JCB Dump Truck 31 Fault Code: Short Circuit Battery Voltage Or Open Circuit At Engine Speed Input
Short Circuit Battery Voltage Or Open Circuit At Engine Speed Input · ai-assisted, editor-reviewed · Last updated 2026-07-13
TL;DR
Fault code 31 means the TCU (transmission control unit) has measured a voltage higher than 7.00V at the engine speed input pin, which points to a short to battery voltage or an open circuit on the speed sensor wiring. This is an electrical fault affecting the engine speed signal used by the transmission control system.
Medium severity. This is a signal-integrity fault rather than an active mechanical failure, but the transmission control system depends on an accurate engine speed input to shift correctly. Diagnose it before extended operation, since an incorrect or missing speed signal can cause erratic shifting or protective transmission behavior.
What does JCB Dump Truck error code 31 mean?
Fault code 31 is set when the TCU measures a voltage higher than 7.00V at the speed input pin that reads engine speed. Under normal operation this pin should show a lower signal voltage as the speed sensor pulses; when the TCU instead sees a voltage near battery level, it interprets that as either a short circuit to battery voltage or an open circuit on that input line.
This code affects the transmission control system's ability to know actual engine speed, which is used to manage shift points and clutch/torque converter control logic. It does not directly point to an engine problem, but to the wiring, connectors, or sensor feeding that signal into the TCU.
Because the fault is electrical in nature, it usually comes down to a damaged cable, a bad connector, or a failed sensor rather than a transmission mechanical issue.
Common causes of 31
- Cable from the speed sensor to the TCU is defective and has come into contact with battery voltage, causing a short.
- Cable has no connection to the TCU, creating an open circuit at the speed input pin.
- Speed sensor itself has an internal defect.
- Connector pin is contacted to battery voltage or is physically broken.
How to troubleshoot JCB Dump Truck 31: first checks
- Inspect the cable routing between the TCU and the speed sensor for chafing, pinching, or damaged insulation that could allow contact with a battery voltage source.
- Check all connectors in that circuit for corrosion, bent or broken pins, and secure, fully-seated connections.
- Test the speed sensor itself for an internal defect using appropriate sensor test procedures.
- Verify continuity of the cable from the TCU to the sensor to rule out an open circuit condition.
- Confirm connector pins are not shorted to battery voltage and are not physically broken.
How the code clears
No separate clearing step is listed for this code. JCB's listed actions are to check the cable from the TCU to the sensor, check the connectors, and check the speed sensor; once the fault is repaired, the code should clear once the TCU no longer measures the out-of-range voltage on the speed input pin.
Frequently asked questions
What does fault code 31 mean on a JCB dump truck?
It means the TCU measured a voltage higher than 7.00V at the engine speed input pin, which indicates a short circuit to battery voltage or an open circuit in that speed sensor wiring.
Is fault code 31 a serious problem?
It is an electrical signal fault rather than an immediate mechanical failure, but the transmission control system relies on this speed signal, so it should be diagnosed promptly to avoid shifting problems.
What usually causes fault code 31?
JCB lists four possible causes: a defective cable shorted to battery voltage, a cable with no connection to the TCU, an internal defect in the speed sensor, or a connector pin that is shorted to battery voltage or broken.
How do I start diagnosing fault code 31?
Start by checking the cable from the TCU to the speed sensor, inspecting all connectors in that circuit, and testing the speed sensor itself, since these are the three checks JCB specifies.
Will fixing the wiring clear fault code 31 automatically?
No separate clearing step is listed. Once the short or open circuit is repaired, the voltage at the speed input pin should return to normal and the fault should stop being active.
Can a bad connector alone cause fault code 31?
Yes. A connector pin that is contacted to battery voltage, or one that is broken, is specifically listed as a possible cause.