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Old 03-15-2014, 03:50 PM   #1
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Hard starting Onan

I have a 7.5kw Onan gen in my 04 sahara that is very hard to start. My problem is I don't think my glo plugs are getting juice to heat them up. everyone tells me that when i depress the start button the gen should not start turning over until the glo plugs are hot, when i push my start button the gen starts spinning at that time and is very hard to start . Is there a fuse on the glo plugs? or some sort of relay that i should check or change. I don't want to damage my starter. THANKS
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Old 03-15-2014, 04:21 PM   #2
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My 7.5 does not try to start until the light stops blinking on the start switch.
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Old 03-15-2014, 04:24 PM   #3
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Good luck solving your problem.
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Old 03-15-2014, 04:37 PM   #4
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Welcome

You have summed up the issue correctly in that the glow plugs require heating (with current through them) for the initial start and if the starter has to crank a lot before the compression is sufficient to cause ignition you have the potential to burn out the starter motor.

It is possible that your glow plugs are heating while the engine is cranking and the delay mechanism is not working, I am not an expert, but I assume there must be something that provides feedback based on the thermal status of the cylindar preheating because the time differs depending on temperature. This feedback system would then enable voltage to the starter relay/selonoid when the plugs have reached the start threshold. It would appear that your starter is getting voltage applied the instant you push the start button.

I would trace the wire from the starter relay to see what activates it. Sorry, do not know if there is a fuse for the glow plugs but it makes sense that any circuit must be protected. I would look for the delay mechanism.

Good luck,
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Old 03-15-2014, 04:44 PM   #5
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From my research of the schematic, I don't see how Onan causes the delay-before-start function. But I'll include the links that I've researched below. There are fuses on the Onan that should be checked. Perhaps one of the relays shown there is sticking "on" .... hence your immediate start problem (rather than the needed delay).

http://www.cumminsonan.com/www/pdf/m.../981-0161C.pdf

http://www.justanswer.com/rv-motorho...tart-stop.html
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Old 03-15-2014, 07:13 PM   #6
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http://www.irv2.com/forums/f54/gener...arts-2537.html

Check out this older thread for some great information.
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Old 03-15-2014, 07:22 PM   #7
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On the gen itself there is a small panel with a screw or two on the top. Take it loose, there are dog ears holding it on on the one side. Pull it up out of the way and you will find 3 fuses a 20 or 30 amp for the glow plugs as nd two 15 amps or what ever they are. The glow plugs no go will make it very hard to start. Mine were gone and had to replace them, if your lucky it will b e a fuse. If in warm weather or having already warmed th e gen should turn over right away. If cold out it will take several seconds before it turns over but should start on two or three turns over(very quickly). Have you tried holding the right or bottom of the start switch for a few seconds to see if it is a fuel problem, This should click and prime the gen.

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Old 03-16-2014, 05:33 AM   #8
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Rob, can you post your exact model number?

Depending upon which "Spec" your unit is will make a difference on how the unit controls the glow plugs.

On certain models the glow plug circuit utilizes the water temperature sensor to determine whether or not it's cold enough to initiate the glow plug heating sequence. The time needed to heat the glow plugs is a preset routine built-in to the controller based upon the water/coolant temperature.

Unfortunately, there is no logic in any of the models controllers to determine if the glow plugs are actually working or not, but only if there is a short in the glow plug circuit. For example, you can pull the fuse(s) for the glow plugs and in a properly operating unit, it would still attempt to pre-heat the glow plugs for a timed period based upon the coolant temperature before opening the fuel pump rack then energizing the starter. It has absolutely no clue that the glow plugs didn't actually energize. However If the glow circuit was shorted you would get an error code "18".

I would suspect either your water/coolant temperature sensor has failed and is reading incorrectly making the unit think the water temp is over 80 degrees or so which would cause the unit to bypass the pre-heat routine and turn over almost immediately after pressing START or the onboard controller-inverter has failed and is simply not engaging the glow plug sequence at all.

Unfortunately the controllers are e-x-p-e-n-s-i-v-e, but very easy to access - the coolant sensor is not. Obtaining access to the coolant temperature sensor requires removing the top cover and 3 side covers. There are tests available for the coolant sensor to see if it's reading properly before replacing it. There are no end-user tests available for the controller :-(

Keep us posted and good luck.
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Old 03-16-2014, 05:48 AM   #9
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Old 03-17-2014, 02:39 AM   #10
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Rob I had the same exact problem. Ordered a water temp sensor from cummins. Took the top cover off for acess. put the wire on new sensor and set it on top of the motor. Worked as it should. If you ohm the sensors you will see a large difference. Do not try to take old sensor out, as the chance of the cast housing breaking is very high. Even though the new sensor doesnt read the water temp anymore, I figure the old one didnt either. At least the gen starts every time. I will have it installed when I go in for service. Will be a spendy repair if the housing breaks. I know how my luck runs. lol Dale
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