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Old 09-15-2017, 04:23 AM   #1
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Short in Onan generator start system

1993 vintage Onan 20 kw has developed a short in the start system such that pressing start /preheat rocker switch will instantly trip the red beaker in either mode . Anyone else ever have this problem and if so what did you find for cause ?

I've already examined inside the main electrical box and checked continuity on all switches and trips and found no smoking guns yet. Wonder if one of the sensors has gone bad .... overheat sensor seems ok but are there others like for low oil level or such ? Also, who to call ?

FWIW, the problem started when the gen had been running perfectly for a few hours and just shut don suddenly. I found the white D.C. Breaker was tripped but resetting it didn't help as the red D.C. Rocker switch continues to trip the instant gen start or gen preheat is pressed.
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Old 09-15-2017, 08:12 AM   #2
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You need the control schematic and then go through it methodically to find the fault. One possibility is a fault in the remote wiring/panel, something as simple as a shorted hour meter or gauge can do this. A good first step is disconnect the remote cabling and try it with the local switch.
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Old 09-15-2017, 09:37 AM   #3
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Excellent... I was hoping it could be something that simple and related to vibration cause thanks and yes have schematic !

But re remote cabeling do you mean the gen on rocker switch at the lower helm ? Everything else gauges and all are at the gen itself

FWIW, it will trip the red breaker the same at lower helm rocker as well

And there are no f lybridge gen switches
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Old 09-15-2017, 09:57 AM   #4
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If you found the DC breaker has tripped , obviously should be a DC issue rather than AC (which is possibly easier and cheaper), Activating the start sequence will place power to

1) The fuel pump (priming pump)
2) The 12 volt relays
3) The sender units
4) The excitation circuit (not sure on that older model)
5) The glow plugs

Over the years on my Onan 12/15 KVA I have had three similar issues, but mine as a later model has a blinking response on the rocker switch where you count the number of flashes-which tells you where to look!

1) Shorted Fuel Pump
2) Hot wire to glow plug touching block
3) Glow plug shorted

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Old 09-15-2017, 11:04 AM   #5
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This thing (MDL4) has a mechanical lift pump, but if that has been replaced with electric pump, that is a good place to start. Stanadyne pump just has a little solenoid for shutdown, easy to check for short. Glow plugs not in play when running, so not likely as it tripped when running. Shutdown switches if shorted to ground can do this, I think.
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Old 09-15-2017, 09:21 PM   #6
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Quote:
Originally Posted by Ski in NC View Post
This thing (MDL4) has a mechanical lift pump, but if that has been replaced with electric pump, that is a good place to start. Stanadyne pump just has a little solenoid for shutdown, easy to check for short. Glow plugs not in play when running, so not likely as it tripped when running. Shutdown switches if shorted to ground can do this, I think.
Yep 20MDL4B...mechanical pump...
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Old 09-16-2017, 11:54 PM   #7
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Quote:
Originally Posted by Ski in NC View Post
This thing (MDL4) has a mechanical lift pump, but if that has been replaced with electric pump, that is a good place to start. Stanadyne pump just has a little solenoid for shutdown, easy to check for short. Glow plugs not in play when running, so not likely as it tripped when running. Shutdown switches if shorted to ground can do this, I think.
Just to note, the circuit is tripping prior to running (not running) there fore the glow plugs would be initiated during the starting cycle so a good place to look after the basic wiring, (easy to just dis connect the glow plug wire?) and see if the fault is still present,

Cheers Steve
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Old 09-17-2017, 06:16 AM   #8
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But, first issue was shutdown while running (see post) #1
DC breaker tripped.
Good place to start
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Old 09-17-2017, 11:36 AM   #9
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Just to note, the circuit is tripping prior to running (not running) there fore the glow plugs would be initiated during the starting cycle so a good place to look after the basic wiring, (easy to just dis connect the glow plug wire?) and see if the fault is still present,

Cheers Steve
Short in glow plugs makes the most sense now that you mention it, but last night I removed the first wire going to the series of 4 glow plugs and that didn't help. Also removed all wires from gauges (one gauge at a time) and reset breakers...didn't help. Also substituted red rocker breaker from good generator, didn't help.

I'm thinking the round start solenoid in there may be shorting....thoughts ?
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Old 09-17-2017, 11:47 AM   #10
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My onan genset shut down after running for a couple of hours. Turned out to be a loose (dirty) connection on the start solenoid. It did not trip any breakers though. You might try starting it while holding the plunger on the solenoid in by hand. If it starts leave go of the plunger and see if it quits.
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Old 09-17-2017, 11:52 AM   #11
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My onan genset shut down after running for a couple of hours. Turned out to be a loose (dirty) connection on the start solenoid. It did not trip any breakers though. You might try starting it while holding the plunger on the solenoid in by hand. If it starts leave go of the plunger and see if it quits.
Couldn't get a real good look at it but one of the round types that looks like it has no plunger access. I could swap known good one for (maybe) bad for tests, but major PITA access and wiring wise.
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Old 09-17-2017, 12:21 PM   #12
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This thing does not have an external shutdown solenoid. Stanadyne pump with internal shutdown solenoid. Easy to disconnect and check for short.

Have you disconnected remote harness? Easy if the type with a plug at control box, harder if hard wired.

I don't have schematic at hand, so can't tell if a shutdown switch can cause a breaker trip, but should be easy to disconnect one at a time and see if fault clears.

Good you got glow plugs off the list. Was not high on the list as they would not cause the initial running trip.

If this has a battery charging alternator with the motorcycle type regulator, that might do it, if so easy to unplug.

Not suspecting starting solenoid, that is not energizing in preheat, and you get the bkr trip there, right?
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Old 09-17-2017, 12:37 PM   #13
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Not suspecting starting solenoid, that is not energizing in preheat, and you get the bkr trip there, right?
Good point.
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Old 09-17-2017, 01:47 PM   #14
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This thing does not have an external shutdown solenoid. Stanadyne pump with internal shutdown solenoid. Easy to disconnect and check for short.


Forgot to mention I did check that...removed the injector solenoid wire. What is interesting is when the wire is connected there is very little resistance between positive stud and ground...like 4 ohms....wereas the good generator has way more resistance in same situation, like 400 ohms. But when the positive wire is disconnected from both injection pumps, the ohm readings are the same on each....i.e. injection solenoid on bad generator is ok.
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Old 09-17-2017, 02:45 PM   #15
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This might be too obvious, check the raw water strainer basket. When I don't have sufficient flow it will shut down the generator and trip at each restart.
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Old 09-17-2017, 08:56 PM   #16
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This might be too obvious, check the raw water strainer basket. When I don't have sufficient flow it will shut down the generator and trip at each restart.
Seems like if that were the case it would take at least a few seconds before tripping the breaker. Mine trips instantally when I press either the start or preheat positions of the black rocker switch.
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