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Old 09-16-2012, 01:08 AM   #1
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Another Onan Generator Issue

Have a Onan Quite Diesel 7500 genset, model HDKAJ. It has been very hard to start since we bought the MH in 2009. Had previously serviced the genset, new fuel filter, and air filter, changed oil and oil filter. New air and fuel filters made no difference. Always hard to start. This past week, with the help of an experienced retired diesel mechanic we determined that the problem was with the glow plugs. Cold hard start. No working glow plugs. Determined that the glow plugs were good, but no 12v to glow plug buse bar during start cycle. Did not have the benefit of a service manual for this unit at the time. Traced the wire from the glow plugs back to a connection that led to an enclosed area that I have since learned houses a control circuit board.

This is as far as our search for an answer has taken us so far. I now know that the glow plugs are good, that the wire feeding the buse bar is connected and not corroded, and that there is no 12v current to this wire during the start up cycle. The diagnosis so far is taking me to a failed control board, or possibly a problem with a coolant heat sensor. From a PDF copy of the service manual for this unit I have learned that the timing of current to the glow plugs during the start cycle is dependent on the temperature of the coolant. The colder the coolant the longer the glow plug heat duration. Coolant temp above 120 degrees no glow plug heat in the start cycle.

Could the problem be in the coolant heat sensor, (which I have yet to identify), or is the problem more likely in a failed control board. If it is the control board has anyone had experience in replacing or repairing this item? I do not have a parts lists for this genset. Does anyone have a PDF parts list for this unit, or can anyone identify the control board part number for this genset?
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Old 09-16-2012, 11:21 AM   #2
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Couldbe, and you are probably on the right track.

If you press and hold the switch in the Stop position, it should manually activate the glow plugs. Does that work? Mine gives a faint clicking sound when I hold the switch down and it continues as long as the heat cycle is running. When it stops clicking, it should be ready to start.
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Old 09-16-2012, 12:11 PM   #3
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Didn't think to check for voltage at the glow plugs holding the start/stop switch in the stop position while we had the top panel off and access to the engine. Only checked it in the start cycle. Had everything opened up at a camp ground last week when we were trouble shooting the problem with a friend. Buttoned it all up to come home. Will check that when I get a chance. Am thinking it is going to be the control board. Would like to hear from anyone who has "been there, done that", or who can help find Part #, has PDF parts list for this make/model, or information/links on repair or replacement of control board.
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Old 09-16-2012, 09:47 PM   #4
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We have the same model as you and the same problem, only ours is difficult to start only when it's less than 60 degrees F. Onan service pulled our genset ($500) and diagnosed the problem as a failed inverter/control board. They wanted $1500 for a new one or $750 for a rebuilt one.

Sorry I don't have the part number, we didn't replace ours, a quick shot of starting fluid works wonders when it's chilly and a lot cheaper. Just don't use too much.

The inverter/control board is multifunction so it could be only a small portion is defective (i.e temperature sensing) and everything else works fine. Some people have had success in getting the board repaired.
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Old 09-16-2012, 10:08 PM   #5
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ON the generator remove the control access plate. It's just above the start switch. Inside there should be three fuses.......#F3 is the glow plug fuse.
The plate also has the coolant fill cap under it.
PM me if you need more info.
BTW......Onan recommends pressing the STOP side of the switch until primed every time to start. According to my info this action does not affect the glow plugs but does prime the unit.
Hope this info helps you.
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Old 09-16-2012, 10:12 PM   #6
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Quote:
Originally Posted by Martind4 View Post
We have the same model as you and the same problem, only ours is difficult to start only when it's less than 60 degrees F. Onan service pulled our genset ($500) and diagnosed the problem as a failed control board. They wanted $1500 for a new one or $750 for a rebuilt one.

Sorry I don't have the part number, we didn't replace ours, a quick shot of starting fluid works wonders when it's chilly and a lot cheaper. Just don't use too much.
$750 ~ $1500, ouch!! The retired diesel mechanic who helped me diagnose the problem last week at the campground suggested a simple work-a-round by wiring the glow plugs to a 12v hot lead through a starter button, and disconnecting the plug from the controller, thereby heating the glow plugs manually for a few seconds before or during the start cycle. At $750 ~ $1500 that might be a way cheaper way to go, except there is also the dash start location to consider.
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Old 09-16-2012, 10:21 PM   #7
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I thought about that option but I was at the mercy of the Onan shop. We don't have a gen slideout and I don't have the equipment to remove the 700# beast myself so i could take the time to jury rig a glow plug switch.
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Old 09-16-2012, 10:23 PM   #8
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Quote:
Originally Posted by KIX View Post
ON the generator remove the control access plate. It's just above the start switch. Inside there should be three fuses.......#F3 is the glow plug fuse.
The plate also has the coolant fill cap under it.
PM me if you need more info.
BTW......Onan recommends pressing the STOP side of the switch until primed every time to start. According to my info this action does not affect the glow plugs but does prime the unit.
Hope this info helps you.
We pulled and looked at all three of those fuses, but didn't know what each controlled. All looked ok, but now that you have highlighted that one of those fuses is to the glow plugs, I am going to double check the fuses again and the associated wires, thanks. Are the fuse holders identified in any way do you know. Will I know which is #F3?
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Old 09-16-2012, 10:27 PM   #9
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Quote:
Originally Posted by Monaco38 View Post
We pulled and looked at all three of those fuses, but didn't know what each controlled. All looked ok, but now that you have highlighted that one of those fuses is to the glow plugs, I am going to double check the fuses again and the associated wires, thanks. Are the fuse holders identified in any way do you know. Will I know which is #F3?
From very poor memory I think they are labeled. But you could ohm all three to be sure.
Check you PM's
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Old 09-16-2012, 10:55 PM   #10
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Quote:
Originally Posted by Martind4 View Post
I thought about that option but I was at the mercy of the Onan shop. We don't have a gen slideout and I don't have the equipment to remove the 700# beast myself so i could take the time to jury rig a glow plug switch.
Fortunately ours is on a slide and it could be any easy work-around, if only temporarily.
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Old 12-09-2012, 07:38 PM   #11
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I had a problem of my onan 7500 not starting, getting a code 36. I finally unplugged the diesel input fuel line, had my wife try to prime/choke the genset by pushing the bottom of the start button. I put my finger on the fuel pump where I had disconnected the fuel line. I got very little suction, which told me the fuel pump wasn't up to par. Bought one from Cummins (about $100), installed it and now everything's fine.
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Old 12-10-2012, 02:25 AM   #12
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Join the club. Mine gets difficult to start well above freezing. I can usually get it going by using the system that works with big two-stroke diesels - operate the starter for a few seconds, let it rest, operate starter again, rest and so on. Theory is the compressing of the air heats up the air and that passes heat to the cylinder walls thus acting as a weak pre-heater.
Anywhere near freezing and out comes the aerostart.

Problem is my Onan is tucked way up under the front and not something I can get out myself - and I refuse to pay an experienced tech to play musical chairs swapping stuff until it either works or doesn't work.

I know the fuse is OK and judging by all the white smoke as it is turning over, there is no shortage of fuel. Problem is the location makes it impossible to get anywhere close to the top of the engine to check supply to the glow plugs. I assume there is a glow plug for each cylinder and it wouldn't be likely for all three to have failed.
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Old 12-10-2012, 09:54 AM   #13
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re the 12v switch idea. CAUTION. I had a deisel car in the 80's and I tried that trick. Turned out on that engine the glow plugs were supplied a pulsed 12v source and I had to replace all 8 burned out glow lugs.

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Old 12-10-2012, 02:12 PM   #14
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Though probably not related to this specific issue. I will toss in the reminder that this era's coaches may have had fuel lines that do not like the ULSD changes.

We replaced our fuel line to both the generator and the Hyrdo Hot, with the latest and greatest Gates. (Believe it was the Barricade line, but that was 2 1/2 years ago now.)

For those of you that have ongoing fuel delivery problems, factor the age and type of hose you have in your fuel line.

OP - Do a search here and on RV.Net for Fire Up (Scott's) post on his QD7500, he quite a bit of work on his due to the tossing maganate problems. His pictures and detail write up might come in handy for you. (But, I hope not!)

Best of luck on this one! I'll follow this post with interest.
Best to all,
Smitty
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