JCB Dump Truck D6 Fault Code: Error At Valve Power Supply VPS2
Error At Valve Power Supply VPS2 · ai-assisted, editor-reviewed · Last updated 2026-07-21
TL;DR
D6 sets when the transmission control unit (TCU) commands Valve Power Supply 2 (VPS2) on but reads it as off, or commands it off but still reads it as on. It points to a wiring or supply fault between the TCU and the gearbox, or an internal TCU problem.
High severity. VPS2 feeds power to transmission valve solenoids. A mismatch between commanded and actual state can cause wrong gear behavior or a transmission that won't shift correctly, so treat this as a stop-soon issue rather than something to ignore for the rest of the shift.
What does JCB Dump Truck error code D6 mean?
D6 is logged by the TCU when there is a disagreement between what it tells VPS2 to do and what it actually measures at that circuit. If the TCU switches VPS2 on but voltage isn't present, or switches it off but voltage is still there, the control unit flags D6 because it cannot trust that the valve supply is behaving as commanded.
VPS2 supplies power to solenoid valves in the transmission that control shift behavior. If this supply circuit is unreliable, the gearbox may not shift when and how it should, so this fault matters even if the truck seems to be running fine at the moment it appears.
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 to the TCU is missing.
- Internal defect in the TCU itself.
How to troubleshoot JCB Dump Truck D6: first checks
- Check the fuse feeding the VPS2 / TCU circuit before doing anything else.
- Inspect 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.
- Inspect the connectors from the gearbox to the TCU for corrosion, loose pins, or moisture intrusion.
- Confirm permanent power (KL30) is actually present at the TCU with the key in the appropriate position.
- If wiring, connectors, and KL30 supply all check out, consider the TCU itself as the likely fault and plan for replacement.
How the code clears
No separate clearing step or reset procedure is listed for this code. Once the wiring, connector, fuse, or power supply issue is corrected (or the TCU is replaced if it is internally defective), the fault should stop being active; verify with a rescan after repair.
Frequently asked questions
What does VPS2 stand for on a JCB dump truck?
VPS2 stands for Valve Power Supply 2, a power feed from the TCU to solenoid valves in the transmission. D6 is specifically about this circuit; a related circuit, VPS1, is covered by a separate code.
Is D6 the same as D5?
No. D5 covers Valve Power Supply 1 (VPS1) while D6 covers Valve Power Supply 2 (VPS2). They are separate physical circuits feeding different valve supplies, so a fault on one does not necessarily mean a fault on the other. If you have both codes, check both circuits independently.
Can a bad fuse cause D6?
Yes. Checking the fuse for the VPS2 circuit is the first recommended check. A blown or intermittent fuse can cause the TCU to see the supply as off even when it commanded it on.
Will D6 stop my truck from shifting gears?
It can. VPS2 powers solenoid valves that control transmission shifting, so an unreliable supply can lead to incorrect or missing shifts. Treat D6 as a reason to get the truck diagnosed soon rather than keep working it hard.
Do I need to replace the TCU to fix D6?
Not necessarily. JCB lists cable and connector faults (shorts to battery voltage or to ground) and a missing KL30 permanent power supply as more common causes. TCU replacement is listed as the action only after fuse, wiring, connectors, and power supply have been checked and ruled out.
What is KL30 and why does it matter for D6?
KL30 refers to the permanent (always-on) power supply feeding the TCU. If this permanent supply is missing, the TCU cannot reliably control or read VPS2, which can trigger D6.