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Kohler 12 Res Natural Gas Gen OS fault

Trekrider2001

Registered
This is my home generator set. It won't run now. It starts, kicks in like its starting and then stops. The display says OS, for Over Speed fault, but that has to be bogus as the unit barely started. Runs for about 1 second. Any ideas? My guess is that it is fuel starved, so I'm looking at the input. I have a dedicated regulator feeding fuel off a 2 lbs meter (the house has its own regulator). I put this in years ago when the gen was hard starting, hence, my current theory of fuel starvation. I measured the voltage at the gen fuel solenoid and see 12 volts. During cranking I see the voltage drop to about 9 volts but I have no min reading voltmeter so this is a close guess. My theory now is that the voltage drops too low for the solenoid and it cuts off the gas right when it needs it the most. The battery is now 8 years old but it tested OK when I took it into the shop, but I think a new battery will fix this. Anybody have a better idea?

Update: changed the battery, no joy... ideas please.
 

Joe Batts

Registered
Check your gas pressure feeding into the regulator on the gen. It should be at or damn near close to 7-11" water column (26"=1lb.). Check your vents on the gen monted regulator to see if they're clear. Check to make sure the bolts mounting down the carburetor aren't loose as the gen mounted regulator needs to develop vacuum off the outlet of it to start fuel flow (you can usually just grab the air cleaner and see if it moves). Check your carb linkage to make sure it's free and not binding up (try running the throttle by hand if necessary to be sure the throttle is operating properly). 8 vdc or higher for battery voltage during cranking is usually fine. Check your plugs to make sure the unit isn't running on 1 cylinder (make sure plugs are seated and gapped properly - .030). Start here and let us know.
 

Trekrider2001

Registered
Ran through the check list above:
1) Gas pressure: I don't have a gauge, but gen has its own feed from the house. The main regulator is a 2 lbs system and the gen has it own regulator. So I adjusted that 2 turns higher and then one turn lower. No change in gen operation. I is now returned to what it was.
2) Vents on gen regulator and the utility one are all clear
3) bolts on carb all tight
4) carb linkage moves be hand and during the starting process
5) I replaced the Battery. With new battery I get ~10 volts at the fuel solenoid
6) I replaced and gapped the plugs to .030
Additional info:
A) when you look down the throat of the carb you see a disk. This is normally closed. during cranking, this opens fully. When it shuts down, this closes again. Is this correct? Your comment says that it should develop a vacuum, but as the gas enters under this disk, opening it fully would give you the least vacuum. What is that disk? In a gasoline carb, I would call it a choke and it should be closed during starting. Do NG carbs work the opposite?
B) I noticed that the oil level was a bit high so I drained out about 4 oz and now it is just at the full mark. But no change in the generator operation.
C) the snubber diode across the fuel solenoid measures 0 ohms in the forward and 5 M-ohms in the reverse when it is removed from the circuit. I take that as a good diode.
 

Joe Batts

Registered
Yes, the butterfly goes full open when trying to start, the venturi in the carb helps develop the vacuum needed during cranking.

You could try checking for spark with an inline spark tester to make sure you have spark at both cylinders

Make sure you have gas flowing through the gas valve, the gas regulator on the gen has a removable plug on it, apply 12vdc to the gas valve & typically this would also be a great time to also hook up a gauge to read actual gas psi. Otherwise just check for flow.

Check your speed sensor on the opposite side of the engine coming out of the fan shroud will be a red & black wire, disconnect those wires & check AC voltage during cranking. Should be about 3.0 vac or higher.
 

Trekrider2001

Registered
Red an black wires measure 3-4 volts when cranking. I tried to remove the plug on the square box after the regulator (the one where you plumb for NG or LP). It is so tight, I fear I might break something if I force it. I don't have a gas pressure gauge anyway. Spark must be there because the engine "starts" and runs for about 1 second then shuts down with the OS fault code.
 

Joe Batts

Registered
OS or overspeed means the engine speed is too fast (more than 3600 rpm). Just looking at this complaint, I would be concerned with if the engine is somehow getting stuck mechanically to go too fast or is there some kind of control problem doing it. If you try to run the machine by hand, can you keep it running? If you put a voltmeter that reads frequency on it across L1 & L2, is this a legitimate overspeed? Does it actually shut down at around 70 hz like it's supposed to? If it runs ok by hand, I would begin checking the stepper motor, 38.5 ohms across 1a, 1b & across 2a, 2b. Check pin connections at the controller, stepper motor plug, etc. If these check ok, we might start looking at a controller.
 

Birken Vogt

Email NOT Working
Another possibility is that the overspeed sensing part of the unit has gone flaky and is killing it right away before it can come up to speed.
 

LWB250

Registered
I don't think you have a fuel problem, but just as a data point:

If you suspect a vacuum problem or a clogged vent/bad secondary regulator, remove the air cleaner and choke the mixer with your hand by covering it during cranking. This will increase the manifold vacuum substantially and help to open the regulator.

Dan
 

pegasuspinto

Registered
+1 on getting a speed reading for real. In any case, over speed is a shutdown fault and there won't be any delay. That is why it quits. Either it is overspeeding or the control thinks it is.
 

Trekrider2001

Registered
Thanks! I'll try the hand choke in the morning. As to the over speed going flaky, where it that measured? Seems I would have to take half the thing apart to even get at the sensor.

---------- Post added at 09:02 PM ---------- Previous post was at 09:00 PM ----------

And I really don't think the thing is actually going overspeed. It barely has enough time to even start, let alone go too fast.
 

pegasuspinto

Registered
It's probably measured either by the output frequency or the ignition system. I doubt there is a sensor, and even if there is, I would seriously doubt it's giving a false reading. Most likely, it's over speeding, next would be control malfunction, last would be the sensor.

If it is sensing the AC output directy, and it is for SURE not overspeeding, i'd check to see if anything is causing the AC output to be excessivly......dirty?
 

Isaac-1

Registered
Last Subscription Date
01/12/2014
If you do pull the pickup, make sure to clean the tip, as metal shavings can build up on th tip and cause problems.
 

Trekrider2001

Registered
There is a small inspection window under the oil cooler. I can see the magnet on the flywheel. It seems pretty strong as my screwdriver stuck right to it. But it looks "crappy". For something that is supposed to not hit anything or wear on anything, it sure looks beat up. Now the question is, where is the speed pick up? Are there instructions somewhere on how to change that? It looks like the pickup might be on the opposite side from the oil cooler (ie the right side when looking toward muffler). As suggested above, it looks like a pain to get everything out of the way. Best if I had full instructions.

Also, to clarify some of the questions above, there is no possibility to get any frequency reading as the unit does not run that long. Only a second or so. What does "run by hand mean"? When I try to run it, I use the manual run switch right on the unit, not the control on the transfer switch.
 

Trekrider2001

Registered
Ok.. I found the speed sensor, but there is no way to get it out unless you take the fan shroud off which seems impossible without taking the wall down which means you have to take the half completely apart. Really? Say it an't so!
 

Birken Vogt

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Yes you have to take the wall down, and it is a big hassle. But not that big, just label where the wires and hoses go.
 

Trekrider2001

Registered
How do I actually know that the speed sensor is actually bad? There are suggested tests (>1.7 volts while cranking) but that does not make sense from an engineering point of view. I assume that the speed sensor sends pulses to the control board and these are very accurately and quickly timed to adjust the step motor for the throttle. If the control board lost its mind, you could have the same condition... No? I don't have a scope, but that would be nice. Any ideas on how to ensure it is the pick up and not the control board?
 

Birken Vogt

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I like the good ole BK Precision dual trace scope, and I see them on ebay for less than 150 bucks all the time. I carry mine around in my truck and even though many tests could be accomplished without it, it is extremely easy to use and see what is going on directly. So I say just go buy one and play with it, and you will find other uses for it shortly.
 

pegasuspinto

Registered
If the sensor makes voltage it is good. It just can't fail many ways. It's just a magnet with wire wrapped around it. They either go open, or they get damaged by something hitting them(and go open). It's a tiny AC generator. The voltage check is just to make sure it's close enough to the flywheel. I suppose you could check the frequency, but if you could do that, maybe you would of checked the output frequency a long time ago ;)

In any case, the sensor may STILL not have anything to do with the overspeed sense. A lot of times they just drive the governor.
 
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