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Onan 12.5 Generator shuts down on engine start-up


Mike Wahl
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I've been able to use our new to us 2001 Signature over the past 3 weekend and we are loving it.  One of the things I noticed is that when I start my main engine, the Onan 12.5 generator shuts down. Is this the way it is suppose to work or is there something wrong?  

 

Edited by Mike Wahl
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It is not how it is supposed to work. Your gen DC power runs off chassis batteries. Same batteries used for starting the engine. If the DC voltage drops too much at that time, the genny will stop. Check the batteries, connections, voltage at that time  See if it happens with boost switch ON, that should confirm it 

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I was able to spend a little time last night working on my Generator.  I had it shut down once over the weekend for over-temp with the single flash of the light.  While it was running I could hear the fan belt squealing occasionally.  I was able to tighten the fan belt and washed out the radiator.  I will see if this fixes that issue.

I then started the generator and after it was warmed up started the engine and the generator shut down.   I shut down engine and restarted the generator and then started the engine with the battery boost switch on and it didn't shut down.   I forgot my good meter at work yesterday so tonight I can check battery and also do a DC amp draw on the starter. 

Ivan, from your post I'm wondering if its the engine starter drawing heavy amps.   The starter is an original and it looks like it had give problems in the past and someone used the old hit it was a hammer trick to get it engage.  I can't find anywhere in the documentation that it was fixed.  I found this when I was doing engine maintance last month.  I have a new Denso Heavy Duty Starter 428000-4420 on order that should be here this week.    The Denso is a gear reduction starter verse a direct drive in the original Delco starter.

I will also check out the battery's.  They are getting the age of replacement so that could also be an issue. 

Ivan, Do you know at what DC Voltage the generator shuts down?

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Mike, I have never found low DC voltage limit (there is only a high DC code) so I can't tell. The microprocessor probably runs on 5v but fuel actuator gets the full voltage, who knows how the voltage regulation is done. I think your 'boost test' confirms the suspected situation so you know where to look from here. Your starter could be out of norm as you mentioned too. I am curious what you will find out when you do the measurements, sounds like you got a good handle on it.

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  • 2 weeks later...

I did some more checking and when I loaded tested the chassis batteries, one of the two batteries was getting weak.  I have changed both batteries and it has corrected the issue with the generator shutting down when starting.  I'm working on my replacement starter, because I have a high amp draw with the starter.   The one I had on order has been back-ordered with no estimated ship date.  Looking at other options.

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To begin with I didn’t put a 4440 on because it was only 10 teeth instead of 11 but finally figured out the gear is off-center in the hole and it was the correct starter for my ISX. Bought it off eBay from DB electric and it’s probably a better price than what you were quoted.

I assume you have a ISM and getting the top boat out was the hardest part when I changed to a Denso because of the flex in a long extension to get to the bolt. The Densowiill start it with just one battery where the Delco had a hard time with three batteries.

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