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Onan: Onan BFA 4.0 Spec (A) Diagnostic and Tips

JollyGreenGiant

Registered
Hello folks,
I recently had the privilege of coming into ownership of an older genset. I believe my thread description is accurate but I will begin by providing model and serial numbers for reference (4.0 BFA 1R/16004A #A783418527). I have started the process of bringing it back to life as I believe it is a good candidate to do so. Compression checked great on both cylinders, slip rings on generator side cleaned up very well, brushes were in good shape, and wiring was complete. During its previous life the start/stop run switch on the genny was damaged and so I purchased a replacement Onan 308-0383 and installed it just as I uninstalled the broken one. I replaced the positive and negative battery wiring with some 6 gauge, new terminals, and connected an automotive battery with 675 CCA and 12.6V. I cleaned the fuel pump, replaced the old oil, and fitted a new filter. Although I intend on doing quite a bit more to put it into operation, I decided now was a good time to spin it over and take compression readings. First sign of problem occurred when pressing the start rocker in. I hear my fuel pump clicking but get no movement at the generator side. I checked my F1 and F2 5A fuses and both were good. I took the plugs out and turned the flywheel by hand to see if it wasnt just at the top of the compression stroke when it shut down , no dice. I pulled my truck up and ran jumper cables from it while running straight to the right side of the solenoid and a ground, still no dice. I decided to attempt to bridge the solenoid posts with a pair of vice grips and low and behold this genny spun around like a top. I might add, this was after the failed attempt to jump start it and only with its battery power. What I am hoping to get some insight on is what may be causing it not to crank without bridging the solenoid? The easy answer is a bad solenoid right? Is there a way to test the solenoid while still in the control box or does it need to be removed? Are either of the relays I see below the start/stop run switch responsible for cranking this unit? I appreciate any input and look forward to sharing more of the experience along the way.
 

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Kevin K

Subscriber
Last Subscription Date
07/12/2019
Congratulations on your new Onan. The BFA produces 4Kw at 120 volts only and was mostly used in RV's. Your Spec A version is slightly different than the later Spec B & C models and uses a different starter solenoid.

I'm attaching the factory production order and wiring diagram for your set. You can download the manuals here: http://www.skilledcrafting.com/onanfiles/

900-0337 service
965-0222 parts
965-0122 operator
965-0612 install

It is common for people repairing these sets to install the wrong starter solenoid. Your solenoid should have three terminals, the heavy battery and generator end terminals and one small "S" terminal. One end of the starter solenoid coil is connected to the battery post internally, so be sure someone did not switch the battery and generator end wires. To crank the engine, the small "B" terminal is grounded by the start switch. Possibly the switch is wired wrong. To test it connect one end of a test light or voltmeter to the small "S" terminal, the other end to ground. You should see 12 volts. When the start switch is pressed the voltage should go to zero.
 

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Billy J Shafer

Subscriber
Age
69
Last Subscription Date
09/03/2019
You need to break up the question. Very hard for some of us to read. But looks like Kevin has your answers.
 

JollyGreenGiant

Registered
Congratulations on your new Onan. The BFA produces 4Kw at 120 volts only and was mostly used in RV's. Your Spec A version is slightly different than the later Spec B & C models and uses a different starter solenoid.

I'm attaching the factory production order and wiring diagram for your set. You can download the manuals here: http://www.skilledcrafting.com/onanfiles/

900-0337 service
965-0222 parts
965-0122 operator
965-0612 install

It is common for people repairing these sets to install the wrong starter solenoid. Your solenoid should have three terminals, the heavy battery and generator end terminals and one small "S" terminal. One end of the starter solenoid coil is connected to the battery post internally, so be sure someone did not switch the battery and generator end wires. To crank the engine, the small "B" terminal is grounded by the start switch. Possibly the switch is wired wrong. To test it connect one end of a test light or voltmeter to the small "S" terminal, the other end to ground. You should see 12 volts. When the start switch is pressed the voltage should go to
Thank you for sharing these insights and material for me. I have taken a second look at the solenoid and it appears to be as you described and the parts book shows with two larger posts at either side and one smaller post on S terminal. I will use the wiring diagram to do some tracing and as you suggested check for voltage on the S terminal when depressing the start rocker. Do you happen to know if the two relays are onan specific or can automotive replacements be used? Thank you again
 
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JollyGreenGiant

Registered
You need to break up the question. Very hard for some of us to read. But looks like Kevin has your answers.

I apologize if my initial post was confusing. I tried to add as much information as possible. Kevin has done a great job providing information and essentially instructed me how to test the solenoid.
 

KPack

Subscriber
Last Subscription Date
04/03/2020
Congratulations on your new Onan. The BFA produces 4Kw at 120 volts only and was mostly used in RV's. Your Spec A version is slightly different than the later Spec B & C models and uses a different starter solenoid.

I'm attaching the factory production order and wiring diagram for your set. You can download the manuals here: http://www.skilledcrafting.com/onanfiles/

900-0337 service
965-0222 parts
965-0122 operator
965-0612 install

It is common for people repairing these sets to install the wrong starter solenoid. Your solenoid should have three terminals, the heavy battery and generator end terminals and one small "S" terminal. One end of the starter solenoid coil is connected to the battery post internally, so be sure someone did not switch the battery and generator end wires. To crank the engine, the small "B" terminal is grounded by the start switch. Possibly the switch is wired wrong. To test it connect one end of a test light or voltmeter to the small "S" terminal, the other end to ground. You should see 12 volts. When the start switch is pressed the voltage should go to zero.
Can you take that solenoid apart and change it?
I drilled the rivets out of one on a Bgm and changed it.
 

JollyGreenGiant

Registered
Can you take that solenoid apart and change it?
I drilled the rivets out of one on a Bgm and changed it.
Yes the solenoid mounts to the inner back of the electrical box with two standard bolts. I went ahead and did test the voltage at the S terminal on the solenoid as Kevin had advised and confirm that it does have 12.5v constant and when depressing the start rocker drops to .30v. Since this is the case, I am less inclined to believe either the solenoid or the start/stop run switch wiring is causing the issue at hand. I am not a fan of throwing parts at a project, especially when they need to be bought from Onan $$$. I am very interested in know if either of the two relays are responsible for cranking and if so are they testable/interchangable with basic automotive relays?
 

Kevin K

Subscriber
Last Subscription Date
07/12/2019
I am very interested in know if either of the two relays are responsible for cranking and if so are they testable/interchangable with basic automotive relays?
Yes, K2 and K3 are standard automotive relays.
Yes, K2 and K3 can be tested.
No, K2 and K3 are not responsible for cranking.

K2 supplies power to the ignition system and fuel pump when the cranking cycle starts and prior to the oil pressure safety switch S3 closing.

K3 is the stop relay, and provides power to the ignition system and fuel pump after the oil pressure closes the safety switch. It also functions as the stop relay to remove power from the ignition system when the STOP switch is pressed.
 

zuhnc

Subscriber
Last Subscription Date
12/08/2019
When pressing the start/stop rocker to the start position, did the solenoid click? Did you test for 12V on the generator side of the solenoid when pressing the start switch? Use a voltmeter for these tests. Also, if the solenoid DOES click, measure the voltage on the battery side of the solenoid; should not drop more than 2 volts or so. zuhnc
 

VelocityDave

Subscriber
Last Subscription Date
06/23/2019
JollyGreenGiant in your posts you stated that when you depress the start switch it grounds the S terminal on the large start solenoid. This is what it should do. You said the S terminal has 12 volts when the start switch is not pressed. This is also how it should be. You also jumped across the two large terminals with visegrips and the engine spun over. This is what it should have done. If the large start solenoid was good the engine should spin over when you press the start switch.
The large solenoid switch is bad. What KPack was telling you might save you from buying a new solenoid. Lots of people here have removed the rivets from the solenoid and cleaned or rotated the contacts repairing the solenoid.

David
 
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JollyGreenGiant

Registered
Yes, K2 and K3 are standard automotive relays.
Yes, K2 and K3 can be tested.
No, K2 and K3 are not responsible for cranking.

K2 supplies power to the ignition system and fuel pump when the cranking cycle starts and prior to the oil pressure safety switch S3 closing.

K3 is the stop relay, and provides power to the ignition system and fuel pump after the oil pressure closes the safety switch. It also functions as the stop relay to remove power from the ignition system when the STOP switch is pressed.
Thank you for clarifying for me and I apologize as I seem to have misunderstood your previous reply regarding the rivets and servicing the solenoid. I will take a look at it today.
 

JollyGreenGiant

Registered
When pressing the start/stop rocker to the start position, did the solenoid click? Did you test for 12V on the generator side of the solenoid when pressing the start switch? Use a voltmeter for these tests. Also, if the solenoid DOES click, measure the voltage on the battery side of the solenoid; should not drop more than 2 volts or so. zuhnc

Sorry for late reply I can confirm there is no voltage on the generator side of solenoid when depressing start rocker. I do not see voltage drop on battery side of solenoid and S terminal acts normally starting at 12.44 and dropping to .3. I believe as others have suggested the larger solenoid is bad.
 

Power

Registered
Before I did much more, I would remove wires from S terminal and battery terminal. Clean both terminals thoroughly, and connect battery terminal to battery + and S terminal to -. If solenoid does not kick in, and there is no spark when connecting S terminal to battery, coil is probably bad. I do not consider that repairable. If there is a spark, solenoid plunger may be stuck. I consider that repairable, and dissassemble solenoid for repairs.
 

Vanman

Subscriber
Last Subscription Date
07/10/2019
I had one that wouldn’t work because it was stuck due to being overheated and melted internally, even though the coil still tested good.

I replaced it with a continuous duty one, with an isolated coil, allowing me to connect it as required to the original controller circuit.

Keith
 
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