Onan Control Board Operation

The contents of this document are based upon personal experience gained by "Hands-On" vehicle maintenance over many years. They are "One Man's" viewpoint & do not prepresent authorized data pertaining to the GMC Motorhome.
It is the Reader's responsibility to establish his/her position associated with each subject matter before vehicle repair and/or modifications are accomplished.

ONAN MOTOR GENERATOR
GENERAL INFORMATION

The Onan Motor Generator is powered by a two cylinder - four cycle horizontally opposed gasoline engine. An automotive type starter is used to start the engine. The starter is powered by a battery located in the motor generator compartment. A permanent magnet flywheel alternator and solid state voltage regulator is used to charge the battery on the initial coach configuration (dedicated onan battery, most have been upgraded to share battery with house and are charded via the engine alternator and AC to DC converter).

Lubrication is provided by an oil system with a spin-on type oil filter.

The Motor Generator's electric fuel pump draws fuel from the vehicle's main fuel tank.

A Control Circuit Board provides control functions for operation including the following:

(a) Ignition start and stop control
(b) Fuel pump and fuel solenoid control
(c) Automatic starter disconnect
(d) Circuit fuse protection
(e) Automatic delayed los oil pressure condition shut-down
(f) Electric choke operation
(g) Remote control capability

3

MOTOR GENERATOR WIRING DIAGRAM
Large View

CIRCUIT BOARD SCHEMATIC DIAGRAM
Large View

FIGURE 1
START CIRCUIT SCHEMATIC DIAGRAM
Large View

STARTING CIRCUIT [Figure 1]

Pressing the "START" switch (S3 or the Remote Switch) will power the starter relay [K1] by applying a ground through the "HOLD RELAY" contact K2a (normally closed contact)

When the "Start" relay [K1] is energized it activates:

(a) Starter solenoid and starter motor [B1]
(b) Electric choke solenoid [E1]
(c) "Run Relay" [K3] thru CR5 and R2

The "Run Relay" contact [K3] (normally open) applies power to the following:

(a) Fuel pump [E2]
(b) Fuel solenoid [K4]
(c) Ignition coil [T1]

Resulting action: Engine starts and runs

FIGURE 2
RUN CIRCUIT SCHEMATIC DIAGRAM
Large View

RUNNING CIRCUIT (Figure 2)

The output (AC voltage) of the alternator [G1] increases as the engine speed increases. This AC voltage is converted to DC voltage by Diode [CR1], Resistor [R1], and Capacitor [C1] and is applied to the "Hold Relay" coil [K2].

The "Hold Relay" [K2] is energized when the applied voltage reaches 10 to 11 volts DC (typical). One contact of [K2} opens to disable the start circuit ([K2a] - See Fig. 1) and the second contact [K2b] closes to apply power to the following:

(a) "Run Relay" coil [K3]
(b) Remote control panel
(c) The low oil pressure (LOP) circuit.

FIGURE 3
STOP CIRCUIT SCHEMATIC DIAGRAM
Large View

STOPPING CIRCUIT (Figure 3)

Pressing the "STOP" switch (S3 or Remote Switch) will short out the "Run Relay" coil [K3] which de-energizes and removes power from the following:

(a) Fuel pump [E2]
(b) Fuel solenoid [K4]
(c) Ignition coil [T1]

Resistor [R2] limits the short circuit current thru the "Run Relay" coil [K3] and Diode [CR7]

As the engine speed decreases, the AC voltage output of [G1] decreases to a potential where the "Hold Relay" [K2] de-energizes and drops out (opens). This action removes power from the "Run Relay" coil [K3], the remote control panel and the low oil pressure circuit. The action also resets the [K2a] contact to the starting position for future starts.

Resulting action: Engine stops and resets starting circuit.

FIGURE 4
LOW OIL PRESSURE (LOP) CIRCUIT SCHEMATIC DIAGRAM
Large View

LOW OIL PRESSURE (LOP) CIRCUIT (Figure 4)

The engine opl pressure activates the LOP switch [S2] to an open condition (no connection) for normal engine operation. However, if a low oil pressure condition exists, the LOP switch activates the LOP circret by applying a ground to control board terminal 12.

Power (+) is applied to the LOP circuit by the "Hold Relay" [K2} after the engine starts. With power applied and the LOP switch closed (low oil pressure condition), the circuit is complete and capacitor [C3] starts to charge thru resistor [R3]. When capacitor [C3] reaches the level of the voltage divider resistors [R5 - R6], the programmable unijunction transistor [Q1] fires (conducts) and the charge on capacitor [C3] is conducted thru resistor [R4} to the gate of the silicon controlled rectifier [SCR-CR8], which puts it in a conducting state [on]. Resistor [R7} and capacitor [C4] prevents unintentional operation of the SCR [CR8].

Typical time delay is 3 to 5 seconds

The energizing current that holds the "Run Relay" coil [K3] during "Engine Run" is routed (shunted) to ground via the diode [CR6], the SCR [CR8] and the LOP switch [S2], effectively shorting out the "Run Relay" coil [K3] which then opens (drops out), shutting down the engine as noted in the "Stopping Circuit" description (Fig 3).

Note: LOP circuit is reset (initialized) upon power removal

EMERGENCY START / STOP PROCEDURE

Bypass circuit board (all functions except fuse) as follows:

! Jumper terminal 9 to 5 (fused power) - This will energize the fuel pump, fuel solenoid and ignition coil
! Press the "Start Switch" [S3] or temporarily jumper terminal 1 to 7 - This will engage the starter. Remove jumper as soon as the engine starts. Note: Do not press switch or jujper terminal 1 to 7 while engine is running.
! Remove jumper from terminal 9 to 5 for engine stop

CAUTION: This emergency / test operation does not provide starter disconnect or Low Oil Pressure shutdown protection and should not be used without monitoring the motor generator.

For Emergency and Test Situations Only

14

LOW OIL PRESSURE (LOP) CIRCUIT TEST PROCEDURE

Verify board low oil pressure (LOP) circuit operation as follows:

Engine running under normal conditions
Jumper terminal 1 to 12 and measure time required for engine to stop
Typical delay time should be 3 to 5 seconds
Remove jumper and re-start engine
Board test complete
Circuit board is OK with 3 to 5 seconds time delay
Circuit board Malfunction if no time delay or if engine does not stop

LOP switch and wiring test

Remove wire from board terminal 12
Measure resistance to ground of wire 12
Without engine operating, should be near zero ohms
With engine operating, should be high resistance (open)
Test complete: Attach wire to terminal 12 and start engine

Note: A malfunction LOP circuit could cause permanent engine damage due to operation with loss of oil pressure.

15

REMOTE CONTROL PANEL FUEL PRIMING CIRCUIT

Modify remote control panel wiring with the following:

5 amp fuse and fuse holder
5 amp diode (50prv)
Momentary switch (5 amp)
Hook-up wiring

Locate local +12 volt source and connect fuse holder

Drill hole in panel and mount momentary switch

Connect fuse holder to anode end of diode

Connect cathode end (with band) of diode to switch (input)

Connect switch output to start / stop switch terminal that is activated with +12 VDC while the Onan is running
Also wire is present between switch terminal and RTM if so equipped

Press and hold momentary switch for 10 to 30 seconds to prime carburator prior to start function

Circuit diagram:

12

CONTROL BOARD REMOVAL AND REPLACEMENT PROCEDURE

Pull out Onan drawer and remove board metal protective cover (Right side / 2 screws) and lightly spray board terminals with WD-40, 2-26 or equal.

Remove wire from board terminal 11 (Input Power), Tape wire and set aside.
! Note: Care should be taken in removing wires due to fragile terminal design.

Remove one wire at a time and verify wire marking of terminal number.
! Note: Wires are marked with terminals numbers.....Don't cross them.

Verify remote control wire function (upper terminals) before install.
! Carefully identify each wire function with ohmmeter and an assistant to activate the remote control switch. Verify the wires are not interchanged (crossed) or shorted to ground within the coach.

Install control board with 4 mounting screws.

Attach each wire to board terminals using positive ID of wire markings.
! Remove tape from wire 11 and attach to board terminal 11.
Install cable ties and board metal protective cover

Press / Hold "Start Switch" and verify Run / Stop operation.

17

GENERAL REPAIR PROCEDURE

Clean circuit board and terminals with degreaser, soap and water (avoid water in the switch and relays). Low heat to dry board

Test, isolate, remove and repair malfunctioning component(s).

Replace CR5 diodes with 1N4004 (400 volt, 1 amp) or equal.
Due to potential damage from the choke solenoid high voltage "Kick Back". Also replace CR1 and 6 with IN4004 diode or equal.

Replace CR7 diodes with 1N4004 (400 volt, 1 amp) or equal
Due potential damage from CR5 failure (also replace CR1 and 6)

Replace K2 and K3 if contacts are burned or non-functional

Apply electronic printed circuit board protective conformal coating (acrylic resin, type AR) to printed circuit side (bottom side) and non-terminal area of component side (top side)...several coats

Apply anti-oxidant grease on all terminals (protective coating)

Test and verify all circuit board functions

18

AFTER MARKET CONTROL BOARDS

Stay away from after market boards

No documentation of circuit

No valid trouble shooting procedure

Onan company repair centers will not work on a generator with a bogus control board. It must be replaced with a genuine Onan control board before repair @ $$$$$$

19

GENERAL CONTROL TROUBLE SHOOTING GUIDE

Clean battery cables (both + and - cables / both ends)
! Apply corrosion prevention material to cable ends and mating surfaces.

Remove upper board terminal wires (Leave disconnected for test)
! Verify operation of remote control panel and that wires are not shorted to ground.

Remove wire from board terminal 12 (Leave disconnected for test)
! Verify oil pressure switch and wire operation (not shorted to ground).

Remove wires (Typically 2 or 3) from voltage regulator. Note wire location
! Tape up single terminated wire and 2X terminated wires separately
! Permanently stow (voltage regulator is not required).

Disconnect 12 pin connector [J2]
! Clean and re-shape female sockets for round shape.

Verify fuse [F1] is 5 amp and is functional
! Terminal 5 to 1 or each fuse holder clip to 1 is +12VDC

Verify fuel present at carburator:
! Disconnect at fuel pump and jumper board terminal 9 to 5

Verify ignition functional - spark present
! Jumper board terminal 9 to 5 and make and break points

Verify voltage / resistance levels

Terminal

12 to 1 greater than 10 ohms (starter on and wire on)

10 to 1 less than 0.5 volts DC

8 to 11 26 to 30 volts AC (Onan running)

10 to 1 greater than 10 volts DC (starter on)

9 to 1 greater than 10 volts DC (starter on)

1 to (-) bat near 0 ohms

11 to 1 greater than 10 volts DC (starter on)

20 - 21

TROUBLE SHOOTING AIDS

TRYS TO RUN WITH START SWITCH ON

Battery charge verified, battery cables clean and functional, control board fuse (5a) OK, and oil level verified

Attach jumper board terninal 9 to 5 - should hear fuel pump running

Press start switch and Onan runs

Remove jumper and Onan continues to run

Suspect low battery voltage between terminal 11 to 1 (with starter on)
! Must be greater than 10.5 VDC (typically 11.5 VDC

Suspect [K1] starter relay or wiring faulty
! Must have greater than 10.5 VDC terminal 10 to 1 (starter on)
! Temporary jumper terminal 11 to 10 and try starting (remove jumper)

Remove jumper and Onan will not continue to run

Suspect low AC voltage (terminal 8 to 11) must be 26 to 30 VAC running
! Disable / disconnect Onan voltage regulator wiring
! Remove single wire and tape up without touching ground or other wires
! Remove double wire with adapter (keep connected) and tape up without touching ground or other wires

Suspect remote control panel or wiring faulty
! Remove wires from upper terminals 1, 2 and 3 and try running again.

Suspect faulty low oil pressure (LOP) switch or wiring failure
! Remove wire from board terminal 12 and try running again

Suspect control board faulty
! Test and / or repair board as required

Press start switch and Onan will not run

Suspect fuel or ignition problem
! Repair as required

22

WILL NOT TRY TO RUN (STARTER OK)

Battery charge verified, battery cables clean and functional, control board fuse (5A) OK and oil level verified

Attach jumper board terninal 9 to 5 - should hear fuel pump running

Press start switch and Onan runs
! See "Trys to run with start switch on" chart: "Onan continues to run"

Press start switch and Onan will not run (jumper attached)

Suspect fuel problem (probe pump wire for voltage)
! Remove carburator fuel hose at fuel pump
! Fuel must flow out of pump with jumper attached
! If not, fuel pump faulty, hose faulty or no fuel in tank

Suspect ignition problem
! Remove ignition point cover and hit starter so points are closed
! Open and close points. You should have a spark
! If not, Suspect wiring, coil or points faulty

Suspect faulty low oil pressure (LOP) switch or wiring failure
! Remove wire from board terminal 12 and try running again
! If it now runs, Suspect LOP switch or wiring faulty

Suspect remote control panel or wiring faulty
! Remove wires from upper terminals 1, 2 and 3 and try running again
! If now functional, Suspect Remote Control Panel or wiring faulty
! Probe wires from upper terminals 1, 2 and 3 to verify Control Panel operation and wires are not crossed or shorted to ground.

23

REMOTE CONTROL PANEL IS NON FUNCTIONAL

Battery charge verified, battery cables clean and functional, control board fuse (5A) OK and oil level verified

If any of the Remote Control Panel functions are non responsive

Suspect the 4 wire connector faulty (located in Onan compartment floor.
! Eliminate the 4 wire connector by hard wire bypass

Press Remote Control Panel start and stop switch - Onan should respond. If not:

Remove wires from upper board terminals 1, 2 and 3
! Jumper upper terminal 1 to 3 - Onan should start

If functional:
Suspect Remote Control Panel or wiring faulty

If non functional
Suspect control board faulty

! Jumper upper terminals 1 to 2 (Onan running) - Onan should stop.

If functional:
Suspect Remote Control Panel or wiring faulty

If non functional
Suspect Control Board faulty

24

NO STARTER ACTION

Battery charge verified, battery cables clean and functional, control board fuse (5A) OK and oil level verified

Press Control Board start switch and no starter action

Jumper [K1] starter relay "S" terminal (small) to chassis ground

Starter action
! Control board or wiring faulty

No starter action
! Apply +12 VDC to [K1] starter relay "S1" terminal (large, left side)

Starter action
! [K1] relay faulty

No starter action
! Apply +12 VDC to starter solenoid "S" terminal (slip-on terminal)

Starter action
! Wiring between [K1] and starter solenoid faulty

No starter action
! Starter faulty

Note: If starter ever stays engaged after Onan starts:

Pull slip-on terminal wire from starter solenoid to stop starter
! If starter stops - Suspect [K1] Relay
! If starter will not stop - Suspect starter solenoid sticky (clean & lube)
! Remove starter battery cable to stop starter.

25

WILL NOT RESPOND TO STOP SWITCH

Battery charge verified, battery cables clean and functional, control board fuse (5A) OK and oil level verified

Press control board's stop switch to find it non functional and will not stop Onan

Remove wires from board's upper terminals 1, 2 and 3

Attach jumper between upper terminals 1 and 2

Non functional
! Suspect faulty control board

Functional
! Suspect faulty Control Board

To force stop function

Remove wire from board terminal 11, 9 or 1

26

GENERAL REPAIR COMMENTS

Keep WATER OUT of Onan
If it gets wet, dry it out before starting (24 hours minumum)

Clean connectors and board terminals with electrical cleaner

! CRC Inc. #2-26 Electrical grade contact cleaner or equal at Home Depot Home Center Electrical Dept.

Repair [J1] and [J2] with cleaner and carefully close female sockets (back to original shape) for better contact (hardwire if necessary)

Verify low oil pressure (LOP) circuit as noted herein (could save an engine which is low in oil pressure)

Before you declare the control board faulty, verify:

Fuse [F1] (5 amp) is functional
[J1] and [J2] connector is functional (clean and repair)
Wire terminals and board terminals are functional (no broken or shorted wires)
LOP functional (not shorted to ground)
Voltage regulator [VR1] is disabled - terminal 8 to 11 = 28VAC +/- 2

Board Repair and Technical Assistance

Duane Simmons

WHEN ALL ELSE FAILS

READ THE MAINTENANCE MANUAL