GeneratorAC weird problem... stumped

[aaron_o_]

Hey guys long time lurker, still somewhat new to motorhomes, first post, go easy on me. 🙂

Here's my issue, bear with me, Well first, RV is 2000 Gulfstream SunSport, diesel pusher on Spartan chassis.
Onan diesel 7500 generator. Runs fine, and had it checked and was told it was "100% perfect", he put it on a load bank and ran 50 amps for 10 minutes, worked flawlessly. I wasn't there, but he's a reputable generator repair place as far as I understand. Now, it'll run the water heater fine, I don't know what kind of load those pull, but I'm sure it's not too light.
Here's the problem... when I try to run an AC unit, either one, it runs at first, then you can hear the compressor kick on, then after a short period of time, could be 15 seconds, could be 45, you hear the compressor "kick out" and then the generator revs high for a short period then returns to idle. Repeat this cycle as many times as you like. It will just keep doing this.
Here's the kicker... both units, both separately and together, run fine on shore power, whether it be Georgia Power or my Honda EU6500 quiet generator. All weekend long.
Thinking it could be the switch, I first cleaned the contacts, no help, then swapped the inputs. So the generator was actually feeding in where the shore power would normally be feeding into the switch. No help.
Then I ran a totally separate wire set(stole from my 220 welder extension cord) direct from junction box in front of drivers feet, back through cabin, into input to switch in back. I had 121-123 volts into switch and out of switch. Also had this with coach wiring, forgot to mention that earlier. It does drop a second when compressor kicks on, I figured that was normal and expected.
I also tried the Supco hard start capacitor on one of the units with no help. The units are old, but appear to work fine on shore power, so I've somewhat ruled them out, but not totally.
I'm no electrical guru, especially RV systems, but I'm not your average dummy either. I may have left something out and hope it's something totally simple I'm not aware of with RV's, so give me your thoughts and/or questions and I'll answer them as best I can. Any help is appreciated. I will take it to someone if I have to eventually, but I'm afraid to take it just anywhere and hear the horror stories of people just shotgunning on the customers dime. I can't afford that.
Thank you in advance.

[aaron_o_]

I think I talked to them a while back and they said they don't mess with the diesel stuff. I'll try again though, maybe things have changed.

[Jim]

aaron o. wrote:
Yeah, that's it. Thanks for the easier link. 🙂 Hey can I borrow yours for testing?

Sure, just drive over here to Merrill, Oregon, park in the next space, and we'll work on it.

On Edit: Here's a link to Flight Systems. They repair Onans and have troubleshooting guides though I can't find our model in their list of docs. They've been known to help people with their Onan issues via email. "Flight Systems..."

[tropical36]

aaron o. wrote:
Hey guys long time lurker, still somewhat new to motorhomes, first post, go easy on me. 🙂

Here's my issue, bear with me, Well first, RV is 2000 Gulfstream SunSport, diesel pusher on Spartan chassis.
Onan diesel 7500 generator. Runs fine, and had it checked and was told it was "100% perfect", he put it on a load bank and ran 50 amps for 10 minutes, worked flawlessly. I wasn't there, but he's a reputable generator repair place as far as I understand. Now, it'll run the water heater fine, I don't know what kind of load those pull, but I'm sure it's not too light.
Here's the problem... when I try to run an AC unit, either one, it runs at first, then you can hear the compressor kick on, then after a short period of time, could be 15 seconds, could be 45, you hear the compressor "kick out" and then the generator revs high for a short period then returns to idle. Repeat this cycle as many times as you like. It will just keep doing this.
Here's the kicker... both units, both separately and together, run fine on shore power, whether it be Georgia Power or my Honda EU6500 quiet generator. All weekend long.
Thinking it could be the switch, I first cleaned the contacts, no help, then swapped the inputs. So the generator was actually feeding in where the shore power would normally be feeding into the switch. No help.
Then I ran a totally separate wire set(stole from my 220 welder extension cord) direct from junction box in front of drivers feet, back through cabin, into input to switch in back. I had 121-123 volts into switch and out of switch. Also had this with coach wiring, forgot to mention that earlier. It does drop a second when compressor kicks on, I figured that was normal and expected.
I also tried the Supco hard start capacitor on one of the units with no help. The units are old, but appear to work fine on shore power, so I've somewhat ruled them out, but not totally.
I'm no electrical guru, especially RV systems, but I'm not your average dummy either. I may have left something out and hope it's something totally simple I'm not aware of with RV's, so give me your thoughts and/or questions and I'll answer them as best I can. Any help is appreciated. I will take it to someone if I have to eventually, but I'm afraid to take it just anywhere and hear the horror stories of people just shotgunning on the customers dime. I can't afford that.
Thank you in advance.

Along the way, think engine here and maybe for whatever reason, it wants to bog down under a perfectly normal load.

[aaron_o_]

Yeah, that's it. Thanks for the easier link. 🙂 Hey can I borrow yours for testing?

[Jim]

Turns out, I have the same genset.

Very interested in how this all turns out for the OP.

Here's a clicky for the above link to a new inverter: Genset Inverter

[aaron_o_]

msturtz wrote:
Yes the inverter Onan part #327-1461 (about $950) is a large silver box that is connected to the generator by fairly large copper wires. The box usually has fins on it for cooling. It will also have large wires coming back out of it that connect to the output load connection. It can be replaced separately from the generator components. Onan will only sell the inverter module with the generator control box so you will need to replace both. It is possible that you have a bad capacitor on the inverter module thus causing the inability to start your AC units but you can still run resistive loads. You may need wiring diagram 611-1236 to R&R the inverter / control boards.

This is what mine looks like. http://www.ebay.com/itm/Genuine-Onan-300-6058-96-Diesel-Generator-Inverter-Factory-New-300-6058-02-/231584937278?hash=item35eb89b53e:g:Oj0AAOSwrklVcenJ&vxp=mtr

[aaron_o_]

After quick search, it seems the inverter is part of the controller box, last I checked, around 1500. Trying to stay away from that. Dangit!

[msturtz]

aaron o. wrote:
msturtz, that's a lot of good info, thanks! I posted the model and s/n earlier but here it is again...7.5HDKAJ11451D
S/N J990 000244
I'll try the heater thing. Will turning the water heater on be enough of a load to do the same? It does bump up the rpm's a little when I run that. And they remain increased.
Is the inverter part of the "control box" or something I can replace individually?

Yes the inverter Onan part #327-1461 (about $950) is a large silver box that is connected to the generator by fairly large copper wires. The box usually has fins on it for cooling. It will also have large wires coming back out of it that connect to the output load connection. It can be replaced separately from the generator components. Onan will only sell the inverter module with the generator control box so you will need to replace both. It is possible that you have a bad capacitor on the inverter module thus causing the inability to start your AC units but you can still run resistive loads. You may need wiring diagram 611-1236 to R&R the inverter / control boards.

[aaron_o_]

msturtz, that's a lot of good info, thanks! I posted the model and s/n earlier but here it is again...7.5HDKAJ11451D
S/N J990 000244
I'll try the heater thing. Will turning the water heater on be enough of a load to do the same? It does bump up the rpm's a little when I run that. And they remain increased.
Is the inverter part of the "control box" or something I can replace individually?

[aaron_o_]

Bikeboy57 wrote:
Before you rewire the entire coach.......

If you measure the voltage at the transfer switch when the AC is trying to start, what do you get? 120+, dropped some when compressor kicked on but not super low, 110 maybe? I think lowest number I saw was like 107.

The point is that measuring there divides your issue into half. If it's OK, then the problem is between the xfer switch and the AC. If it's low, then measure at the gen output while starting the AC. Keep measuring at different points at the system until you find the low voltage instead of guessing at the problem. That's a good point, but I don't have a real good way to measure there as it's wire nutted. I guess I could test right at the generator, but it's hard to run from AC to front of RV to test. I'll have to get some help I guess.

Given the AC units run on shore and an external gen, your assumptions about the generator are valid. I just think it best to do the diagnostics with a voltmeter.Agreed, that's what I've been doing.

[aaron_o_]

msturtz wrote:

aaron o. wrote:

msturtz wrote:
Did the AC run properly on the welder wiring?

No

Where did you make the connection? At the AC or at the panel feeding the AC?

From junction box off of generator to input of switch.

[msturtz]

Can you post the exact model off of the data plate. I suspect this may be an early QD series generator that has a built in inverter. They work very differently from standard generators. The generator creates either split phase or 3 phase AC variable frequency electricity. This is rectified to DC and the inverter is recreating 60 Hz AC split phase 120 VAC electricity. It is possible that the inverter portion of the generator has malfunctioned and cannot provide sufficient surge current. I suspect that this is the case because you state that the generator returns to idle. A 1800 RPM or 3600 RPM generator without an inverter won't do that because the frequency is tied to the RPM of the generator. You may try this to test. Plug in a 1000 watt or 1500 watt heater or blow dryer or 500 watt halogen light running on the generator. Then try to start the AC unit. I know that it sounds counterintuitive but that amount of load will cause the inverter to ask the generator to operate at a higher RPM thus giving the AC unit additional start voltage and provide less voltage sag. In an ideal world you would put this on an oscilloscope and watch the waveform as the AC compressor motor tries to start. It is likely you will see a major voltage drop and the compressor protection circuit kicks off. The locked rotor load on a compressor is significant. You can also try a boosting transformer such as a Hughes Autoformer. That isn't a "fix" just a away to determine if the voltage is sagging so much that the compressor can't start. It will be difficult to see the voltage problem on a standard multi-meter because it can happen so fast. I would try however.

[Bikeboy57]

Before you rewire the entire coach.......

If you measure the voltage at the transfer switch when the AC is trying to start, what do you get?

The point is that measuring there divides your issue into half. If it's OK, then the problem is between the xfer switch and the AC. If it's low, then measure at the gen output while starting the AC. Keep measuring at different points at the system until you find the low voltage instead of guessing at the problem.

Given the AC units run on shore and an external gen, your assumptions about the generator are valid. I just think it best to do the diagnostics with a voltmeter.

[msturtz]

aaron o. wrote:

msturtz wrote:
Did the AC run properly on the welder wiring?

No

Where did you make the connection? At the AC or at the panel feeding the AC?