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Old 11-13-2015, 11:05 AM   #1
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Very Strange Issue Onan 7500 Diesel generator

Generator stop running and when I test power into the unit I find 12v at the back of the unit. However there is no power at the lighted switch nor at any of the three fuses that I can access under the access panel. The weird thing is that this morning I put a test lamp of the fuses again and found power there, but before I could even try to start it the power mysteriously was gone and the test light no longer lights up? It is a 2004 unit with low hours and always worked well. Chassis batteries are pretty new.
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Old 11-13-2015, 11:23 AM   #2
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Bad Ground or dirty connection
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Old 11-13-2015, 12:11 PM   #3
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Give this a try,
Running across the top of the generator is the control umbilical harness. Unplug the connection in this cord, spray it with some Electrical Contact Cleaner, put a dab of Dielectric Grease on the pins and reassemble.

In addition to the generator, I was having strange things happening with my dash gauges. Spent an afternoon pulling every plug on the firewall, cleaning and reassemble. Now, the Gauges work fine. All it takes is poor contact on one of these pins to cause trouble.
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Old 11-13-2015, 05:07 PM   #4
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I cleaned the grounds and even tried to jump it from my car from the back of the generator to rule out battery problems. None of that worked. Sporadically power will return to the switch. If I push it down to prime, it primes and does its thing. As soon as I push it up to start, I loose all power to the switch and nothing happens.
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Old 11-13-2015, 11:31 PM   #5
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One other thing you might look at is the battery cable and connection at the generator. It is possible the 12V cable coming into the generator has been pinched. During a service on mine, I saw where the cable had been pinched due to lack of support to carry the cable. When the generator was pulled out for service and rolled back in,the cable was being pinched.
That's about all I can offer at the moment.
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Old 11-14-2015, 07:53 AM   #6
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Generator gremlins. I used to have a Generac that shut down every time I went around a curve.
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Old 11-14-2015, 08:48 AM   #7
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What model is your RV?

Tracking the heavy wires that provide power to the generator should not be extremely difficult. If you have not done any battery connection maintenance I would start there. First take several pictures of the wires and batteries. Then clean and tighten every connection you can find.
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Old 11-14-2015, 08:59 AM   #8
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What Dennis said in post #3... only leave the plug disconnected and try starting/running with the gen mounted switch. That plug eliminates all your coach start wiring/switches. Many places in that circuit to cause problems you describe.


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Old 11-14-2015, 08:05 PM   #9
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I don't mean to "steal" this thread but could someone tell me where the battery cables enter the above generator.

Thanks, Mike
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Old 11-14-2015, 09:14 PM   #10
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Rear of the gen, down low. Do not over torque cables... they mount in a plastic block.
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Old 11-15-2015, 09:03 AM   #11
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I feel like I eliminated coach wiring or battery problems by putting jumper cables on the terminals at the rear of the generator and trying to jump it from my car. The same symptoms happen when I do this. I made a small video to show the problem.
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Old 11-15-2015, 09:38 AM   #12
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Quote:
Originally Posted by evotech View Post
Generator stop running and when I test power into the unit I find 12v at the back of the unit. However there is no power at the lighted switch nor at any of the three fuses that I can access under the access panel. The weird thing is that this morning I put a test lamp of the fuses again and found power there, but before I could even try to start it the power mysteriously was gone and the test light no longer lights up? It is a 2004 unit with low hours and always worked well. Chassis batteries are pretty new.

Has it gotten cold since the last time you started. Hold the start switch down for an extended period. I've seen it take up to a minute while the glow plugs work when ambient temp is low. Can't hurt.


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Old 11-15-2015, 09:49 AM   #13
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[QUOTE=evotech;2825728]I feel like I eliminated coach wiring or battery problems by putting jumper cables on the terminals at the rear of the generator and trying to jump it from my car. The same symptoms happen when I do this. I made a small video to show the problem.QUOTE]

Actually you only provided another power source. That did not eliminate coach wiring. Removing the pigtail that goes to the coach is the only way to isolate the genset. If that is disconnected the set can function normally IF there are no gen problems and the problem is in coach wiring.

You could have the big green box sitting on the ground and jump it from your car and it would run...it would be perfectly happy not to be connected to anything. That pigtail contains stop functions that could be a shorted wire or a switch causing your problem. Easy and cheap to at least try it. If it is in coach wiring that's when you real fun will start.
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Old 11-15-2015, 10:02 AM   #14
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Since you are losing power to the small wires as you try to start, It sounds like a loose main power wire. Take the ground wire loose and clean the metal surfaces. Reattach try to start. Take the main battery cable to generator loose. clean metal ends reattach. Try to start. Make sure you do the positive wire to the generator (at the battery)
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