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Old 09-23-2016, 05:01 PM   #1
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Air conditioner problem..maybe generator.

Hi all.
So last night we are watching a movie on the TV. Nice evening, cool enough to not have the air conditioner on for a change. Just the trusty Onan generator running. So suddenly the air conditioner display starts blinking all sorts of lights and there are clicks coming from the control box. I'm like what the hell it's not even on, but jump up and shutoff the breaker. Sure enough today it's dead as a door nail! Called Dometic and another control box should be here by Tuesday. Weird though how the circuit board apparently got fried when the unit was not even turned on? It's been working just fine for over a year. Even the guy at Dometic didn't have an answer for that one. So I'm wondering, how does that happen? The generator is an Onan 4K
MDKC 1987 vintage. The air conditioner is. Dometic 16k just over a year old.
I'm thinking maybe a power problem with the gen? Reason is in addition to what happened, on generator power my battery charger, a master volt 40amp works just fine until absorption phase is reached at which time it, after about a half hour, shuts itself down too. I've never had any problems for years with either on shore power, and I've been running the AC for weeks on generator power and the only issue a few times has been a high pressure fault. After cleaning everything related to water flow, finally putting a fan blowing on the compressor seemed to do the trick...until now...
Any advice, suggestions would be much appreciated.
Thanks
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Old 09-23-2016, 06:58 PM   #2
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MDKC of that vintage is transformer regulated, which is pretty rock solid. Either volts are in range, or output is super low. Can't really spike high like an AVR. Monitor volts and frequency, Hz and see what it is doing. Volts should be 110-125, Hz should be 58-62 depending on load. Start there.
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Old 09-23-2016, 07:28 PM   #3
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Thanks Ski. I measured the volts and they seemed pretty steady at 115. Not sure how to measure the Hz. But I'll check into that, thanks!
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Old 09-23-2016, 10:01 PM   #4
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Hertz or frequency is how many AC cycles per second. Your machine is described as synchronous, so at 1800 revs per minute, it is making 60 cycles per sec. Since engine governor has a little slop in it, Hz will go up a touch at no load, and down a touch with heavy load. Thus the 58 to 62Hz range.

Some better hand held voltmeters have a Hz setting, or you can phototach crankshaft. 60Hz is 1800rpm. 62 is 1860. 58 is 1740.

Not likely that your gennie caused those finicky electronics to freak out.
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Old 09-24-2016, 04:46 AM   #5
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Possibly an intermittent current delivery problem? Check that your AC wiring from the generator to transfer switch is in good condition.
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Old 09-24-2016, 06:23 AM   #6
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A Kill-A-Watt is a handy device to have on board. Inexpensive and it will display Hertz too.
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Old 09-24-2016, 07:17 AM   #7
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Thanks for the replies as it gives me a direction to investigate. The thing that really has me baffled is that the unit was off when all this occurred. I guess being old school my perception is that when a switch is off, it's off. So in this case how does any current get to the unit to begin with? Is there something about electronic switches I'm not aware of?
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Old 09-24-2016, 07:43 AM   #8
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Sounds like the control panel for the a/c. I am going through something similar on my Marine Trader. I am waiting for the "new" control panel. Worth giving it a shot.
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Old 09-24-2016, 07:48 AM   #9
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Quote:
Originally Posted by MYMT View Post
Thanks for the replies as it gives me a direction to investigate. The thing that really has me baffled is that the unit was off when all this occurred. I guess being old school my perception is that when a switch is off, it's off. So in this case how does any current get to the unit to begin with? Is there something about electronic switches I'm not aware of?
Unless power is completely shut off whether at home, or on the boat, assume it is on and proceed with caution. When in doubt call a specialist.
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Old 09-24-2016, 08:51 AM   #10
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As long as the breaker is on, the control panel is powered so that when you push the start button it can respond.
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Old 09-24-2016, 10:22 AM   #11
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Thanks Bob, that makes sense.
Two other questions I'm trying to research:
One is if the generator chassis is the ground for the generator power, including its start battery, doesn't the generator chassis need to somehow be connected to earth, like the engine block is to water through the driveline etc?
Also, having the generator battery ground connected to the same bus as say the house bank ground, (that bus being grounded to the engine block), is there a way just through the negative connections a bad battery in the house bank can drain the generator battery?
..and thanks Donna, but if I could afford to call a specialist whenever I was in doubt, I'd have a much bigger boat and probably a crew to run it.lol But it is the circuit board for the AC that got fried, (I hope the control panel is still fine) I don't want to chance frying another one if I can help it.
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Old 09-24-2016, 10:40 AM   #12
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Just having the grounds tied together will not drain the genset battery unless there is a common connection on the positive side.
When we bought our boat we were told that there was a separate start battery for the genset in addition to the house bank and engine start battery. There were indeed three batteries so it sounded good. Bringing the boat home we anchored out one night and the next morning all three batteries were dead. Even though there was a separate start battery, all three were tied together at some point. Took a lot of tracing and rewiring to straighten that out.
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Old 09-24-2016, 11:39 AM   #13
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Thanks Bob. My generator positive cable runs directly from its battery to the starter solenoid and the neg cable from the generator chassis to the neg bus through which everything else connects to the engine block. That's the way it's been and I thought it should be but was told otherwise lately. So what I did was disconnected the gen battery neg cable from the bus and connected the gen chassis neg to its battery.
So basically the generator battery cables was positive to starter, and negative to chassis. Somehow that I believe led to the problem, thus my question about how the generator is grounded. The AC worked without a problem till I changed that connection, and I'm trying to figure out why. I really appreciate you taking the time to respond. Forgive my ignorance, but as with a lot of things I know just enough to get into trouble, but not enough to get back out...
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Old 09-24-2016, 11:55 AM   #14
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Who told you to make and change and why? There is nothing wrong with having all the negative terminals connected together. I suspect that there may be a grounding issue with the AC connections which lead to the problem when you made the change.
You may want to check the AC wiring from the genset to the electrical panel/selector switch.
Do you have a manual for the genset? That should have wiring diagrams for the AC power connections.
In the mean time I would connect it back the way it was if that has been working all along.
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Old 09-24-2016, 01:23 PM   #15
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Thanks Bob. I have changed it back to the way it was with all the negative terminals connected together. I think you are probably right about the grounding issue with the AC connections. It had always worked before just fine. The reason I changed it was because I was told that just the negative connections alone could drain good batteries if something went bad, but obviously that isn't the case. I should have researched it to verify it, but I didn't.
I don't know where to begin to look for a possible grounding connection problems so I guess I'll leave good enough alone. But since you seem pretty knowledgeable, any ideas on why the battery charger works just fine on gen power until absorption phase is reached then goes into fault mode? The Manuel says wrong AC voltage, and I measured the voltage and no load is on avg 114.3 - 114.7 Under a load it is 112.5 - 112.7
Momentarily I've seen it as low as 110.9 and as high as
112.9. Do you think that is enough fluctuation to cause the charger to go into fault mode?
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Old 09-24-2016, 01:45 PM   #16
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The gennie ground system should not be tied to the earth ground. The chassis is the ground...
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Old 09-24-2016, 02:11 PM   #17
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If the charger works up to that point and then faults, sounds more like a charger problem. Have you tried to contact the charger manufacturer? Also try to google it, it's amazing how many times other people have had the same problem before you and have a solution.

As far as the grounding issue is concerned, it should be covered in the genset manual.
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Old 09-24-2016, 02:55 PM   #18
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You say the tsat was off ,but power to the a/ c unit was on correct? If so yours a/ c unit had 120 v to the board ,and was fried by the power surge. So then you get up and turned off the breaker you stopped the surge to the unit. Don't know what happened or why but unit will have power to board as long as breaker is on.
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Old 09-24-2016, 03:09 PM   #19
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Thanks for the replies. I'll have to call Mastervolt on Monday and see what they have to say about it.
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