and then drops out when the second air conditioner kicks on, the manual’s answer to codes 13, 15, 22 and 38 is the same: fewer appliances, and for code 38, “Have air conditioners and other appliances checked for proper operation. (A locked compressor rotor can cause very low power factor.)”
The two Arizona limits
Heat. The A035D009 manual: “As ambient temperature increases, rated generator set engine power decreases approximately 1% for every 5.5 °C (10 °F) above 25 °C (77 °F).” At 117 degrees that is 4 percent gone before you switch anything on, and the manual adds, “It may be necessary to run fewer appliances at higher altitudes and ambient temperatures.”
Altitude. The same manual: “Power decreases approximately 3.5% of rated power each 305 m (1000 ft) of increase in elevation.” The KYD manual prints the table for a 4,000-watt set: 4,000 watts up to 500 feet, 3,720 watts at 2,500 feet, 3,300 watts at 5,500 feet. Above 5,500 feet the table takes off another 140 watts per 1,000 feet, which is the arithmetic behind a set that ran two air conditioners in Yuma and struggles with one in Flagstaff or Show Low. For the HGJAB series the manual says to “reset the carburetor for the current altitude” between 0 and 5,000 feet, and it warns that “Operating the generator set at low altitude with a high altitude setting can cause power loss, overheating and engine damage.” Above 5,000 feet for an extended stay, “a high altitude kit is required.”
Exercise it, or it will not start when you need it
The single maintenance rule the manual repeats most: “Exercise the genset at least 2 hours each month if use is infrequent. Run the genset at approximately 1/2 rated power.” It explains why: “Exercising a genset drives off moisture, re-lubricates the engine, replaces stale fuel in fuel lines and carburetor and removes oxides from electrical contacts and generator slip rings.” And it adds that “A single two hour exercise period is better than several shorter periods.” A snowbird rig that sat in a Mesa storage lot from April to October without this is the one to exercise before the first cold night, not after.
Carbon monoxide
The manual’s safety page is not optional reading in a desert where people run the generator all night for the air conditioner. “Never occupy the vehicle while the genset is running unless the vehicle is equipped with a working carbon monoxide detector.” “Inspect for exhaust leaks at every startup and after every eight hours of running.” And: “Do not operate the genset if there is an exhaust leak or any danger of exhaust gases entering or being drawn into the vehicle.” Check the detector’s date before the season, not after.
When to call a tech
Cummins draws the line itself: “Only trained and experienced service personnel with knowledge of fuels, electricity, and machinery hazards should perform genset service.” Anything the code table sends to a dealer, plus a carburetor rebuild, a stator or brush problem, or a control board, is that work. A mobile tech can do most of it in the generator bay at your site; the techs below say generators are among the systems they work on. Bring them the model number, the hour meter reading, and the code you saw, and the visit gets shorter.