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Onan Generators

Onan - 12.0 MDJC-3CR/2268AA


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Old 04-15-2019, 10:00:22 AM
scottj32712 scottj32712 is offline
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Default Onan - 12.0 MDJC-3CR/2268AA

This is the standby generator on my boat. It is wired to provide 110V as a backup should the main 20KW stop working.

I adjusted the governor spring and got it to 1700RPM via an optical tachometer. I ran out of adjustment to get 1800RPM and have a new spring to install but need to address the output issue first.

The a/c output on the generator is 117V but the frequency 111hz via a meter. I do not have access to an oscilloscope.

I have removed the two rectifiers and tested them with a meter that has a diode test feature. I am not sure how to intrepid these results as they seem like the diodes are working.

Positive Rectifier -
Positive Lead on F1 and Negative on CR
CR1 - 1
CR2 - 1
CR3 - 1
Negative Lead on F1 and Positive on CR
CR1 - 1030
CR2 - 1020
CR3 - 1020

Negative Rectifier -
Positive Lead on F2 and Negative on CR
CR4 - 990
CR5 - 980
CR6 - 1040
Negative Lead on F2 and Positive on CR
CR4 - 1
CR5 - 1
CR6 - 1

The value of 1 was also displayed when no connection was made or the circuit was open.

So if I understand this correctly these diodes are functioning by only allowing voltage to flow in one direction. If that is the case what other tests can I do to determine why my frequency is so high on the A/C ouput.
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Old 04-15-2019, 10:19:38 AM
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Default Re: Onan - 12.0 MDJC-3CR/2268AA

These are priced pretty cheap and quite accurate for keeping an eye on HZ: https://www.walmart.com/ip/P3-Intern...iABEgJLy_D_BwE
Sometimes the JC series of generators have voltage regulator issues, is there a voltage adjustment pot on your regulator?
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Old 04-15-2019, 10:25:32 AM
scottj32712 scottj32712 is offline
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Default Re: Onan - 12.0 MDJC-3CR/2268AA

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Originally Posted by nothingbutdarts View Post
These are priced pretty cheap and quite accurate for keeping an eye on HZ: https://www.walmart.com/ip/P3-Intern...iABEgJLy_D_BwE
Sometimes the JC series of generators have voltage regulator issues, is there a voltage adjustment pot on your regulator?
I will look at getting one of those.

Yes there is a adjustment pot on the regulator. What I am most worried about is the 111hz on the output. I know it was seriously out of spec because I have an APC SmartUPS battery back up and it was really unhappy with me running on that generator.
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Old 04-15-2019, 10:58:55 AM
Tracy T Tracy T is offline
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Default Re: Onan - 12.0 MDJC-3CR/2268AA

your meter is picking up noise, no way making that many cycles as cycles are directly related to rpm, 1800 rpm= 60 cycles or hertz so @ 1700 rpm you are even lower hertz/ cycles.

this was discussed in another thread, divide your 111 by 2 and you get 55.5 cycles/ hertz which sounds right for 1700 rpm..... their cure was to install say a 100 watt light bulb to get rid of the noise your meter is picking up.

regarding adjustment (engine rpm) although i am not familiar with the marine version, i suspect it has close to if not the same govenor setup as its air cooled conterpart. where the other end of the spring hooks are notches in the govenor arm that allow more adjustment by moving the spring to the next notch. if memory serves me correct you will need to move one notch upward and then do the fine tuning with the adjustment nut.
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Old 04-15-2019, 11:19:14 AM
I like oldstuff I like oldstuff is offline
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Default Re: Onan - 12.0 MDJC-3CR/2268AA

Output frequency is determined ONLY by rpm of the engine. Yes you may be getting some interference noise/harmonic that confuses your meter. Put a small load on the generator, say a 1000 watt hair dryer or space heater and you'll see the true frequency.

Or,, your method of measuring the rpm is faulty and the machine is running way fast and actually generating that 111 hZ. 1800 rpm will give 60 hZ. As you ran out of governor adjustment I'd bet you tachometer is wonky and you're way overspeed. Go to Home depot, Harbor Freight, hardware store and buy a $20 kill a watt meter and plug it in. That'll show the freq and volts and they're quite accurate. Plug your load into the kill a watt.
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Old 04-15-2019, 11:24:34 AM
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Default Re: Onan - 12.0 MDJC-3CR/2268AA

old stuff made a point i missed, if your tach is faulty you very well could be making 111 hz. these generators run slow and quite, not like a box store generator thats screeaming away @ 3600 rpm.
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Old 04-15-2019, 12:13:29 PM
scottj32712 scottj32712 is offline
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Default Re: Onan - 12.0 MDJC-3CR/2268AA

What I am understanding then is I should adjust the RPM to get 60hz using a kill a watt and then use the adjustment on the voltage regulator to get 120V.
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Old 04-15-2019, 12:34:37 PM
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Default Re: Onan - 12.0 MDJC-3CR/2268AA

yes, speed has nothing to do with voltage output only HZ.

this is not the case for all generators though. the cck onan basically has no regulator and speed affects both HZ & voltage. totally different design
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Old 04-15-2019, 12:45:38 PM
Zephyr7 Zephyr7 is offline
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Default Re: Onan - 12.0 MDJC-3CR/2268AA

I agree with the other posters that your tach measurement is probably off. The output frequency comes from a combination of the mechanical construction of the generator (the number of poles), and the rotational speed of the engine, period. I suspect you’re running at double speed.

You need a RESISTIVE load to smooth out noise. An incandescent light bulb works, or a hair dryer or electric heater. Measure the frequency with a good meter after doing that.

I’d be pretty careful running at that much overspeed for any amount of time. Best to get that speed issue corrected ASAP.

Bill
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Old 04-15-2019, 12:49:08 PM
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Default Re: Onan - 12.0 MDJC-3CR/2268AA

Speed does affect voltage but as stated adjust speed first then voltage and you should be in the ballpark. Cheers Dan
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Old 04-15-2019, 12:53:00 PM
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Default Re: Onan - 12.0 MDJC-3CR/2268AA

The only reason I ask this question. This generator is on the boat and is original from the 1970s. It was only putting out 70v a/c when I started it and measured 900 rpm by the tachometer by adjusting the governor I got it to 117v.
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Old 04-15-2019, 01:19:44 PM
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Default Re: Onan - 12.0 MDJC-3CR/2268AA

need to verify engine speed first, 60 to 70 vac output is commonly a voltage regulation issue. in other words a bad voltage regulator or one or more of its components. anybody know if this is the YD head? if so he needs the trouble shooting manuals. I found them very helpful, easy to understand, and also extreamly thorough!
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Old 04-15-2019, 01:48:44 PM
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Default Re: Onan - 12.0 MDJC-3CR/2268AA

I would think an optical tach would be fairly accurate but maybe not. You really need to get the linked Kill a watt meter before you do any more messing with it. Most hardware stores carry them, they are a bit more expensive like $30.00.
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Old 04-15-2019, 03:38:54 PM
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Default Re: Onan - 12.0 MDJC-3CR/2268AA

Spec. AA should be a YD gen-end.
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Old 04-15-2019, 04:43:01 PM
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Default Re: Onan - 12.0 MDJC-3CR/2268AA

The OP should verify his control board is configured correctly. I chased a frequency issue and bought a new board from flight systems etc to find the board was configured wrong. If you would like pictures of the board configured properly I can post.
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Old 04-15-2019, 07:22:57 PM
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Default Re: Onan - 12.0 MDJC-3CR/2268AA

With a YD gen end speed and voltage are linked and HZ is locked to engine speed. I bought a 12lead DJC a while back and instead of reconnecting the leads to give 120V the owner adjusted the governor down to 37Hz . He wasn't familiar with the voltage adjustments on the VR board. So the OP seeing 70V at 900 RPM sounds about right to me. Once the engine speed is close to correct the voltage regulator can hold it to a fairly close tolerance but if the frequency is off by more than 5Hz or so the VR can't keep up and you get signficant variance in voltage. Cheers Dan
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Old 04-16-2019, 07:41:58 AM
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Default Re: Onan - 12.0 MDJC-3CR/2268AA

Quote:
Originally Posted by consolenut View Post
The OP should verify his control board is configured correctly. I chased a frequency issue and bought a new board from flight systems etc to find the board was configured wrong. If you would like pictures of the board configured properly I can post.
Yes please if you have pictures of a board in correct config. I do not know what the previous owners have done here.
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Old 04-16-2019, 08:47:23 AM
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Default Re: Onan - 12.0 MDJC-3CR/2268AA

I can assure you. No MDJC is going to run at 111 hertz. You are picking up noise from something.
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