How to test your GPM sensor

Identifying Your Sensor

Before you can check the sensor, you'll need to determine what kind of sensor you have.
There are common sensor types used for GPM sensing on Bearcat equipment.

Gear-tooth Sensor

PN: ELC-26148

The gear-tooth style sensor threads into a bracket and reads a toothed ring mounted to the product pump shaft. It is also the same sensor used to read Feet Per Minute (FPM) from the differential housing on Bearcat 501 and 502 Distributer Trucks.

Speed Ring Sensors

PN: ELC-53286

The speed-ring style sensor threads into the side of the hydraulic motor that is driving the product pump input shaft.
It came in 3 flavors:
• 3-pin with black wire
• 3-pin with blue wire
• 4-pin

3-pin to 4-pin Adapter

3 pin sensors are no longer sold and have been replaced by the 4-pin sensor. There is a plug and play adapter harness that makes the 4 pin backwards compatible with both of the 3 pin styles. You'll need either:

• PN: 220725 (has a 3-pin female end)
• PN: 220740 (has a 3-pin male end)

It all depends on what you need to match on your truck's harness.

Testing Gear Tooth Sensors

The sensor 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 bracket.
2. Without striking the sensor, rapidly wave a wrench or similar ferrous metal object as close as possible to the pickup.
3. Have the helper watch the output in the cab for variations in the GPM while you are moving the “wrench”.

Interpreting “Wrench” Test Results

If the GPM reading in the cab changes your sensor and harness are working and the problem may be somewhere else. If no reading is detected, you'll need to check the Harness.

Why does this work?

The Gear tooth sensor works by reading a varying magnetic field created by the passing gear teeth. By moving a ferrous metal object past the pickup of the sensor you are mimicking the input from a passing gear tooth.

Testing Speed Ring Sensors

This type of sensor can be tested like the Gear-tooth Sensor. However, instead of using a ferrous metal object like a wrench you will need to use a magnet of some kind.

Interpreting “Magnet” Test Results

If the GPM reading in the cab changes, your sensor and harness are working, and the problem may be somewhere else. If no reading is detected, you'll need to check the Harness.

Why does this work?

The Speed Ring style sensor works by reading a varying magnetic field created by the passage of magnets located within the speed ring past the sensor. The speed ring is typically press-fit to the shaft inside the hydraulic motor.

How to check the harness?

To quickly determine if the harness from the sensor plug to the cab control is good you can perform a “Tap Test”.
1. Unplug the harness from the sensor.
2. Using a jumper wire, tap the ground and sensor wires together.

Gear-tooth sensor harness

For Gear-tooth Style sensors you would hold the wire on the “C” pin and tap the pin marked “B”

Speed-ring sensor harness

For 3-pin black wire and 4-pin sensors. Hold the jumper on the black wire and tap the white.

For 3-pin blue wire sensors, they used a blue wire instead of a black. The test is run the same way, by holding the jumper on the blue ground wire and tapping the white sense wire.

3. While tapping, have a helper in the cab monitor the output for the GPM sensor.

Interpreting “Tap” tests results

If the reading in the cab changes it means that the harness back to the node, the nodes themselves and the communication bus between the nodes are working. If no change is observed in the cab further diagnosis is required.

Why does this work?

The brief contact made by “tapping” the wires together mimics the pulses created by a working GPM sensor tricking the computer into thinking that it is reading output form a good sensor.

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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.