JCB Dump Truck D5 Fault Code: Error At Valve Power Supply VPS1
Error At Valve Power Supply VPS1 · ai-assisted, editor-reviewed · Last updated 2026-07-13
TL;DR
D5 sets when the transmission control unit (TCU) switches VPS1 on but measures it off, or switches it off and measures it still on. This points to a valve power supply circuit fault between the TCU and gearbox, usually wiring, connectors, or a missing permanent power feed.
High severity. D5 affects the transmission's valve power supply circuit. A mismatch between commanded and measured power state can leave gearbox valve control unreliable, so the truck should be checked before continued heavy use.
What does JCB Dump Truck error code D5 mean?
D5 is a valve power supply (VPS1) fault reported by the transmission control unit. The TCU commands VPS1 on or off and then checks whether the actual measured state matches. If the TCU switches VPS1 on but reads it as off, or switches it off but still reads voltage present, it logs D5.
VPS1 supplies power to a transmission valve circuit. If the TCU cannot trust that this supply is behaving as commanded, it cannot reliably control the associated valve, which can affect shifting or clutch/valve engagement inside the gearbox.
This is fundamentally a wiring and power supply integrity check. The fault does not describe a specific valve function failing, only that the power feed to it is not matching what the TCU expects.
Common causes of D5
- Cable or connectors in the VPS1 circuit are defective and shorted to battery voltage.
- Cable or connectors in the VPS1 circuit are defective and shorted to vehicle earth (ground).
- Permanent power supply KL30 to the TCU is missing.
- Internal defect inside the TCU itself.
How to troubleshoot JCB Dump Truck D5: first checks
- Check the relevant fuse for the VPS1/TCU circuit before doing anything else.
- Inspect cables running from the gearbox to the TCU for chafing, pinching, or damaged insulation that could cause a short to battery or ground.
- Inspect connectors from the gearbox to the TCU for corrosion, loose pins, or moisture intrusion.
- Confirm permanent power (KL30) is present and reaching the TCU as expected.
- If wiring, connectors, fuse, and KL30 power all check out, consider the TCU itself as a possible internal fault.
How the code clears
No separate clearing step or reset procedure is listed for this code. Once the wiring, connectors, fuse, and KL30 power supply are confirmed good and any faults repaired, the code should clear after the TCU re-evaluates VPS1 status on a subsequent key cycle. If the fault persists after all external checks pass, TCU replacement is the listed action.
Frequently asked questions
What does JCB fault code D5 mean?
It means the transmission control unit detected a mismatch between the commanded state and the measured state of VPS1, the valve power supply circuit. The TCU either turned VPS1 on and read it as off, or turned it off and still read voltage present.
Is D5 safe to keep driving with?
It is not an immediate stop-the-engine emergency, but because it affects a transmission valve power circuit, gearbox control may be unreliable. It should be diagnosed before continued heavy operation or towing loads.
What usually causes a VPS1 fault on a JCB dump truck?
JCB lists defective cables or connectors shorted to battery voltage, cables or connectors shorted to earth, a missing permanent power supply (KL30) to the TCU, or an internal TCU defect.
Where should I start checking for D5?
Start with the fuse for the circuit, then inspect the cables and connectors running between the gearbox and the TCU. Confirm KL30 permanent power is present at the TCU before assuming the TCU itself is bad.
Will fixing the wiring clear code D5 on its own?
No separate reset procedure is listed. Repairing the wiring, connector, fuse, or power supply issue should allow the TCU to correctly measure VPS1 again, but if the code persists after all of that checks out fine, the TCU may need to be replaced.
Can a bad fuse alone cause code D5?
Yes, checking the fuse is the first listed action, since a blown or intermittent fuse can prevent VPS1 from measuring correctly even if wiring is otherwise fine.