JCB Dump Truck D5 Fault Code: Error At Valve Power Supply VPS1
Error At Valve Power Supply VPS1 · ai-assisted, editor-reviewed · Last updated 2026-07-21
TL;DR
D5 sets when the transmission control unit (TCU) switches Valve Power Supply 1 (VPS1) on or off and the voltage it measures back on that circuit does not match what it commanded. In plain terms, the TCU tried to control a transmission valve supply and got the wrong feedback, either the supply stayed off when it should be on, or stayed on when it should be off.
High severity. This affects the transmission valve power supply. A mismatched VPS1 state can mean loss of proper valve control in the gearbox, which can cause unexpected shift behavior or loss of drive function. Diagnose before continuing to operate under load.
What does JCB Dump Truck error code D5 mean?
VPS1 is a switched power supply that the TCU controls to operate a valve inside the transmission. The TCU turns this supply on or off as part of normal shift logic, then checks the actual voltage state on that circuit to confirm the command worked.
D5 sets when there is a mismatch: the TCU switched VPS1 on but measured it as off, or switched it off but measured it as still on. This tells the TCU that the electrical circuit is not behaving as commanded, so it cannot trust or continue using that valve supply reliably.
Because VPS1 feeds a valve that affects transmission operation, this fault can lead to incorrect shifting or a valve staying in the wrong state until the fault is cleared and the circuit is repaired.
Common causes of D5
- Cable or connector in the VPS1 circuit is defective and shorted to battery voltage, which can make the TCU read the supply as on when it commanded it off.
- Cable or connector in the VPS1 circuit is defective and shorted to vehicle earth (ground), which can make the TCU read the supply as off when it commanded it on.
- Permanent power supply KL30 is missing, so the TCU has no constant power reference to properly switch or verify VPS1.
- TCU has an internal defect that prevents it from correctly switching or reading back the VPS1 state.
How to troubleshoot JCB Dump Truck D5: first checks
- Check the fuse feeding the VPS1 circuit before doing any wiring work.
- Inspect and test the cables running from the gearbox to the TCU for chafing, corrosion, or damage that could cause a short to battery voltage or to ground.
- Check the connectors from the gearbox to the TCU for looseness, corrosion, or backed-out pins, since connector faults can cause either a short-to-power or a short-to-ground condition.
- Confirm that permanent power (KL30) is present and reaching the TCU as expected.
- If all wiring, connectors, fuse, and KL30 power check out, consider the TCU itself, as it may have an internal defect requiring replacement.
How the code clears
No separate clearing step or reset procedure is listed for this code beyond correcting the wiring, connector, fuse, or KL30 power issue, or replacing the TCU if it is internally defective. Once the underlying electrical fault is repaired, the code should stop being detected as VPS1 feedback will again match the commanded state.
Frequently asked questions
What does JCB fault code D5 mean?
It means the TCU commanded Valve Power Supply 1 (VPS1) on or off and the actual measured state of that circuit did not match the command, indicating an electrical fault in the VPS1 circuit or the TCU itself.
Is D5 safe to keep driving with?
It is not recommended. VPS1 feeds a transmission valve, and a mismatched state can affect shift control. Diagnose the fault before continuing normal operation, especially under load.
What usually causes D5 on a JCB dump truck?
JCB lists four possible causes: a cable or connector shorted to battery voltage, a cable or connector shorted to vehicle earth, a missing permanent power supply (KL30) to the TCU, or an internal TCU defect.
How do I start diagnosing D5?
Start with the fuse for the VPS1 circuit, then inspect the cables and connectors running from the gearbox to the TCU for shorts to power or ground. Also confirm KL30 permanent power is present at the TCU. If everything checks out electrically, the TCU itself may need replacement.
How is D5 different from D6?
D5 covers Valve Power Supply 1 (VPS1), while D6 covers Valve Power Supply 2 (VPS2). They are separate circuits feeding different valves, so a D5 fault should be diagnosed on the VPS1 wiring and connectors specifically, not VPS2, even though the causes and checks follow the same pattern.
Does D5 require replacing the TCU?
Not necessarily. JCB lists an internal TCU defect as only one of four possible causes. Check the fuse, cabling, connectors, and KL30 power supply first, since these are more common and less costly fixes than TCU replacement.