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Onboard scale systems allow optimum, safe use of a truck’s payload capacity both for maximizing loads and preventing the liabilities of overloading.
ApexEgroup LLC
Direct Line:1-330-476-2422
Celebrating 35 years of Service in the Scale Industry
International Sales: ApexEgroupLLC@gmail.com

Onboard Load Cell Testing


Troubleshooting Most Vulcan load cells. 15", 17", 19", 26", Heavy duty cells may have higher ohm readings.
Excitation voltage is 5 to 10 Volts
Things to Check for:
Moisture ingress, Short circuit , Broken wire/component, Excessive heat, Electrical transients, Mechanical overload,Excessive corrosion, Broken cable.
Load Cell Pin Outs
B Plus SIGNAL Green
C Negative SIGNAL White
D Negative EXCITATION Black
Red to Black 349-400 Ohms
White to Green 349-352 Ohms
green to red must be the same as green to black within 1 Ohm
white to red must be the same as white to black within 1 Ohm
Note: For Center Hanger load cells, red to black 349-450 Ohms
Connector Cleaning
1. Clean the load cell connector and cable connector with: spray electronic contact cleaner or cotton swabs and isopropyl alcohol.
2. Dry thoroughly with a hair dryer if needed. DO NOT OVERHEAT.
3. Apply Dielectric grease inside the connector. Use Electrical coating on outside of connector: Liquid Electrical Tape (Cost $7.00 to $8.00)
Dielectric grease is electrically insulating and does not break down when high voltage is applied. It is often applied to electrical connectors, particularly those containing rubber gaskets, as a means of lubricating and sealing rubber portions of the connector without arcing.
Test Procedures and Analysis
The diagram below represents a proposed sequence for testing load cells after a particular system malfunction. Isolate the fault location by moving a relatively small deadweight over each load cell, or by disconnecting load cell by load cell.
Test #1: Sudden change in Zero point
Zero Balance The Zero Balance is defined as the load cell output in a “no-load” situation. Therefore, all weight (including dead load) has to be removed from the load cell. Low capacity load cells should be measured in the position in which the load cell is designed to measure force to prevent the weight of the element giving wrong results. The load cell should be connected to a stable power supply, preferably a load cell indicator with an excitation voltage of at least 10 volts. Disconnect any other load cell for multiple load cell systems. Measure the voltage across the load cell's output leads with a millivolt meter and divide this value by the input or excitation voltage to obtain the Zero Balance in mV/V. Compare the Zero Balance to the original load cell calibration certificate (if available) or to the data sheet.
Analysis Changes in Zero Balance usually occur if the load cell has been permanently deformed by overloading and/or excessive shocks. Load cells that experience progressive zero output changes per time period are most likely undergoing a change in the strain gage resistance because of chemical or moisture intrusion. However, in this case the insulation resistance and/or the bridge integrity will also be compromised.
Test #2: Unstable readings, random change in Zero point
Insulation Resistance The insulation resistance is measured between the load cell circuit and element or cable shield. Disconnect the load cell from the junction box or indicator and connect all input, output and sense (if applicable) leads together. Measure the insulation resistance with a megohm meter between these four or six connected leads and the load cell body. Repeat the measurement between the same 4 or 6 leads and the cable shield. Finally measure the insulation resistance between the load cell body and cable shield. Never use a megohm meter to measure the input or output resistance, as it normally operates at a voltage which exceeds the maximum excitation voltage by far!
Analysis The insulation resistance of all load cells should be 5000 MΩ or more for bridge circuit to housing, bridge circuit to cable screen and housing to cable screen. A lower value indicates electrical leakage, which is usually caused by moisture or chemical contaminations within the load cell or cable. Extremely low values (≤1 kΩ ) indicate a short circuit rather than moisture ingress. Electrical leakage results usually in unstable load cell or scale reading output. The stability might vary with temperature.
Test #3: Scale reads overload, incorrect or not at all
Bridge Integrity The bridge integrity is verified by measuring the input and output resistance as well as the bridge balance. Disconnect the load cell from the junction box or measuring device. The input and output resistance is measured with an ohmmeter across each pair of input and output leads. Compare the input and output resistance to the original calibration certificate (if available) or to the data sheet specifications. The bridge balance is obtained by comparing the resistance from –output to –input, and –output to +input. The difference between both values should be smaller than, or equal to 1 Ω.
Analysis Changes in bridge resistance or bridge balance are most often caused by a broken or burned wire, an electrical component failure or internal short circuit. This might result from over-voltage (lightning or welding), physical damage from shock, vibration or fatigue, excessive temperature, or from production inconsistencies.
Test #4: Erratic readings when load is applied or removed
Shock Resistance The load cell should be connected to a stable power supply, preferably a load cell indicator with an excitation voltage of at least 10 volts. Disconnect all other load cells for multiple load cell systems. With a voltmeter connected to the output leads, lightly rap on the load cell with a small mallet to mildly shock it. Exercise extreme care not to overload low capacity load cells while testing their shock resistance. Watch the readings during the test. The readings should not become erratic, should remain reasonably stable and return to original zero readings.
Analysis Erratic readings may indicate a failed electrical connection or a damaged glue layer between strain gage and element as a result of an electrical transient.

General Load Cell Testing

1. Power off mode: Using an digital ohm / volt   meter, measure the following resistance's on the load cell cable.
A. + Excitation and - Excitation & Input Should be slightly higher than output
B. + Signal and  - Signal Output
C.+ Excitation and + Signal
D.+ Excitation and - Signal
E.- Excitation and + Signal
F.- Excitation and - Signal
The input and output resistance should match the manufacturer's specs. The other resistance readings should all be the same and approximately 75% of the input or output resistance's.
2. Using an ohm meter, measure the following resistance's.
A. All leads to case.
B. All lead to shield.
The resistance should be greater than 5,000 ohms. If  it is less, there is a chance for signal loss or voltage loss due to leakage to ground, etc.
3. Power on mode: Using an digital ohm / volt meter, measure the following voltage outputs on the load cell cable Signal Leads. Note: Max output will depend on Excitation voltage and mV per Volt of load cell. This test 15 volts Excitation and 3 mV per Volt. Sometimes you can only get and accurate reading by disconnecting the signal lead and taking your measurement.
A. Measure Excitation voltage. Make sure it meets factory specs for indicator and load cell.
B. Unloaded:  Depends on dead load of scale. Should be close to 1 mV to 5mV, but can be more.
C. With load:  at 3 or 4 test loads. Max load (full scale for 3mV per Volt load cells with 15 V excitation voltage) 45mV. If over you have a problem.
• (full scale for 2mV per Volt load cells with 15 V excitation voltage) 30mV. If over you have a problem.
• (full scale for 2mV per Volt load cells with 10 V excitation voltage) 20mV. If over you have a problem.
• (full scale for 2mV per Volt load cells with 5 V excitation voltage) 10mV. If over you have a problem.

For meter isolation test, you will have to use a load cell simulator.

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Celebrating 35 years of Service in the Scale Industry.
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