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 2026-07-21
TL;DR
Fault code 31 sets when the transmission control unit (TCU) sees a voltage higher than 7.00V at the engine speed sensor input pin. This points to a wire shorted to battery voltage or an open circuit on the engine speed sensor signal line feeding the TCU.
High severity. The TCU relies on engine speed input to manage shift logic and torque converter control. Running with a bad engine speed signal can cause harsh, incorrect, or unpredictable shifting, so this should be diagnosed before the truck goes back into regular hauling duty.
What does JCB Dump Truck error code 31 mean?
Code 31 is set by the TCU when it measures a voltage higher than 7.00V at the engine speed input pin. The TCU expects a normal varying signal from the engine speed sensor as the engine turns. When the pin sees a voltage that high, the TCU reads it as an invalid signal rather than real engine speed data.
This is an input high or open type of fault. In this failure mode, the signal wire has either been shorted to a source of battery voltage, or the circuit has gone open so the pin floats and settles at an abnormal high reading. Either way, the TCU can no longer trust the engine speed data it is getting and will fall back on default or limited transmission control behavior.
Common causes of 31
- The signal cable from the TCU to the engine speed sensor is defective and has come into contact with battery voltage, effectively shorting the signal line high.
- The cable has lost its connection to the TCU entirely, creating an open circuit on the input.
- The engine speed sensor itself has an internal defect.
- A connector pin in the circuit is contacting battery voltage, or the pin itself is broken, again producing an abnormally high or open reading at the TCU.
How to troubleshoot JCB Dump Truck 31: first checks
- Inspect the cable routing between the TCU and the engine speed sensor for chafing, pinching, or contact with any wiring or terminal that carries battery voltage. Look closely at spots where the harness crosses near power feeds or ground straps.
- Check all connectors in this circuit, at the TCU, along the harness, and at the sensor itself, for corrosion, backed-out pins, bent terminals, or a pin that could be bridging to a voltage source.
- Verify the cable actually has continuity and connection all the way to the TCU. A broken or disconnected wire will set this code just as readily as a short to voltage.
- Test the engine speed sensor itself for an internal defect. If the wiring and connectors check out but the abnormal high voltage persists, the sensor is the next suspect.
How the code clears
No separate clearing step or reset procedure is listed for this code. After repairing the wiring, connector, or sensor issue, the TCU should stop reading the abnormal voltage and the fault should clear on its own once the circuit is restored to normal operation. Cycle the key or run the engine and recheck for the code after the repair.
Frequently asked questions
What does JCB fault code 31 mean on a dump truck?
It means the TCU measured a voltage higher than 7.00V at the engine speed sensor input pin, which the TCU interprets as either a short to battery voltage or an open circuit on that signal line.
Can I keep driving with fault code 31 active?
You can typically limp the truck, but transmission shifting decisions depend on accurate engine speed input. Expect abnormal shift behavior and get the wiring, connectors, and sensor checked before continuing regular work.
How is code 31 different from code 33?
Code 33 is a logical error at the engine speed input, meaning the signal is present but does not make sense against other data. Code 31 is a voltage-level fault, an actual short to battery voltage or open circuit, not a logic mismatch.
How does code 31 differ from code 34, 37, or 3A?
Those codes describe the same short-to-battery-or-open failure mode but on different sensor circuits: 34 is the turbine speed input, 37 is the internal speed input, and 3A is the output speed input. Code 31 is specific to the engine speed input pin.
What usually causes this fault?
JCB lists a defective cable shorted to battery voltage, a cable with no connection to the TCU, an internally defective speed sensor, or a connector pin either shorted to battery voltage or broken.
Do I need to replace the speed sensor to fix code 31?
Not necessarily. Start with the wiring and connectors between the TCU and the sensor, since a shorted or open cable or a bad connector pin is a common cause. Only test or replace the sensor itself if the wiring checks out.