How to test your FPM Sensor

Surging or Inconsistent Shot Rate

One of the most common causes of surging or inconsistent shot rates is bad or missing sensor signals. Shot rate is calculated by reading the Feet Per Minute (FPM) and Gallons Per Minute (GPM) sensors. Bad data from either sensor can cause problems.

Remember that a sensing system has 3 main parts.

• The signal generator (ring gear teeth)
• The sensor
• The output signal carrier (wire harness)

How to check FPM sensors?

FPM sensors can be tested by providing artificial input to the sensor pickup while having a helper monitor the output on the cab control. To do this you can conduct a “Wrench Test”.

1. Remove the sensor from its housing.
2. Without striking the sensor, rapidly wave a wrench or similar ferrous metal object as close as possible to the pickup.
3. Have a helper watch the corresponding output in the cab for variations in the reading.

Interpreting "Wrench" test results

If the test causes the read out in the cab to change your sensor is likely good, if not, the sensor or harness is bad and should be replaced.

Why does this work?

The FPM sensor works by reading a varying magnetic field created by its proximity to the ring gear in the differential. By moving a ferrous metal object past the pickup of the sensor you are mimicking the input from the gear teeth passing by the sensor pickup.

How to check the harness?

To quickly determine if the sensor or the harness is bad you can conduct a “Tap Test”.

1. Unplug the harness from the sensor.
2. On the harness side use a paperclip or something similar as a jumper wire, tap the white sensor wire and the black ground wire together (terminals "B" and "C" on the Amphenol plug)
3. While tapping have a helper in the cab monitor the output for variations.

Interpreting "Tap" test results

If the test effects the sensor readout in the cab it means that the harness back to the node, the nodes themselves and the communication bus between the nodes are working. If no output observed on the readout in the cab further diagnosis is required. To rule out the external harness you can try running the tap test on the control box end of the sensor harness.

Why does this work?

The brief contact made by “tapping” the wires together mimics the pulses created by a working FPM sensor. This tricks the computer into thinking that it is reading output from a good sensor.

For a full installation guide for FPM sensors click here.

If you need to order parts, click here.

If you need to look for a manual, click here.

Disclaimer: The preceding information is for educational purposes only. Repairing, maintaining, or modifying your equipment can be dangerous. It is your responsibility to ensure that you are aware of all the dangers involved, that you are working in a safe manner and that your repairs, maintenance, or modifications are performed in accordance with all applicable laws and regulations.