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Michael
Michael, Master Mechanic
Category: Boat
Satisfied Customers: 257
Experience:  Over 25 years experience working on everything from PWC's to Mega Yachts
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I have a 1982 Onan Marine MCCK 6.5 Spec H generator that starts

Customer Question

I have a 1982 Onan Marine MCCK 6.5 Spec H generator that starts but then stops when you release the stop switch?
Submitted: 1 year ago.
Category: Boat
Expert:  Michael replied 1 year ago.
Hi, Thank you for your question regarding your Onan generator.... There are a few things on these older units that can cause them to shut down like that: First, try unplugging the exhaust temperature sending unit located on the exhaust elbow or riser... then start the generator and let go of the start switch and see if it keeps running... If not, plug it back in and try the same thing with the temp sensor located on the engine near the thermostat housing. If it still dies then try the engine oil pressure switch located down low on the engine. It should be one of these sensors that has failed causing the engine to shut down. Give this a try and get back to me with your findings. Also, if you need more help finding them I can try to send you a diagram but they are pretty easy to find as this is a fairly basic engine. Mike
Customer: replied 1 year ago.
I inl have two sensors; one by fly wheel and I've on engine block by the control box. I disconnected one, then the other and same thing need to hold down start to keep it running.
Expert:  Michael replied 1 year ago.
Your unit should have an exhaust overheat sensor on the exhaust elbow/riser. I would show you a diagram but Onan diagrams are almost impossible to find online. If for some reason you do not have this sensor then you may be looking at a new control board.
Customer: replied 1 year ago.
I don't don't have a control board either. I have a solenoid, 3 relays, a DC power block, an AC power block to the rear, start switch, and two resistors; one small and one large.
Customer: replied 1 year ago.
See attached photos
Expert:  Michael replied 1 year ago.
ahh ok, thanks for the pics... then what you need to do is trace the wire from the positive side of the ignition coil to the relay... attach a voltmeter to the terminal that the wire from the coil goes to, start the engine and watch the meter when you release the start button... if the voltages goes away when you release the button then it's either the relay or the start switch... you can also test the start switch the same way by connecting the volt meter to the "ign" terminal on the switch and see if it has power when you release the switch. Try this and get back to me with your findings and we'll take it from there.
Customer: replied 1 year ago.
I tried all 3 relays with a new relay and same thing. There are two switches for the generator, one is remotely located. It does the same thing regardless of the switch being used. Also, I traced the ignition cool wire, it first goes to what looks like a resistor (see photo)
Expert:  Michael replied 1 year ago.
Try temporarily bypassing the resistor with a jumper wire to see if it will stay running...
Customer: replied 1 year ago.
No luck
Expert:  Michael replied 1 year ago.
ok, the next thing to try is to unplug the sensors again one at a time and short the wire to ground... if the sensor is a "normally closed" style then this would neutralize it. Also, did you check the fuses on the box? they must be replaced with same spec "slow blow" style Mike
Customer: replied 1 year ago.
All of the fuses are good, not slow blow style though. I tried grounding the sensors already. Could the resistor be bad?
Expert:  Michael replied 1 year ago.
yes, it's coming down to that now... not much else that could be wrong here if you have found all the sensors....as a test you could also try hooking up 12v straight to the ign coil and start the generator and see if it keeps running.
Customer: replied 1 year ago.
It does keep running when I do that, but it doesn't produce power.
Expert:  Michael replied 1 year ago.
If it has been sitting for a long period of time it is possible that the voltage regulator needs to be excited to get it to start working again... This is a method that sometimes works and sometimes doesn't and can damage a good voltage regulator so we are not supposed to advise this method but you are running out of options. The method requires jumping 12v directly to the hot lead on the regulator but I do not have the diagram for that so it would be difficult for me to tell you how to do it. You could try putting an A/C voltmeter on the regulator output to see if your getting 115V there.
Customer: replied 1 year ago.
I've used it a lot this season, it hasn't sat. Where is voltage regulator
Expert:  Michael replied 1 year ago.
on the end of the armature shaft on generator assembly.... has a cover plate over it most likely
Expert:  Michael replied 1 year ago.
Were you able to find/test the voltage regulator? Mike
Customer: replied 1 year ago.
It was the bridge rectifier
Expert:  Michael replied 1 year ago.
ok, so you got it to stay running and produce voltage?

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