Onan MDL4 reset pops when trying to start

[52CMY]

I'm new to this forum and amazed at the great info i've acquired as I've lurked in the shadows for the last year since I purchased my new-to-me 52CMY. I'm saddened to learn in the last 48hrs how unsupportable my old Onan 20 MDL4 has become over the last 10 years, if I have parts issues.
It's time for my first new thread:

Onan MDL4 genset has run fine until the other day and it cranks but won't start. Within a 1/2 sec of selecting START, the Fault/Reset breaker pops.

I see in the owners manual page 3.4 that only two fault sensors are immediate (Overspeed and Exhaust Temp sensors). I've disconnected the Exhaust Temp Sensor and the Fault breaker still pops. So I'm guessing it's the overspeed sensor? I can't easily get to overspeed sensor without many hours of labor pulling other equipment (ER fire suppression system and racors)...

Anyone have this issue or know a way I can disconnect the overspeed lead at the control panel with easier access and quicker troubleshooting? Thanks!

 

[Sadey]

For parts call Yachts and Diesels in Fort Lauderdale, 954-584-0042. They are a nirvana of Diesel engine parts. They are known for rebuilding many of the motors or parts for these old motors. They are currently rebuilding the Marquipt davit I purchased. Kinda of a jack of all trades for anything mechanical.
Good Luck, I have the same geny.

 

[Captain Ralph]

There is a wire lead that goes to the Stanadyne fuel injector pump. Disconnect this wire and see if the safety breaker still trips. This would be your run/stop to the pump.
Small coil that does short out.

If symptoms change, ensure the voltage (12,24,32Vdc) before ordering a new coil (if still available) or whole injector pump (if still available).

Have heard the clothes dryer elements (air heater) and air heater relay can cause this safety breaker to pop also.

Good luck.

 

[52CMY]

Capt Ralph,
I disconnected the 3 wires connected to the Standyne Fuel pump and the Fault reset button still pops within 1/2 of selecting START (no change).

My control panel Fault Reset is labeled " Lo Oil Pressure, Hi Coolant Temp, Hi Exhaust Temp, and Overspeed".
I've disconnected Exhaust and Oil pressure sender wires (no change). now faced with disconnecting Ospeed and Coolant but can't get to either wire at sender due to cramped location (52CMY). Thinking of opening Control panel lid but I've just got 8 inches of headroom there too.

any other guidance?

 

[dottieshusband]

If you're speaking of the little pop out breaker, they do go bad. I've replaced mine twice in 10 years.

 

[52CMY]

What were the symptoms of a bad Fault Reset button that you experienced? My Fault reset is popping is when I select START--the Engine continues to crank without starting after the button pops out.
Please confirm this is the component that failed?


I've pulled the front and the top of Onan soundshield and top and front of Control box. Sadly the headroom and lack of access on back and sides of GenSet is too limited to make this easy.



Anyone have experience tracing wires or know which terminals inside the Control box are for the four shutdown sensors (low oil, overtemp x2, and overspeed). I'm still guessing my overspeed sensor is bad but I can't get to that sensor without more extensive surgery on inaccessible panels.



I see the four white wires coming up through the bottom of the control box but they all disappear under the circuitboard pictured above.

I guess this may be more of a post for an engine forum like SmokeStak but figured there may be some knowledge here. Thanks so much!

 

[52CMY]

If you're speaking of the little pop out breaker, they do go bad. I've replaced mine twice in 10 years.

Update. I ordered a replacement FAULT RESET Shunt CB (Cummings Part number 0320-1139 from Cummings Southern Plains). Installed and now CB pops after ~3 seconds of cranking, as engine fires and begins to start. (so slightly different behavior but same issue)
Here's the new CB in pic below.

The white wire out of the back of the CB is wired to the LEAD Terminal on the circuit board (A11). The connection on the left (Note directional arrow down/OUT) is wired to 2B Terminal on the circuit board. The connection on the right (note arrow up/in) is wired to LINE on circuit board.
If I disconnect 2B lead, generator fires right up.

So the troubleshooting continues. Any help would be appreciated.
As my Overspeed sensor is out of reach without removing genset, I'm thinking of just running this old girl without the FAULT RESET functionality until I replace unit with modern GenSet hopefully soon. I would lose auto-shutdown for Low Oil, O-temp and O-speed. Thoughts?

 

[Jims43DC]

Out of curiosity when you have it running with the breaker disconnected and you connect it does it then pop the breaker, or does it only pop when your trying to crank it

 

[MadHatter53]

Out of curiosity when you have it running with the breaker disconnected and you connect it does it then pop the breaker, or does it only pop when your trying to crank it

im thinking it will run.

 

[52CMY]

Incorrect. When I reconnect that wire (the 2B wire), the CB pops and the engine dies.

The 2B lead from the A11 circuit board always a ground. I'm guessing it should be an open ungrounded wire that only grounds with an OTemp, Ospeed or low oil.

im thinking it will run.

 

[52CMY]

Jim,
Sadly, CB pops and engine dies when reconnected. the 2B wire from the A11 circuit board is always to ground which seems to be causing the fault.

Out of curiosity when you have it running with the breaker disconnected and you connect it does it then pop the breaker, or does it only pop when your trying to crank it

 

[52CMY]

I forgot to close out this thread from a while back. The Fault button was popping because: My halon system pressure was a bit low.
The halon system low pressure switch is wired to the SHUTDOWN terminal of Control board ("A11 board" in the wiring schematics.) The control board SHUTDOWN terminal shares two wires: the Overspeed Switch (S4) and the Halon low pressure switch signal (the red wire was cleverly poorly labelled but was depicted on the Hatteras Blueprints. If either wire is grounded, the Fault will pop.
So the mystery ground that was popping the FAULT shunt CB (CB12) via 2B was coming into the GenSet from the Halon Switch.
My halon pressure over the last 18 months has continued to drop now into the recharge region so it's time to upgrade to a modernized CO2 fire suppression system.
Thanks for the help on this!

 

[MadHatter53]

That’s some useful information for everyone.