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Old 10-23-2021, 07:25 AM   #1
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Odd air A/C pump issue only when running genset

I've been helping a friend of mine get his boat ready for a long cruise. The boat has a 16000 BTU Cruiseair reverse cycle A/C unit for the salon and another for the rest of the boat. The boat also has a Kohler 8kw genset.



  • The genset control board failed recently and was replaced by a certified Kohler technician. The genset was fully tested and given a clean bill of health. Since then, the salon A/C unit would not stay running on the genset. It would run just fine on shore power. The other A/C unit runs fine on both the genset and shore power.
  • When the genset is running, the salon A/C will run for about 15 seconds, then it just shuts down.
  • We swapped the starting capacitor just in case it was weak (they are readily available for 12 bucks, and super-easy to swap), no change.
  • An A/C technician was hired to look at the A/C unit. He verified the voltage and frequency going into the A/C unit was good from the genset.
  • The A/C tech swapped the A/C control board. It did not help.
  • The A/C tech rigged it up so that the A/C compressor is now triggered to run from the A/C raw water pump control output rather than the compressor control circuit, and the A/C unit now runs just fine on the genset or shore power. The raw water pump is still connected to the same circuit, BTW.
  • The A/C tech cannot explain why it won't work normally on the genset when connected normally, but says it will work as it is now rigged indefinitely.


Just curious if anyone has any ideas.
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Old 10-23-2021, 07:50 AM   #2
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My inclination would be to replace the solenoid for the compressor circuit. I'm assuming he wired the compressor to the cooling water pump solenoid. Possibly a slight reduction in voltage from the generator as opposed to shore power has the solenoid dropping out.

That's my thought anyway.

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Old 10-23-2021, 09:28 AM   #3
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Like OCdiver said. There was too much voltage drop on the original circuit. The Generator is delivering less voltage, enough to be with in specs but less than shore. This difference turned out to be just enough to trip something.
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Old 10-23-2021, 07:22 PM   #4
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It might be worth considering replacing the wires feeding the unit from the gen. to reduce any Vdrop.
Also adjusting the Gen. speed to raise the V just a bit without going over that rating. Gens usually drop a volt or so when loaded as the governor lags a bit allowing the Hz to drop. All it maybe would take is a couple of Hz and thus V.

Use a voltmeter to check what the unit is receiving from each power source.
As suggested a few volts may be just enough. The V & Hz may not be all but the actual wave shape affects power also. Small gen. often produce a narrower wave form delivering less real power. Unfortunately that would likely take a scope to see the difference.
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Old 10-24-2021, 07:05 AM   #5
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We turned up the genset voltage 5 volts to 127 and it still does the same thing when compressor is connected normally. The voltage at the compressor drops momentarily to 123 when the compressor kicks on, then stabilizes back to 127. The voltage jumps to 131 when the compressor kicks off. We turned the voltage back down to normal after that and nothing has changed. So strange... Thanks for the replies.
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Old 10-24-2021, 07:32 AM   #6
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MM
Is this correct:
-The genset control board failed. Why?
-A new board was installed.
-After that the AC won't run on the genset.
-But the AC runs fine on shore power.
- A re-wiring was done and unit works fine on the genset.


Did the tech re-wire so that all ACs are tied into the cooling pump? On our vessel the three Cruise AAir compressors will not run unless pump breaker turned on too. A check of the compressor installation wiring diagram may indicate the new wiring setup is preferred.

Is there a standard 3 way switch for Shore - Generator - Off?
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Old 10-25-2021, 08:29 AM   #7
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Quote:
Originally Posted by sunchaser View Post
MM
Is this correct:
-The genset control board failed. Why?
-A new board was installed.
-After that the AC won't run on the genset.
-But the AC runs fine on shore power.
- A re-wiring was done and unit works fine on the genset.


Did the tech re-wire so that all ACs are tied into the cooling pump? On our vessel the three Cruise AAir compressors will not run unless pump breaker turned on too. A check of the compressor installation wiring diagram may indicate the new wiring setup is preferred.

Is there a standard 3 way switch for Shore - Generator - Off?

Not sure why the genset control board failed, it kept shutting down the genset and throwing a strange "code" on the LED display. The code was not listed in the service manual and the service technician did not know what it meant.


The boat owner reported that the AC untit ran fine on the genset prior to board replacement.


Both AC units in the boat have always been tied to the one cooling water pump. The only change in wiring was that the "faulty" AC now no longer turns on the AC compressor and cooling pump separately. According to the technician, the controller always turned the compressor and cooling pump on at the same time, but with two different output circuits. Now both the compressor and the cooling pump are connected to the same output circuit. The obvious answer is that the compressor output circuit on the AC controller is faulty, but the the controller has been replaced with no change.



Like I said, strange, right?
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Old 10-25-2021, 02:45 PM   #8
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Just last week troubleshot a boat that had two brand new Dometic AC's and a brand new NL 12kW gennie. AC's ran on shore but not on gen. I checked gen output and it was 124/248Vac, Hz 62.3 unloaded. A little high Hz, but pretty normal.

For giggles set governor down to 60.5Hz unloaded. AC's worked fine then. Stupid for Dometic to make their control system so picky.

Boatbuilder had called Dometic and asked if the Hz was a problem, they told him flat out that it was fine at 62.3. I told him to call them back and slap them about the head and shoulders.
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Old 10-25-2021, 03:12 PM   #9
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By running the raw water pump off the compressor trigger the inrush currents for the two motors were added overloading the generator temporarily. Starting the raw water pump first and then the compressor solved the issue.
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Old 10-25-2021, 04:03 PM   #10
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"Soft Start" on one or both A/C???
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Old 10-26-2021, 12:03 PM   #11
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Quote:
Originally Posted by gsholz View Post
By running the raw water pump off the compressor trigger the inrush currents for the two motors were added overloading the generator temporarily. Starting the raw water pump first and then the compressor solved the issue.

Not that it matters, but the compressor pump is connected to the raw water pump trigger. Regardless this unusual configuration was used as a band aid to get the system to work.
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Old 10-26-2021, 12:04 PM   #12
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Quote:
Originally Posted by Ski in NC View Post
Just last week troubleshot a boat that had two brand new Dometic AC's and a brand new NL 12kW gennie. AC's ran on shore but not on gen. I checked gen output and it was 124/248Vac, Hz 62.3 unloaded. A little high Hz, but pretty normal.

For giggles set governor down to 60.5Hz unloaded. AC's worked fine then. Stupid for Dometic to make their control system so picky.

Boatbuilder had called Dometic and asked if the Hz was a problem, they told him flat out that it was fine at 62.3. I told him to call them back and slap them about the head and shoulders.



The boat is on it's way south now, but next Spring, I'll adjust the frequency and see if it helps. Thanks!
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