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Old 09-03-2017, 03:35   #1
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Onan mdkd no AC

Hi all,
I have a Onan mdkd 6.5 kva .transformer regulation around 1990 model 3300 hrs.
Just replaced my water pump">raw water pump & a few hoses. When I started her up the starter motor kept running. Shut her off found its not making any 240 volts as it has a remote start needs AC power to register to swith off starter motor.
I pulled out the Bridge rectifier & tested it. It was ok but a bit dodgy so replaced it & the capacitator. Still no AC so I pulled the back off and removed the half moon diodes. Tested them & they are ok 4.16 ohmes one way nothing the other way. I removed the rear bell & pulled the bearing as well will replace it tomorrow.
Im now at a loss as to why it isnt making AC power. Any ideas love to hear from you.
Mike
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Old 09-03-2017, 23:13   #2
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Re: Onan mdkd no AC

I would be very surprised the ac output had anything to do with the start soliniod. Are you sure?
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Old 10-03-2017, 01:17   #3
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Re: Onan mdkd no AC

Yes very sure it has a Blue Seas remote starter with a timer starter as well. There is no 12 volt alternator on the Onan so it relies on the 240 volt AC to cut in to turn off the starter. Its quite common & I had the tech down this afternoon who fitted the blue seas system . He thinks my stata windings look burnt. I have the Onan man coming down Tuesday to inspect. If it needs rewiring there is a company nearby that can do it. I will have to pull the stata off which means turning the genset 90 deg to get it off. That requires removal of the raw water & fuel lines plus all the power out wires. Better here in town that out at sea i suppose.
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Old 10-03-2017, 01:30   #4
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Re: Onan mdkd no AC

Hi Mike, never worked on a Onan, but if I think what you're saying is......
To disconnect the starter in Auto mode, there needs to be a feedback from the Altenator voltage. This should close a relay or something and disconnect power from the solenoid. Novell way of doing it?

Can you run the set in Manual mode, turn key to start, then back to run?
If not, wire a Switch in series with the Solenoid so you can disconnect starter motor.

I would check the Fuse, transformer connections and continuity, as this will be powering your AVR.

If you find out the excitation voltage, you might be able to seperately excite the Altenator from a 12V or 6V Battery. (Sometimes used this method to test up to 1000kVA Altenators)

Connect the battery to the field wires and have a switch in series. You can then energise the field when engine running and should give you a output if all is well. You will need to check polarity using the battery. Don't worry if the volts are high or low (unless excessively high) as you are proving the excitation side of the altenator.

If no output, then your going to look deeper, but as you've check the rotating diodes, only really leaves the Rotor. I take it this is a brushless machine?

If you have output, then problem is on the Transformer/ AVR side. Check if there is a small plug in relay etc. On the old SR4 Altenators Caterpillar used this as a "Build Up Relay". All volts out of the altenator were fed directly back into the exciter until there was about 110 VAC output, at which time the relay would operate and the AVR took over. To get the 110VAC, there was a small Nicrohm piece of wire, which dropped the coil volts. If this failed the Altenator would produce high volts and blow the AVR Fuse.
If your removing the Rotor, you may loose the magnetism and have to flash the fields anyway.
Hope this helps
Cheers
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Old 10-03-2017, 01:40   #5
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Re: Onan mdkd no AC

Thanks Ding Duck ( how did that come about lol) . I have to attend a family get together tomorrow & taking Sunday off . Then the Onan man Andrew from KLM
Electrics is coming down to check it out. My usual engineer dropped by for a beer & a look. He fitted the remote start & auto switch for ship shore power. Everything is ok on his side. Im hoping its the transformer voltage regulator not the windings. Yes it is a brushless & will need to be zapped when back together.
Boats it never ends
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Old 10-03-2017, 01:54   #6
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Re: Onan mdkd no AC

No worries Mike, hope it is something small (& not too expensive)
The name? There was a comic strip called "The Swamp" and I had a pilots licence, hence the connection.
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Old 12-03-2017, 04:46   #7
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Re: Onan mdkd no AC

Does your alternator require self excitation? If so, it has been known for alternators not to self excite in cold weather. Happened to us doing trials in UK on ferries built for Hong Kong in one of the coldest winters on record.
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