Generac QTO 4842 48kW generator: RPM sense loss error

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Joined: Sat Apr 27, 2019 1:24 pm

Sat Apr 27, 2019 1:33 pm


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...

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Location: Clawson, MI

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: ... 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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