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Old 10-04-2019, 06:15 AM   #1
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Old Onan Generator problems

Good morning, we recently purchased a 1981 trawler with an ONAN diesel generator. We have not been able to start it. There is 13v to the starter but no response when either start button is activated. We have jumped the starter solenoid and it turns over but does not start. The shut down solenoid was removed to determine if that was the problem, then no start with button but turns over and does not start by jumping the solenoid. If anyone has any tips or information about these older ONAN generators it would be greatly appreciated. Thanks
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Old 10-04-2019, 06:28 AM   #2
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The starter not working could well be due to a faulty solenoid. The plunger could be stuck, for example. (This happened to me this summer and I was able to free it by moving with my hand and lubriucating with a penetrating oil.)

The engine not starting could be due to various reasons. If it has not been run in a while you should check the fuel supply and carefully bleed the fuel system, first at the filters using the hand lever in the fuel pump, and then by cracking open each injector and turning over the engine with the starter.

Make sure you have plenty of power to the starting battery (preferably connected to the charger or jumped to another battery bank in the boat) and that you do not overheat the starting motor,
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Old 10-04-2019, 06:28 AM   #3
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Did it start when you had the boat surveyed? Do you have a model number for it?
Sounds like a combination of issues.
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Old 10-04-2019, 06:55 AM   #4
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1981 if original is probably the MDJx series. There are schematics of the control system on line I think, or boat may have literature on board. A matter of getting a meter and picking through the circuit. There are some fuses on the control box and some relays inside that control the start sequence.
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Old 10-04-2019, 07:48 AM   #5
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As Ski said, most likely an MDJx unit. They're older and a bit "special" at times like most older Onans, but they're durable and very solid generators provided you're willing to tinker with them occasionally.
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Old 10-04-2019, 08:08 AM   #6
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Check the shutdown bypass circuit. The low oil shutdown needs to be bypassed until the engine starts and builds oil pressure. If not the generator will crank by not start.
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Old 10-04-2019, 08:12 AM   #7
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Quote:
Originally Posted by Woodland Hills View Post
Check the shutdown bypass circuit. The low oil shutdown needs to be bypassed until the engine starts and builds oil pressure. If not the generator will crank by not start.
Good catch. And to add to that, the shutdown circuit should also include a high temp switch and possibly a high exhaust temp switch (not relevant to getting it started, but might be if you have issues keeping it running).
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Old 10-04-2019, 09:47 AM   #8
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+1 on online literature first and foremost. I have that vintage Onan, and it has the manual preheat - it loves about 30-35 seconds of preheat then comes to life quickly.

But like others have pointed out that would be after other checks are complete, especially fuel bleeding.
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Originally Posted by Ski in NC View Post
1981 if original is probably the MDJx series. There are schematics of the control system on line I think, or boat may have literature on board. A matter of getting a meter and picking through the circuit. There are some fuses on the control box and some relays inside that control the start sequence.
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Old 10-04-2019, 09:48 AM   #9
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Make sure you have fuel to the generator. Change fuel filter. Provide 12V to fuel pump and make sure it pumps fuel. Bleed the fuel lines.

Caution: Be careful about extended cranking. Your muffler can fill up and water can enter the generator.
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Old 10-04-2019, 09:58 AM   #10
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Caution: Be careful about extended cranking. Your muffler can fill up and water can enter the generator.
Definitely monitor for this. But depending on the exhaust layout, it may or may not be an issue. My similar vintage gas Onan (installed with the Onan-provided water lift muffler) moves enough air while cranking to push water up and over out of the water lift, so it doesn't over-fill.
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Old 10-06-2019, 02:01 PM   #11
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If a diesel has fuel, enough warmth, good compression, and turns over quickly, it will start. A common problem is failure of the stop solenoid. The stop solenoid shuts a valve in the injector pump, cutting off fuel when the key or run button is off. Most Onans have a wire going to the top of the injector pump. That's the stop solenoid circuit. Under the top cover is the solenoid. When you bleed the air, clear fuel needs to come out of the injector tubes where they attach to the injector.
After you've bled the injector pump, put rags around the injectors to catch fuel, and loosen the nuts holding the tubes to the injector. Crank the engine. Clear fuel should come out. The engine won't start with the tubes open because there isn't enough pressure to open the injector. Once clear fuel is coming out, close the nuts and try to start. (The injector has a spring loaded valve near the bottom to prevent exhaust gas from entering the injector.)
A diesel needs about 500F to ignite. It is achieved by compressing the air in the cylinder. Poor compression (bad rings or valves) let too much air to escape to reach ignition temp. Heat helps with a cold or worn engine. If the engine has glow plugs, make sure they are getting power. Warming the engine or using a hair dryer near the air intake helps, but you shouldn't need it in Florida. Also make sure the stop solenoid is getting power. When the run switch is on and the starter is energized, the low oil/high temp safeties are bypassed. On Onans I have had, the glow plugs come on automatically. I usually put in a switch so I can control them manually.
A 12v electric fuel pump makes bleeding easier if the engine has a mechanical fuel pump with a lever. It can be inline with the stock pump and will push fuel thru the stock pump to the injector pump. It doesn't have to be on once the engine starts, but makes a good emergency pump. Pic is the kind I use, about $20 on eBay.







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Old 10-06-2019, 05:34 PM   #12
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The Onan J series diesels are known for being a bit tough to start cold, so expect to hit the glow plugs for 20 - 30 seconds even on a warm day. Without them, it'll start poorly and might not start at all.
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Old 10-12-2019, 05:17 AM   #13
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I have also had starting issues with our old but recently installed Onan. Thought it was electrical issue but had relay tested and it was fine. Disabled the sensor in exhaust, thermostat and oil pressure to be sure and still no go. The stop solenoid pulled about two amps so appeared to be working. Then moved on to fuel system. Filters check,12 volt pump check, bled at pump and injectors. Then onto glow plugs. There was no current flowing so tested each individual and 1.9 ohms in two and 155 on third but no current would flow even if each one isolated. Phoned my sparky guru and he said sometimes the carbon can build up on the plug and give a false indication of continuity. He suggested all new glow plugs which arrived in 24 hours. Installed and it pulled 50 amps initially decreasing to about 42 after 20 seconds. Fired up like a new one
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