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Old 08-27-2016, 10:32 PM   #1
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AC problems with generator running

1999 dutch star DP with 7500 watt onan generator.
AC units work fine on shore power but compressors kick in and out while on generator, doesn't matter if one or both is running.
Cummins shop did 90% load test on gen set for 2 hrs and all is good they say, gen set puts out 119.3 volts on both sides to and thru transfer switch.
Cleaned all connections and took apart transfer switch contacts and they looked like new yet.
Replaced a corrosive junction block on gen set and cut wire ends down to nice clean wire.
AC units draw around 23 amps on shore power but when on gen set its about the same until the kicking in and out starts and have seen a 45 amp draw and has killed get set once.
AC units seem to work for about 3-4 minutes on gen set then start the kick in and out process and will make gen set change rpm but voltage seems to stay the same.
Any ideas would be great since we are heading out over labor day weekend and won't have shore power.
Cummins shop says AC units are getting bad but I'm having a hard time believing that at this point cause of no issues while on shore power.
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Old 08-28-2016, 05:23 AM   #2
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Quote:
Originally Posted by dirt racer View Post
1999 dutch star DP with 7500 watt onan generator.
AC units work fine on shore power but compressors kick in and out while on generator, doesn't matter if one or both is running.
Cummins shop did 90% load test on gen set for 2 hrs and all is good they say, gen set puts out 119.3 volts on both sides to and thru transfer switch.
Cleaned all connections and took apart transfer switch contacts and they looked like new yet.
Replaced a corrosive junction block on gen set and cut wire ends down to nice clean wire.
AC units draw around 23 amps on shore power but when on gen set its about the same until the kicking in and out starts and have seen a 45 amp draw and has killed get set once.
AC units seem to work for about 3-4 minutes on gen set then start the kick in and out process and will make gen set change rpm but voltage seems to stay the same.
Any ideas would be great since we are heading out over labor day weekend and won't have shore power.
Cummins shop says AC units are getting bad but I'm having a hard time believing that at this point cause of no issues while on shore power.

Do you have the same issue with each AC running by itself on the generator? One or both may have compressor issues causing high amp draw at compressor startup.Your start capacitors may be bad on the AC's, causing large surge when compressor tries to kick in. Shore power may have enough power to handle a momentary hard start surge where the generator does not. Incorrect freon charge, worn compressor, defective start capacitors, dirty coils or filters can cause hard starting. If the AC's are original equipment, 17+ years, may be time for replacement.

I would start by servicing the AC units. You can replace them yourself if needed for around $750 each.
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Old 08-28-2016, 06:31 AM   #3
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Let's get this straight. Current draw on shore power is 23 amps, that puts it's at 11.5 per AC unit. this is good. AND they work fine.
But switch to gen set current draw goes to 45 surge.
AC works find on shore power, but not on Gen set. Then something is wrong with wiring or the gen set cannot handle the load. Now you can have a bad rotor, armature, or a loose connection at the gen set junction box. YOU should look around the gen set, locate the a/c wires coming out and see if there is a junction box. The really only way to really test the Gen Set is with a Load other then the AC units. So unless the shop did this, then they really did no test it.
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Old 08-28-2016, 07:00 AM   #4
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I would be checking your transfer switch for a bad connection on the generator side.

The transfer switch normally will have the generator as the primary feed. If you are hooked to shore power a different set of contacts is used. Once generator starts the transfer switch will switch to generator feed.
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Old 08-28-2016, 10:39 AM   #5
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I# good on shore power then they are good.

Many engine folks are just that...not power trained.

Engine runs and supplies power...so what!

If the frequency is just a bit HIGH, say 70 CPS instead of 60 CPS the compressors will try to run faster which requires much more power, think driving 70 instead of 60.

If you have a voltmeter with a frequency measurement then easy.

Or if you have an OLD electric clock with a motor in it you can do like the old power companies did back in the day and set the clock to a reference and check to see if still "on time".

A faulty governor could be missing speed causing frequency to be off enough for the compressors to get hot enough to kick their over temp protector then when they cool start all over again.

Simple to check and maybe simple to fix.
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Old 08-28-2016, 06:39 PM   #6
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Original post indicated the gen set was load tested and the transfer switch was checked out.

I wouldn't think the problem would be in the wiring between the transfer switch and the ACs as they work on shore power. I assume the transfer switch selects input source with output to electric panel being the same terminals regardless of source, this may be incorrect.

If the AC units draw continuous 23 amps on shore power and continuous 45 on gen set, them the generator and coach wiring are suspect even though the gen set was load tested and the transfer switch inspected.
If the 45 amp draw is cycling, the gen set could have a regulation problem or the AC(s) are likely hard starting. If the ACs are drawing 22.5 amps each continuously, why aren't they tripping the circuit breakers in the electrical panel? Can you check the amp draw at each AC breaker rather than total amps? This could give you some helpful info as to whether one AC is the issue, or, if they both behaving the same the generator is probably the culprit.

Another simple check would be to connect to shore power, run ACs and check temperature at AC's discharge, looking for an approximate 20 degree drop from return air temp. Fans could be running making you think the AC is working when it is just circulating air. Temp gauges are cheap.

It might be worth pulling the covers on the roof ACs and doing a visual check of coils and with the unit running check for short cycling of the compressor. Just labor to do this no cost involved. Good to check the coils periodically anyway.

Hope it is something simple and inexpensive to repair.
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Old 08-28-2016, 09:05 PM   #7
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Bit of a update here,, thanks for the replies and great info for sure.
I got my brothers portable 7K gas generator today and did some hook ups with it.
With it hooked up to shore power going from 50 to 30 to 110 on generator both units worked perfect for over a hour then i took my welder 220 extension cord apart and wired it to where the coach wiring for generator come to the block on gen set and bought a 30 amp 4 pole twist lock for portable generator and AC units ran perfect hooked up that way also.

So at this point i think I have eliminated having a wiring issue and a transfer switch problem and believe it has to be the generator on coach problem.
More investigation to come.
I have already taken covers off AC units and very clean and seem to be working as they should, even tho they are older units but really not sure if they are original since they are 2 different brands.
Units only draw large amps after they ave tried to cycle on generator.
Today i started the gen on coach and units ran for about 5 minutes then the gen jumped way up in rpm and thats when the units started to kick in and out compressor wise and then gen seems to surge rpms and eventually the units will cycle normal for about another 2 minutes or so and then rams change all over again.
Sorry if this is hard to get but hard to explain whats going on also.
Thanks again for all the advice.
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Old 08-28-2016, 09:13 PM   #8
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Change in rpm is change in frequency unless inverter unit.

Standard ac power is 60 cycles per second.

Translated to rpm is 3600.

If at idle is some rpm then increase in load the genny will get louder but actual rpm should be close.

If fine on another genny then wiring should be fine.

If you have any good ac motor that can be plugged in yiu could listen to it run while ac is operating to see if it changes or find a frequency counting device.

Does a kill a watt do frequency?
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Old 08-28-2016, 10:41 PM   #9
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Quote:
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Units only draw large amps after they ave tried to cycle on generator.
Today i started the gen on coach and units ran for about 5 minutes then the gen jumped way up in rpm and thats when the units started to kick in and out compressor wise and then gen seems to surge rpms and eventually the units will cycle normal for about another 2 minutes or so and then rams change all over again.
Sorry if this is hard to get but hard to explain whats going on also.
Thanks again for all the advice.
To me this is starting to sound like issues with your generator. On my previous coach my generator would start surging. When it surged, tho, I would take a voltage reading at the 120V outlets and get a way higher reading than 120V. Not good. Replaced the board on that generator and all worked fine since.
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Old 08-28-2016, 10:56 PM   #10
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TQ60,, its a inverter unit from what i find online typing in the gen numbers.
Does tha t mean inverter is the problem?
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Old 08-29-2016, 02:55 AM   #11
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TQ60,, its a inverter unit from what i find online typing in the gen numbers.
Does tha t mean inverter is the problem?

If everything works well on shore power and alternate generator then your on board generator sounds like the culprit. Do you know the model number of your on board Onan generator?

Your brothers 7k generator is some where near 30 amps @ 240 volt? If so, means you hooked up one leg of his generator to your RV?
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Old 08-29-2016, 05:40 AM   #12
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I had similar problems a few months ago. Someone told me it was either low voltage to the thermostat or corrosion in the connector to the thermostat. Your thermostat is powered by 12 volts.

I went through all my batteries, topped off water, cleaned and tightened all terminals and tested them with battery analyzer and they were all in good shape. Since then I have not had any intermittent A/C issues. I suspect it was terminal corrosion and loose connections. That's a good place to start.
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Old 08-29-2016, 06:20 AM   #13
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triplewide,,,i hooked up to his 4 prong 30 amp plug in on his gen.
brenthg,, I have went thru batteries and cleaned all terminals and checked for loose or corrosive wires and have found nothing that was a red flag.
Gen set on coach starts perfect and runs great till AC units are turned on.
I will say that the gen set on coach will change rpm to 4 levels but very small levels while no load and then does this all over again. Not sure that is normal or not but a friend of mine has a newer 12K onan diesel and it never changes rpm no matter what the load.
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Old 08-29-2016, 07:33 AM   #14
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triplewide,,,i hooked up to his 4 prong 30 amp plug in on his gen.
brenthg,, I have went thru batteries and cleaned all terminals and checked for loose or corrosive wires and have found nothing that was a red flag.
Gen set on coach starts perfect and runs great till AC units are turned on.
I will say that the gen set on coach will change rpm to 4 levels but very small levels while no load and then does this all over again. Not sure that is normal or not but a friend of mine has a newer 12K onan diesel and it never changes rpm no matter what the load.
Have you taken voltage readings through a 120V outlet while the generator is running (load or no load). Does the voltage stay constant?
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