The year, make, model and engine info would help.
For an NPR the rpm sensor is located left rear of engine by the frame rail. Its actually the engine speed per service manual but when you need to replace the sensor the parts department calls it the rpm sensor. Good luck.
crank sensor
There is a grey 4 pin plug on top of the motor, close to the firewall, it controls the RPM, Speedometer, and Temperature gauges. Plug may become loose, causing no RPM needle movement.
The rpm is acquired on almost all computer controlled vehicles from the crank sensor. drl
Here's a link that may help. The RPM sensor is also known as a crank position sensor. http://www.volvoclub.org.uk/faq/EngineSensors.html#Crank_Postition_Sensor
A 2003 Mercury 115 that runs rough at a low rpm might be having a problem with the MAP Sensor or the Oxygen Sensor. The MAP Sensor is going to be the easiest one to replace because it is usually located plugged into the EFI (Electronic Fuel Injector), a fancy would for carburetor.
it is located at the side of the rotary injection pump
The RPM sensor and speed sensor serve different purposes, though they can sometimes be related. The RPM sensor measures the rotational speed of an engine or component, typically expressed in revolutions per minute (RPM). In contrast, a speed sensor measures the vehicle's speed, often in miles per hour or kilometers per hour. While both sensors provide vital information for vehicle performance, they are distinct in their specific functions.
Possibly a MAP sensor issue. the MAP sensor limits RPM's to 2k. If that sounds like the problem, look at the vacuum hoses to the sensor. MAP sensor is located on the left strut tower looking at the engine.
On a Cummins engine, the RPM sensor, often referred to as the crankshaft position sensor (CPS) or speed sensor, is typically located near the flywheel or at the rear of the engine block. Its exact position can vary depending on the specific engine model and configuration. For the 59 Cummins, it is generally found on the passenger side of the engine, near the transmission. Always refer to the specific service manual for your engine model for precise location details.
if it drives normal except when you let off the gas the rpm jumps then it is a map sensor
for the most part rpm is counting from alternator (generator) there should be two wires one for rpm second for waking alternator