Onan 15kw starts but shuts down when releasing the start button

[Bob Quinn]

Any thoughts? It fires right up, voltage comes up fine, but as soon as I let off the start button it shuts down.

Stumped! Any ideas?

Thanks!

 

[JuiceClark]

On mine, it was the oil pressure switch...acted exactly as you said. A sensor is not letting the genny run and shutting it down.

I put a new switch on from Cummins and it still didn't work. Hours of frustration later, I simply gave up and grounded the wire. She's run about 50 problem-free hours since then! Every hour that old genny runs feels like a gift.

 

[Pascal]

That’s pretty much always a failed safety switch. The run switch bypasses the safety switches so when you release the run switch, safeties come on and one of them is shutting it down. You can bypass them one at a time for testing purpose to see which one needs to be replaced. Either oil pressure, overheat or if equipped raw water flow and exhaust tempo always carry a set of spare switches on board

 

[Play'N Hooky Too]

It could also be the centrifugal switch.

 

[Play'N Hooky Too]

I fought the centrifugal switch adjustment on mine for about 6 months and never could get it to work properly. It's impossible (within reason) to fine any replacement parts for these old gensets, so I wound up installing a manual bypass switch to act in place of the centrifugal switch. I just have to flip it to "on" before releasing the start switch.

 

[Bob Quinn]

Thanks everyone. I'm getting in there now.

If I can figure out how to get the boxed in cover off I can start looking at it!

I hate the gen room.

 

[Bob Quinn]

I got the covers off.
Found an oil pressure gauge, holding 40 psi oil pressure so at least that's good.

How to test the sensors? Do they all just short to ground when in the "good" or "o.k." position?
if so I'm assuming just disconnect the wire going to them and jumper to ground?

 

[Play'N Hooky Too]

The oil pressure switch is open until it sees oil pressure (I would start with it).

The high temp switch is normally closed but opens if the temperature gets above its set-point (if it were bad, the engine probably wouldn't start).

The centrifugal switch is normally open. It closes after the engine reaches 900 rpms even after you release the start switch. (unless it is defective, or the point gap is not properly set).

 

[Bob Quinn]

The oil pressure switch is open until it sees oil pressure (I would start with it).

The high temp switch is normally closed but opens if the temperature gets above its set-point (if it were bad, the engine probably wouldn't start).

The centrifugal switch is normally open. It closes after the engine reaches 900 rpms even after you release the start switch. (unless it is defective, or the point gap is not properly set).

Thank you, I will hunt them down.

If anyone has a spare Munchkin that likes to work on generators, send them my way LOL.

 

[ageless]

I went through this same thing, replacement sensors were fairly easy to find and "cheap" enough.

 

[Bob Quinn]

Is there possibly a service manual with a diagram that points to where the sensors are?

I found 4 sensors, one near the top of each cylinder that I am assuming would be temperature sensors, then one on the left side of the engines near the top that is in line with the cut-off fuel solenoid. Maybe that is a temp sensor as well, I don't know.

I don't see where the oil pressure sensor is.....

Thanks!

 

[Bob Quinn]

Found this....

 

[Bob Quinn]

I was able to get a complete set of manuals for the Onan MDJF generators.
If anyone needs them let me know.
Thanks for all your help guys!

 

[ageless]

In case anyone is looking in the future, here's a link to manuals

https://www.manualslib.com/o/onan+mdjf+series.html

 

[rsmith]

It could also be the centrifugal switch.

View attachment 74470View attachment 74471

That’s the typical failure

 

[OVERBOARD]

I have the same exact problem going on 15kw mdjf. I have original Onan manual from 1979 and it states .025” on the centrifugal switch. I have seen .020 to .040 inches. I will forward the wiring diagram that I got from Onan dealer. I was having issues last year with fuel shut down, solenoid chattering, and then burning out. So I rewired the external sensors and now I can’t get it to work when I let go of the switch, just like this post. If possible could someone send me a picture of the terminal board where the sensors are attached and make sure that I didn’t miss wire or something. file:///var/mobile/Library/SMS/Attachments/65/05/570D64CA-537D-4430-BF0A-E0DE3A82C4A4/IMG_0033.jpeg

 

[miboatnutz]

I have the same exact problem going on 15kw mdjf. I have original Onan manual from 1979 and it states .025” on the centrifugal switch. I have seen .020 to .040 inches. I will forward the wiring diagram that I got from Onan dealer. I was having issues last year with fuel shut down, solenoid chattering, and then burning out. So I rewired the external sensors and now I can’t get it to work when I let go of the switch, just like this post. If possible could someone send me a picture of the terminal board where the sensors are attached and make sure that I didn’t miss wire or something. file:///var/mobile/Library/SMS/Attachments/65/05/570D64CA-537D-4430-BF0A-E0DE3A82C4A4/IMG_0033.jpeg

Roy,

See link below.

https://www.smokstak.com/forum/thre...-for-start-disconnect.211060/#google_vignette

Might try jumping on Smokstak as there's lots of info for these old Onans including newer replacement parts for those that may be obsolete. Also, Murphy by Enovation Controls sells an entire replacement shutdown panel which may be a possible alternative to current set up.

https://www.amazon.com/Murphy-Enova...7dc5c1570&ref_=pd_hp_d_atf_ci_mcx_mr_hp_atf_m

Lastly, it looks like the work around for some people has been to replace the centrifugal switch with a timer relay instead of a manual switch. Either way is fine, but the timer relays are adjustable and automatic so the unit will operate the same as current set up.

Good luck!

 

[OVERBOARD]

Thank you for the information. I found out what my problem was. I had a two sets of wires same color, hanging in front of my parallel switch in the generator room. They had been there since I purchased the boat in 2014. So thinking one was for the lower and one for the upper helm station, I hooked the spare set up. This oops on my part caused the generator from running when I let off the switch. LOL. Well those wires were for the hobbs meter that had been missing and only three feet away. Ordered a new gauge and will hook up when it arrives. I kept burning out the fuel solenoid 32V year after year. This was fixed by gapping the centrifical switch to .025, like the original manual on the boat stated. I re-wired all the external sensors with new marine wire. Looks like as is well. Thank you for the information. Roy

 

[Saltshaker81!]

I have the same issue with my MDJF 15KW in my 1981 50C which is a 32V unit, i found a bad contact on the fuel disc relay which was updated to the newer style 8 pin 307-P773 relay. For the remainder of the season i just bypassed the relay and wired in a switch for the fuel solenoid. now the fun part, trying to convert that 8 pin relay to something else because finding one of them seems to be fairly tricky. most online places say they're in stock but actually aren't. I ended up buying a replacement 8 pin square relay with a 6V coil and base and hoping to try and put that in and make everything works as it should again.

I do wish these generators had fuses on the circuits, i got a couple good DC shocks from the unit before when trying to start or shut down the unit before the relay actually failed so i do feel i have something shorting out on the 32v run circuit somewhere but the low oil sensor, head temp sensor and centrifugal switch all seem to be fine and OHM out they way they are supposed too.

-Side note, i also wish all the wiring in the control box wasn't the same color. makes tracing wires and using the diagram a blast