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Old 01-26-2022, 11:26 PM   #1
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Another ONAN thread- Glow Plug Circuit

There have been a lot of ONAN problems on this forum recently. and a lot of knowledgeable responses.
Hopefully some one will have a solution for my years old unresolved issue.

The 20 amp fuse in the OEM glow plug circuit blows when starting the generator.
I took the coach to Cummins/Onan Service Center when this first occurred.
After a $100 exam the tech concluded that a new inverter was needed at a cost of $2800. I passed. I couldn't/don't believe that an electrical engineer would design a system that would require the replacement of a MAJOR component to solve a simple problem in an independent subsystem.
by
I found a separate 12v source and wired a fuse and a push button switch to energize the glow plugs when they are needed. The push button is adjacent to the start switch on the Onan and I have to open the slide to use it.

Most of the time the glow plugs are not necessary in my area and I can start the Onan from inside the coach.

If it is cold and rainy I have to extend the slide and go out in the weather to start the Onan by simultaneously depressing the start switch and the pushbutton glow plug switch.

I could extend the glow plug circuit into the coach but I prefer not to.

I would like to restore the OEM functionality to all of the 3 Onan start switches but not at a cost of $2800.

I apologize for the long explanation and hope it is not too confusing.

All suggestions will be appreciated.
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Old 01-27-2022, 10:38 AM   #2
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Care to post the Gen Model?
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Old 01-27-2022, 10:59 AM   #3
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I can't envision any reason related to the glow plugs that would require replacing the inverter. Unfortunately well trained technicians are hard to find.

I replaced the glow plugs on our Onan 12.5HDCAB generator. I ordered three NGK 5392 Y720U glow plugs for $10.96 each (Rock Auto) vs over $40 each from Cummins.
There is a metal bus bar that spans the three glow plugs and if it touches any metal it can short out.
I would start by installing new glow plugs and carefully inspecting the bus bar and wiring. This is about a 15 min job once you remove the top cover.
Hope this helps.
https://www.irv2.com/forums/f54/onan...ml#post4225313
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Old 01-27-2022, 02:00 PM   #4
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Care to post the Gen Model?
Thanks for asking. Data plate on the genset says: 7.5 HDKAJ/11451D
It is in a 1998 Alpine Coach
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Old 01-27-2022, 02:11 PM   #5
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Does your inverter/ charger work properly once the generator is running?
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Old 01-27-2022, 02:16 PM   #6
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[QUOTE=vito.a;6063061].
There is a metal bus bar that spans the three glow plugs and if it touches any metal it can short out.
I would start by carefully inspecting the bus bar and wiring. Thanks for your response.

I did that before installing the separate fused circuit and pushbutton switch to feed the glow plugs.

The cold engine starts right away on a cold day when I energize the start button and the separate push button for the glow plugs. This indicated that the glow plugs are functioning.

The 20 amp fuse that I installed along with the push button has never blown after several years of operation. If the glow plugs or buss bar were shorted to ground the fuse would blow instantly.
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Old 01-27-2022, 02:33 PM   #7
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Originally Posted by 96 Wideglide View Post
Does your inverter/ charger work properly once the generator is running?
Yes, the inverter/charger works. All electrical systems work on shore power, and
on the generator, including both roof air conditioners.

The only issue is the glow plug fuse blowing when connected to the OEM start circuit.
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Old 01-28-2022, 10:33 AM   #8
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[QUOTE=gmcgene;6063218]
Quote:
Originally Posted by vito.a View Post
.
If the glow plugs or buss bar were shorted to ground the fuse would blow instantly.
I agree. But if one of the glow plug coils have bonded from heat it will drop the resistance enough to occasionally blow a 20a fuse. I just try to eliminate the easy things first.
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Old 01-28-2022, 10:42 AM   #9
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Wonder what this 'tech' would have said if you blindly told him to go ahead and change out the inverter, and the problem persisted !
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Old 01-28-2022, 06:25 PM   #10
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This is a diagram for a 7.5HDKAJ. Note that the glow plug circuit is routed through the controller/ inverter. This is so that the controller can determine glow plug preheat time by reading the coolant temperature sensor. It is not really a separate sub-system from the controller. So, the possible short would be between the fuse and terminal J7 at controller or between terminal J8 and the bus bar at the glow plugs. Otherwise, the short would be internal to the controller/ inverter assembly. Onan only sells the complete controller/ inverter assembly, there may be some aftermarket companies that supply the control board individually.
Attached Files
File Type: pdf Generator glow plug circuit.pdf (183.7 KB, 5 views)
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Old 01-28-2022, 06:54 PM   #11
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Onan Inverter

Ahhhh...! Makes sense now, I was thinking the 120VAC inverter in the MH!
So you have proved the glow plugs are functional, do you know the rest of the controller/inverter module works, then carry on as you have. But try to test the safety shutdowns to ensure they have not failed in some way.

i.e. HWT, low oil pressure, etc.
Then you could wire a fused switch and mount it beside the remote start/stop and use it there also. The colder it is outside, the more time you give the glow plugs before cranking.


EDIT: I see now that the print has a 25A fuse for the glow plug and you said it was 20A... would 5A difference be the cause? Have you been able to measure the actual current draw or try a 25A fuse?
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Old 02-19-2022, 12:28 AM   #12
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I have the same generator and a similar problem. My fuse blows immediately upon installation of the fuse. Not the same as yours (you said yours blows when you try to start), but if this is a common issue then perhaps the inverter tends to fail in regards to glow plug operations.

Long story short, I found a company that will repair the inverter for a cost of $800 + shipping. Does not seem worth it to me.
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Old 02-19-2022, 11:20 AM   #13
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Quote:
Originally Posted by rpaldrich View Post
I have the same generator and a similar problem. My fuse blows immediately upon installation of the fuse. Not the same as yours (you said yours blows when you try to start), but if this is a common issue then perhaps the inverter tends to fail in regards to glow plug operations.

Long story short, I found a company that will repair the inverter for a cost of $800 + shipping. Does not seem worth it to me.
Have you tried disconnecting the glow plug wire and reinstall a fuse to determine if yours is only a glow plug issue?
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Old 02-19-2022, 12:21 PM   #14
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Quote:
Originally Posted by gmcgene View Post
Yes, the inverter/charger works. All electrical systems work on shore power, and
on the generator, including both roof air conditioners.

The only issue is the glow plug fuse blowing when connected to the OEM start circuit.
Could it be as simple as a bad ground? Worth looking into, moisture has a funny way of messing with them. Good luck sir
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