JCB Dump Truck D6 Fault Code: Error At Valve Power Supply VPS2
Error At Valve Power Supply VPS2 · ai-assisted, editor-reviewed · Last updated 2026-07-13
TL;DR
D6 sets when the transmission control unit (TCU) commands the VPS2 valve power supply on or off and the measured state does not match. In plain terms, the TCU tells VPS2 to switch and it doesn't, or VPS2 stays on when it should be off. This points to a wiring, connector, fuse, power supply, or TCU problem in the transmission control circuit.
High severity. D6 affects the transmission control unit's ability to reliably control a valve power supply circuit. This can lead to unpredictable transmission behavior, so the truck should be diagnosed before it goes back into normal duty cycles.
What does JCB Dump Truck error code D6 mean?
D6 is an electrical mismatch fault on the VPS2 (Valve Power Supply 2) circuit controlled by the TCU. The TCU sends a command to switch VPS2 on or off, then checks the actual measured state of that circuit. When commanded on but measured off, or commanded off but measured still on, the TCU logs D6.
VPS2 supplies power to a transmission solenoid valve or valve group. If the TCU cannot trust that this circuit is doing what it commands, it cannot reliably control the transmission function tied to that valve, which is why this fault gets treated seriously even though it is an electrical signal-mismatch rather than a direct mechanical failure.
The root cause is almost always in the wiring between the gearbox and the TCU, the connectors along that path, the fuse feeding the circuit, the permanent power supply (KL30), or, less commonly, an internal fault in the TCU itself.
Common causes of D6
- Cable or connector between the gearbox and TCU is defective and shorted to battery voltage.
- Cable or connector between the gearbox and TCU is defective and shorted to vehicle earth (ground).
- Permanent power supply KL30 is missing at the TCU.
- TCU has an internal defect.
How to troubleshoot JCB Dump Truck D6: first checks
- Check the fuse feeding the VPS2 circuit for continuity and correct rating.
- Inspect cables running from the gearbox to the TCU for chafing, pinching, corrosion, or damage that could cause a short to battery or to ground.
- Inspect connectors from the gearbox to the TCU for corrosion, loose pins, or moisture intrusion.
- Verify permanent power supply KL30 is present and reaching the TCU as expected.
- If all wiring, connectors, fuse, and KL30 supply check out good, consider the TCU itself as an internal defect and plan for replacement.
How the code clears
No separate clearing step is listed for this code. Once the fuse, cables, connectors, and KL30 permanent power supply are confirmed good, or the TCU is replaced if it has an internal defect, the fault should stop being logged on the next duty cycle. If it returns immediately after repair, recheck the same circuit rather than assuming a new fault.
Frequently asked questions
What does VPS2 mean on a JCB dump truck?
VPS2 stands for Valve Power Supply 2, an electrical circuit controlled by the transmission control unit (TCU) that powers a transmission valve. Code D6 means the TCU's command to this circuit and what it actually measures on the circuit don't match.
Is D6 safe to keep driving with?
D6 points to a mismatch in transmission valve power supply control, which can affect transmission behavior. It's best treated as a stop-soon issue rather than something to ignore, since unreliable valve control in the transmission can create unpredictable shifting or operating behavior.
What usually causes D6 on these trucks?
JCB lists four causes: a cable or connector shorted to battery voltage, a cable or connector shorted to vehicle earth, a missing permanent power supply KL30 at the TCU, or an internal defect in the TCU itself.
Where should I start diagnosing D6?
Start with the fuse for the VPS2 circuit, then inspect the cables and connectors running between the gearbox and the TCU for damage, corrosion, or shorts. Also confirm the permanent power supply KL30 is present at the TCU before considering the TCU itself defective.
Does replacing the TCU always fix D6?
No. JCB lists TCU replacement as the last step, only after the fuse, cables, connectors, and KL30 power supply have all been checked and confirmed good. Most D6 faults trace back to wiring, connectors, or a fuse rather than the TCU itself.
Will D6 clear itself after I fix the wiring?
No separate clearing step is listed for this code. Once the underlying wiring, connector, fuse, or power supply issue is repaired, the TCU should stop measuring a mismatch and the fault should not reset on the next cycle.