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thendrix 
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Location: Fairmount GA
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 Topic: Cummins 855 generator problems
Posted: 15 Nov 2023 at 8:09pm |
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Got a 855 Cummins on a generator. It exercises every Wednesday. Today it cranked, ran for a few seconds, and shut down with a cycles over speed alarm, which means it ran over 60Hz. I cranked it back and it ran for a few seconds, maybe 10 to 15, and shut down with the same alarm. I cranked it again and got to watching the governor. It kept speeding the engine up but there was no load on the generator. I watched the tach and it wasn't dropping so I don't see why the governor was opening up. The only real reason I can come up with is a malfunction in whatever controls the governor. For instance, if it's a crank or cam sensor malfunctioning then the governor would read that as low RPM and try to adjust. Once the RPM goes high enough to raise the Hz above the set point then it shuts down. Am I on the right track? Anybody know what controls the governor on this engine? It's an early 80's engine.
Edited by thendrix - 15 Nov 2023 at 8:12pm
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"Farming is a business that makes a Las Vegas craps table look like a regular paycheck" Ronald Reagan
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Lars(wi)
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Posted: 15 Nov 2023 at 8:18pm |
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Is the 855 the mechanical precursor to the N14?
Edited by Lars(wi) - 15 Nov 2023 at 8:18pm
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I tried to follow the science, but it was not there. I then followed the money, and that’s where I found the science.
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im4racin
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Posted: 15 Nov 2023 at 8:51pm |
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The speed is set by the fuel pump. If there is a bubble of air in the fuel or fuel restriction the pump will over compensate and could set those codes. What is the gen set? Onan?
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steve(ill)
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Posted: 15 Nov 2023 at 9:05pm |
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Like them all, but love the "B"s.
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thendrix
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Posted: 15 Nov 2023 at 10:42pm |
 Lars(wi) wrote:
Is the 855 the mechanical precursor to the N14? |
From what I understand, yes it is
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"Farming is a business that makes a Las Vegas craps table look like a regular paycheck" Ronald Reagan
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thendrix
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Posted: 15 Nov 2023 at 10:43pm |
im4racin wrote:
The speed is set by the fuel pump. If there is a bubble of air in the fuel or fuel restriction the pump will over compensate and could set those codes. What is the gen set? Onan? |
Yes it's Onan. I've got a video. If I get caught up tomorrow I'll see if I can get some help posting it. Air is always a possibility but with as good as it cranks and runs I don't think it's the case on this one. But I've been wrong plenty of times before
Edited by thendrix - 15 Nov 2023 at 10:44pm
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"Farming is a business that makes a Las Vegas craps table look like a regular paycheck" Ronald Reagan
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Lars(wi)
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Posted: 15 Nov 2023 at 11:00pm |
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From your description, the engine tach is electronic?
Does this unit have a constant battery charger that keeps constant trickle charge?
I’m thinking possibly the battery(s) may be overcharged. Former employer had an 18v ‘jump box’ and some trucks if the jump box was hooked up too long, would run really funky for a few minutes until the battery charge settled back down below 14v or so.
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I tried to follow the science, but it was not there. I then followed the money, and that’s where I found the science.
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thendrix
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Posted: 16 Nov 2023 at 2:10am |
Lars(wi) wrote:
From your description, the engine tach is electronic?
Does this unit have a constant battery charger that keeps constant trickle charge?
I’m thinking possibly the battery(s) may be overcharged. Former employer had an 18v ‘jump box’ and some trucks if the jump box was hooked up too long, would run really funky for a few minutes until the battery charge settled back down below 14v or so. |
Yes the tach is electronic. We had to replace the controller after the original was hit by lightning. And yes I have a battery tender on both generators at the farm. I don't quite understand how an over charged battery would cause my issue though. I'll see about getting the video up in the morning and maybe it'll explain better than I can
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"Farming is a business that makes a Las Vegas craps table look like a regular paycheck" Ronald Reagan
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Codger
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Posted: 16 Nov 2023 at 5:26am |
I had several Cummins powered gensets I maintained over my employment duration. Most used either a Woodward hydraulic governor setup, or an AMBAC electronic setup. Both were quite accurate in generator settings. Sounds as if your engine is shutting down via the overspeed monitor which should be set at about 65hz or so.
If you'll post photos of the fuel pump and any linkage end points I can further build on your possibilities. Also if you could include the Cummins data tag from the engine, the Onan data plate, and the switchgear, (transfer switch) data plate if you have one attached. Got to know what you are working with to offer proper paths to proceed upon.
I assume you have a multimeter that will measure frequency?
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A career built on repairing and improving engineering design deficiencies, shortcomings, and failures over 50 years now.
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DMiller
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Posted: 16 Nov 2023 at 5:54am |
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It likely has the Set Speed governed PT Pump on it, any trace air will cause that to flutter so start checking lines, filter and head, tank standpipe if so equipped. Had three of those at the nuke we had to test monthly that ran off in ground tanks, had a constant feed/bleed fuel supply to a "Day Tank" at the engines that if had the least amount of air entrapment did exactly that.
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JoeM(GA)
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Posted: 16 Nov 2023 at 4:09pm |
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Allis Express North Georgia
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im4racin
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Posted: 16 Nov 2023 at 6:29pm |
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That uses an automotive pt pump and an external governor. Still I would check for air and fuel restriction
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thendrix
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Posted: 16 Nov 2023 at 8:43pm |
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Thank you Joe! I'll be finished selling chickens at about 5am then we'll be heading to Pigeon Forge TN until next Wednesday. I'll get a chance to dig into it deeper probably next Friday and I'll report back with what I find
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"Farming is a business that makes a Las Vegas craps table look like a regular paycheck" Ronald Reagan
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DMiller
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Posted: 17 Nov 2023 at 6:43am |
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That is called a Isochronous External Governor, the PT pump is only for a Standby engine basis where the housing is used on dozens of apps. Any slight volume of air into that system will drive it nuts and shut it down electronically.
Set up a number of those in the quarries as Independent Shore Power units rather than drape power lines over everything. If fuel storage is below the engine should be a feed and bleed pump to keep it primed and bled out.
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thendrix
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Posted: 25 Nov 2023 at 8:17am |
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Finally got back to the generator. Cranked it assuming if it was an air problem it would have some issues cranking. Absolutely no problems at all of any kind. Cranked ran for several minutes as opposed to several seconds. Smooth as it could be. Could have been maybe some condensation somewhere? Block heather running in cooler weather maybe causing it?
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"Farming is a business that makes a Las Vegas craps table look like a regular paycheck" Ronald Reagan
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SteveM C/IL
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Posted: 26 Nov 2023 at 4:51pm |
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Oh yeah! Fixed itself! You KNOW yer gonna revisit this......probably in crap weather!
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thendrix
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Posted: 26 Nov 2023 at 4:55pm |
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Yeah...I know. Hopefully it'll be on an exercise and not when we need it
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"Farming is a business that makes a Las Vegas craps table look like a regular paycheck" Ronald Reagan
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thendrix
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Posted: 04 Feb 2024 at 6:55am |
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The problem came back and we found it. The controller had a potentiometer added as a speed control because the controller was to new to control the old governor. Pulled the front off of the controller and there was a wire loose from the pot. We're guessing the solder broke from vibration. Looking for a better solution. Thanks everybody
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"Farming is a business that makes a Las Vegas craps table look like a regular paycheck" Ronald Reagan
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DMiller
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Posted: 04 Feb 2024 at 11:13am |
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At least found it. Good to at least know what found.
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thendrix
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Posted: 04 Feb 2024 at 3:59pm |
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Yeah that one has kept me up a little. We're coming to the stormy time of year and having a generator I can't depend on is pretty nerve racking
Edited by thendrix - 04 Feb 2024 at 3:59pm
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"Farming is a business that makes a Las Vegas craps table look like a regular paycheck" Ronald Reagan
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