Onan RV Generator Troubleshooting Guide – Flight Systems, Inc.

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                           A Service of

                 FLIGHT SYSTEMS
               207 Hempt Road  Mechanicsburg, PA 17050
      Ph: 717 590 7330 Fax: 717 590 7327 www.flightsystems.com


RV Generator Troubleshooting Overview                            1
RV Generator FAQs                                                5
Onan RV Generator Charts:
Applications Chart                                               8
Test Information for Troubleshooting                             9
Regulator Pin Assignment Chart                                   10
Control Board P/N Supersedence                                   10
Oil Press/Level Switch Locations                                 10

Onan RV Generator Control Board & Regulator Illustrations
Voltage Regulator Mating Plug                                    11
300-2784/2943 Control Board (Onan Original)                     11
300-2784/2943 Control Board (F.S. Replacement)                   12
300-3056/3687 Control Board                                      12
300-3763/3764 Control Board                                      13
300-4901 Control Board                                           13
300-4902 Control Board                                           14
300-4320 Control Board                                           14
300-4155 Control Board                                           15
Application of External Field Flash using Diode                  15
300-5299 Control Assembly                                        16
RV Generator Measurement Sheet                                   17
                           Issued 6/08; Revised 5/09
Note: While this troubleshooting guide is applicable to most makes of RV Gen Sets, there are many specific refer-
ences to particular Onan Models as these widely used RV Generators are the brand we (Flight Systems) provide
replacement control boards, voltage regulators and test equipment (the G-MAN) for.
What the Engine Control Board Does
The engine control board or module controls the ignition, fuel pump, fuel solenoid (if equipped), field flash and starter.
On LP models, it controls the flow of LP gas to the throttle body. Some models prevent the engine from starting if there
is insufficient oil quantity. Some models prevent continued running if there is insufficient oil pressure. See Trouble-
shooting Chart. NOTE: On models equipped with electronic governors (BGM, NHM Spec. B and later, 300-3764
control board), the oil pressure signal to the control board comes from the governor module. Additionally, most models
require a voltage from the generator to keep the engine running once it has started. See Troubleshooting Chart. All
controls terminate cranking and lock out the starter after the engine has started. The battery charging circuit is part of
the control board on some models, and external on others. NOTE: The controls used on Models KY (Spec. J), KYD
(Spec. A), HGJAB and HGJAC are microprocessor-based and combine the functions of the engine control and
regulator in one module (300-5046, 327-1413 and 300-5374). Because these modules are difficult or impossible to
access without special tools, they are not covered in these instructions.
What the Regulator Does
When the generator is first started, the regulator receives the field flash voltage from the control board and applies it to
the generator field (rotor) to initiate AC voltage buildup in the stator. After the generator output voltage builds up, the
regulator maintains it nearly constant (within a few volts of 120 VAC) under conditions of varying load. It does this by
sensing the 120 volts and constantly adjusting the DC field (rotor) voltage up or down, as needed. For example, if you
turn on a light, the regulator would respond by increasing the field voltage only slightly to compensate for the added
load. If you turn on a microwave oven or an air conditioner, the regulator would increase the field voltage by a much
larger amount, in proportion to the heavier load. The regulator has a built-in protection circuit that reduces the output
voltage of the generator if the engine is running too slow. The purpose of this feature is to protect your appliances from
receiving full voltage at the lowered frequency, as this can cause damage to motors and transformers.
What the Bridge Rectifier Does
The Bridge Rectifier is found on RV generators that do not have an electronic voltage regulator. This will typically be
found on BGE / NHE generators spec A-E. It serves two major functions, field flash and field (rotor) voltage supply.
Upon starting, the control board provides 12 volts DC to the field windings via the + terminal of the rectifier. Once oil
pressure comes up another set of relays closes and opens accordingly, on the control board, and then the rectifier begins
to work. It now takes sensed voltage from the regulating transformer on the AC leads and rectifies it to DC voltage on
the + and  terminals. This is now the DC voltage that supplies the rotor of the generator.
Normal Starting Sequence
This is what normally happens when you start your genset. Pressing and holding the start switch causes the control
board to send 12 volts to the ignition, fuel pump, choke heater, start solenoid and field flash circuits. The engine then
cranks and starts. Oil pressure and generator voltage build up within 1-2 seconds. This causes the control board to
keep sending 12 volts to the ignition, fuel pump and choke heater while cutting off 12 volts to the starter solenoid and
field flash circuits. The start switch can then be released and the engine keeps running. Some models differ slightly in
operation. On models equipped with magneto ignition (KV, Spec. C-F; KY, Spec. A-E), the control board un-grounds
the magneto kill circuit. Model KY, Spec. B-E uses a fuel solenoid in addition to the fuel pump. Models fueled by LP
gas use a fuel shutoff solenoid valve instead of a fuel pump. Several conditions can prevent the engine from starting/
running and are covered under Troubleshooting.
Normal Stopping Sequence
This is what happens when you stop your genset. Pressing and holding the stop switch for a few seconds causes the
control board to cut off the 12 volts to the ignition and fuel pump (and/or fuel solenoid valve) causing the engine to
stop. On models equipped with magneto ignition, the control board grounds the magneto kill circuit. Several condi-
tions can cause the engine to stop by itself and are covered under Troubleshooting.

Determine the control board part number from the board or from the Application Chart, then refer to the Troubleshoot-
ing Chart for test information specific to that board. This troubleshooting guide assumes that the person doing the
work has basic mechanical skills and is familiar with electrical troubleshooting procedures using a multi-meter. Having
a practical knowledge of RV generators is helpful, but not absolutely necessary. DISCLAIMER: Generator servicing
can present certain hazards such as cuts, burns, electric shock, fire, and exposure to exhaust fumes. Take suitable
precautions. Flight Systems will not assume any liability for personal injury or damage to equipment or property as
a result of using this Troubleshooting Guide.

Getting the Engine Running
Before any evaluation of the controls, regulator or generator can proceed, the Onan engine must have oil and fuel, be
in running condition and the 12-volt battery charged. The electronic governor, 151-0752, on Models BGM and NHM,
Spec B and later, must be functioning properly. This guide does not cover engine maintenance and repair procedures
(please refer to the applicable Onan Service Manual for this information). The most common engine problems are
caused mainly by lack of use and/or lack of regular monthly exercise and include the following:

 Low battery voltage because of insufficient charge, worn out battery, faulty cables (partly broken or corroded) or
  poor connections, resulting in slow cranking and hard starting.
 Old or contaminated fuel that has gummed up the lines, fuel filter and carburetor. This can cause clogged jets (mix-
  ture too lean) and/or a stuck carburetor float resulting in an improper mixture (too rich or too lean) or flooding.
 Weak spark and/or fouled spark plug(s) causing hard starting and rough running.
 Stuck automatic choke causing an excessively rich mixture and smoking.
 Stuck oil pressure switch causing shutdown as soon as the start button is released.
 Dirty air filter, causing an excessively rich mixture and smoking.
 Low oil level preventing starting on models equipped with low oil level switch.
 Wiring harness damage from rodents chewing on the wires.
 Corrosion of control board or connections from salty air or road chemicals.


1. CHECK OIL Check engine oil level before starting.

2. GAIN ACCESS Remove panel or cover to gain access to the control board. Some disassembly may be required.
   The exact procedure depends on the model.

3. CHECK BATTERY VOLTAGE Connect the negative voltmeter lead to GROUND and the positive lead to the
   positive battery cable on the starter solenoid and read the battery voltage. The same reading should be obtained at
   the BATTERY POSITIVE pin of the control board. A fully charged battery in good condition should read 12.6 to
   12.8 VDC. Charge the battery if necessary.

CHECK CRANKING Try to start the engine. If it does not crank, check the control fuse (5A Slo Blo, except models
with electronic governor 10A). If the fuse is good, jumper the positive battery post to the coil terminal on the start so-
lenoid. If the engine cranks, there is a poor connection, wiring damage or the control board is defective. If the engine
cranks when the BATTERY POSITIVE pin is jumpered to the START SOLENOID pin on the control board, the board
is defective. NOTE: The battery voltage should not go below 9.5 VDC during cranking.

2. CHECK SPARK If the engine cranks but does not start, remove a spark plug and check for spark during crank-
   ing (spark plug must be grounded). Temporarily jumper the ignition coil positive to the starter solenoid positive
   terminal, or jumper the BATTERY POSITIVE pin to the IGNITION pin on the control board. If you now have
   spark, there is a poor connection, wiring damage or the control board is defective. To determine if it is the control
   board, check for 12 volts at the IGNITION pin during cranking. Other problems such as dirty /worn points or a
   defective coil can cause loss of spark. On models where the oil level switch is hard-wired to the magneto (KV
   Spec. C-F, KVC and KVD), low oil level or a stuck switch will inhibit the spark and prevent starting. On these
   models, it may be necessary to temporarily disconnect the oil level switch if it is stuck closed and not opening. To
   check the level switch, unplug the control board and verify that the MAGNETO KILL circuit on the control har-
   ness is not grounded. Refer to the Troubleshooting Chart.

3. CHECK FUEL SYSTEM If the engine cranks and has spark but does not start, the problem is likely fuel related.
   This can be confirmed by injecting a small amount of starting fluid into the air intake. If the engine fires and tries
   to run, it is starving for fuel. Listen for the fuel pump running during cranking. Check for 12 volts at the FUEL
   PUMP pin on the control board during cranking. If there is no voltage, the control board is defective. The pres-
   sure and flow of the fuel pump (and fuel filter) can be checked by temporarily disconnecting the fuel line to the
   carburetor and jumpering the FUEL PUMP pin on the control plug to battery positive. Take adequate precautions
   when handling fuel. If pressure and flow are normal, reconnect fuel line. Note: If the generator has not been run
   for several months, the carburetor float may be stuck closed and /or the jets and needle valves may be gummed
   up by old fuel that has turned to varnish. These conditions interfere with normal fuel delivery. The automatic
   choke may be stuck closed or binding so that it does not open as the choke heater warms up. On Models BGM
   and NHM spec. B and later, make sure that the electronic governor goes to full throttle one second after cranking
   begins. Any of these conditions will prevent the engine from starting or running smoothly and must be corrected
   before proceeding.

4. CHECK IF ENGINE KEEPS RUNNING The most common complaint is that the engine starts but will not keep
   running when the START switch is released. This happens because the control board will not allow the engine to
   continue running if the generator is not producing voltage or if the oil pressure signal is not present. See CONDI-
   TIONS REQUIRED TO KEEP RUNNING on the Troubleshooting Chart for your model. If the engine will not
   keep running, do the following:

            A1. Generator AC Output Check the generator AC output voltage as soon as the engine starts. Models
            showing "B1-B2 Volts" or "L1 AC Volts" under CONDITIONS REQUIRED TO KEEP RUNNING need
            this AC voltage to keep running. Read between the points shown on the Troubleshooting Chart. If the AC
            voltage readings are good, proceed to "B" below.

            A2. Field Flash If there is no AC, check for 10-12 VDC at the FIELD FLASH pin of the control board
            during cranking. If the field flash voltage is not present, the control board is defective. If the field flash
            voltage is present, the wiring or regulator may be defective. To eliminate the wiring, check continuity
            between the FIELD FLASH pin of the control board and pin 7 of the regulator. If the continuity is good,
            either the regulator or the field circuit (rotor and brushes) may be the problem.

            A3 Field Circuit Check the field circuit by unplugging the regulator and measuring the resistance be-
            tween pins 9 and 10 of the regulator's mating plug. This reading should be 22 to 28 ohms. If this read-
            ing is too high, check the brushes and/or clean the slip rings (use the Slick Stick, or a similar tool). If
            this reading is too low, there may be a short in the rotor. Also, check the resistance from pins 9 and 10 to
            ground. This reading should be very high or infinity. If not, there is likely a ground in the rotor. If these
            readings are all good, the regulator is likely defective.

           A4 External Excitation To confirm that the regulator is defective, turn off the generator's AC circuit
           breaker(s), reconnect the regulator and apply 12 volts through a diode to pin 9 of the regulator plug (see il-
           lustration) while attempting to start the engine. CAUTION: DO NOT attempt this without the diode. If you
           do, there will be fireworks! Remove the 12 volts within 1-2 seconds after the engine starts. If the genset
           now continues to run and produces normal AC voltage, the generator is OK and the regulator is defective.
           If the genset does not continue to run but produces about 40% of normal AC voltage, again the generator is
           OK and the regulator is defective. If no AC voltage is produced, the generator may have a problem that is
           beyond the scope of this guide. Re-check all wiring for security and signs of damage. It may be necessary
           to take the unit to a qualified repair shop.

           B. Check the oil pressure switch as soon as the engine starts. Models showing "LOP to GND" under CON-
           DITIONS REQUIRED TO KEEP RUNNING need the oil pressure switch to be closed to ground when
           running. If the voltage on the LOL/LOP SW pin of the control does not go to near zero, the switch is not
           closing. These switches sometimes stick open if the generator has not been used or exercised often enough.
           Multiple start/stop cycles and/or tapping on the oil pressure switch will sometimes fix it. It may be neces-
           sary to temporarily jumper the switch to ground to keep the engine running. This can be done at the switch
           or at the OIL LOL/LOP SW pin on the control board. Models showing "LOP Open" under CONDITIONS
           REQUIRED TO KEEP RUNNING have electronic governors and require the low oil pressure switch to be
           closed at starting but open during running. When the switch opens, the governor module places a ground on
           the oil pressure input of the control board (P1-5). It is OK to temporarily ground P1-5 on this model to keep
           the engine running. NOTE: See chart for oil pressure and oil level switch locations.

           C. If there is AC voltage present on B1-B2 or L1 AC, as applicable, and the correct oil pressure/level signal
           is present at the OIL LOL/LOP SW input of the control board, and the engine will not keep running, the
           control board is defective.

1. After the engine has been running for a few minutes, the electric choke heater should begin to open the choke. The
   choke will take longer to open fully in cold weather. If the choke does not open, it is either stuck or the choke
   heater is not working. The choke mechanism can be freed up and maintained with "Mouse Milk", a high tempera-
   ture penetrating lubricant.

2. If the engine surges or "hunts" (does not stay at a constant RPM), the cause is likely a gummed-up carburetor or an
   improperly adjusted governor. These conditions should be corrected. If your multi-meter has a "frequency" or a
   "Hz" range, you can check the RPM by measuring the frequency of the 120 VAC at the outlet. It should be approx-
   imately 62 Hz at no load and not go below 58 Hz at full load.

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