Generac QTO 4842 48kW generator: RPM sense loss error

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garyjones
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Sat Apr 27, 2019 1:33 pm

Hi:

I have a Generac QTO 48KW liquid cooled whole-house generator which is about 6 years old. It has performed beautifully under (thankfully) very light use conditions since installation. However, about two weeks ago, I noticed that my generator did not "exercise" at the correct time Thursday evenings. When I checked on the generator, the digital display was showing a fault code. That fault code indicated the following: "Stopped Alarm RPM sense Loss" In looking into this, the most common cause of this problem was an old or poorly operating battery so the battery was replaced with a brand new, 5 year warranty battery. That changed nothing. The generator attempts to start either on manual start button or to the weekly exercise program but shuts down within a few seconds, before starting. Reading through what I can find shows me that the electronic ECM seems to be missing a pulse from the ignition which shows that the motor is turning over, and if there are no pulses within 3 seconds, the ECM shuts down the engine start process. That appears to be exactly what was happening. However, now have no idea what I should check next. Any suggestions or solutions would be greatly appreciated. What is the ECM looking toward to see the rotation pulses showing RPM?

Thanks in Advance...

Gary
Chris
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Sat Apr 27, 2019 5:24 pm

There is what we call a Mag Pickup located on the top of the adapter plate that joins the engine and the alternator assembly. It sends a signal to the controller for RPM during cranking and running to adjust speed and tell the controller what RPM the engine is running at. If this is either dirty or has failed, the unit will throw the RPM Sense Loss alarm.

The pickup can be removed and checked for metal debris stuck to the magnet and if you find some on there, that’s most likely the issue. If you need to replace it, the part can be found here: https://shop.gentekpower.com/products/g ... uid-cooled

Use some caution when reinstalling the mag pickup into the adapter. If the jam nut has moved at all and allows the sensor to be installed further into the housing it will contact the flywheel and ruin the sensor and possibly the flywheel teeth.

The easiest method to use is to rotate the motor by hand until a peak of a tooth is perfectly lined up with the mag pickup hole and then turn the pickup down until it just touches the tooth, then back the sensor out 1.5 turns.
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garyjones
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Tue May 28, 2019 10:36 am

Chris

Thanks for the response and the information. I have not tried to tackle this yet because I got no notice that anyone had responded (Dumb :bigsmile: ). Will follow your advice and see what I see today. Thanks for the response. I will follow up.

Gary
Chris
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Tue May 28, 2019 11:09 am

Sounds good! You can turn on post notifications and also e-mail notifications in your User Control Panel :cool:
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garyjones
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Tue May 28, 2019 11:50 am

I'll do that. Here is a picture that may help...
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RPM sense module.jpg
RPM sense module.jpg (86.35 KiB) Viewed 498 times
Chris
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Tue May 28, 2019 12:00 pm

That shiny stud is just the output for battery charging on the alternator... Was there nothing connected there?

The mag pickup is on the adapter plate connecting the rotor/stator and the engine block.
Generac tech for over 12 years. I'm here to help!
"The only source of knowledge is experience" -Albert Einstein
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garyjones
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Tue May 28, 2019 12:06 pm

OK, I figured that was not the sensor. Does the alternator have to be removed to get to the pick up? I sure dont see where it is located. I will go back out and do some more looking. Thanks for your help.

Gary
Chris
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Tue May 28, 2019 12:41 pm

You'll want to look at the opposite end of the engine where the large alternator (the part that makes power) is connected. You'll see it sticking out of the black iron adapter with a small black wire and gray connector, just like shown in the picture at the link I posted above.
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garyjones
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Tue May 28, 2019 1:59 pm

Confusion in terms. When I hear alternator, then I am looking at the alternator which generates 12 Volts DC. The output end is the "generator" to me. Now I understand. and yes, I see the part there. I am giving thought to how I remove it and get it back installed to the right depth. I think I am going to use spray paint to mark the threads down to where it is screwed in, and then see if that will either let me count the threads below or the paint mark to get it back right. On this 48 KW generator, it looks like it is going to be VERY hard to get my head anywhere in position to see "teeth" down in the hole. Maybe an inspection camera will let me see down the hole.

Thanks for your help Chris.

Gary
Chris
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Tue May 28, 2019 2:44 pm

A BRIGHT flashlight will help. Best way is still to find top of a tooth and line it up center, screw down pickup and then back out 1.5 turns.

PLEASE disconnect the battery while working on the unit. One mishap and you'll be hurt badly!
Generac tech for over 12 years. I'm here to help!
"The only source of knowledge is experience" -Albert Einstein
Looking for Generac parts? Gentek Power offers the full line of Generac replacement parts! Shop Gentek Power Generac Parts
A list of my favorite Generator & Electrical Tools! https://www.amazon.com/shop/gentek_power
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