JCB Dump Truck 9E Fault Code: Open Circuit At Retarder Solenoid
Open Circuit At Retarder Solenoid · ai-assisted, editor-reviewed · Last updated 2026-07-13
TL;DR
Fault code 9E is set when the TCU (transmission control unit) detects a wrong voltage at the output pin controlling the retarder solenoid, a condition that reads like an open circuit. This points to a wiring, connector, or solenoid problem in the retarder control circuit, not a software fault.
Medium severity. The retarder is a braking-assist function, not the primary brake system, but an open circuit here means the retarder cannot be commanded on when needed. Diagnose it within the shift, especially if the truck is used on long grades where retarder braking is relied on to save the service brakes.
What does JCB Dump Truck error code 9E mean?
Fault code 9E means the transmission control unit (TCU) is monitoring the output pin that drives the retarder solenoid and is seeing a voltage reading consistent with an open circuit, meaning the circuit is not completing the way it should.
The retarder is a supplemental braking device, usually working with the transmission to slow the truck down without relying solely on the wheel brakes. If the TCU cannot energize the solenoid because of an open circuit, the retarder function will not engage properly.
This code is about the electrical integrity of the circuit between the TCU and the retarder solenoid. It does not necessarily mean the retarder itself has failed mechanically, though a bad solenoid coil is one of the listed causes.
Common causes of 9E
- Cable from the TCU to the retarder solenoid is defective or has no connection to the TCU.
- Retarder solenoid has an internal defect.
- Connector between the retarder solenoid and the TCU has no connection to the TCU.
How to troubleshoot JCB Dump Truck 9E: first checks
- Inspect the cable routing from the TCU to the retarder solenoid for chafing, cuts, or breaks that could cause an open circuit.
- Check all connectors in the retarder solenoid circuit and at the TCU for corrosion, pushed-back pins, or loose terminals that would prevent a solid connection.
- Check the resistance of the retarder solenoid to confirm the coil is intact and not open internally.
- With the connectors disconnected, visually confirm there is continuity end to end on the wiring harness feeding the solenoid.
How the code clears
No separate clearing step is listed for this code. Once the cable, connectors, and solenoid resistance have been checked and any faulty component repaired or replaced, the fault should stop being detected on the next key cycle or drive cycle. No reset procedure is listed for this code.
Frequently asked questions
What does fault code 9E mean on a JCB dump truck?
It means the transmission control unit has detected a voltage at the retarder solenoid output pin that looks like an open circuit, so the solenoid is not getting a proper electrical connection.
Will code 9E stop the truck from being driven?
The code affects the retarder, which is a braking assist function. The truck can typically still be driven and braked with the standard service brakes, but the retarder assist will not be available, so extra care is needed on long descents.
What usually causes an open circuit at the retarder solenoid?
JCB lists three possible causes: a defective cable with no connection to the TCU, an internally defective retarder solenoid, or a connector with no connection to the TCU.
How do I check if the retarder solenoid itself is bad?
Check the resistance of the retarder solenoid. If the coil resistance reading is not consistent with a healthy coil, the solenoid likely has an internal defect and needs replacement.
Is this an electrical problem or a transmission problem?
It is primarily an electrical circuit issue. The TCU is reporting an open circuit condition on the wiring or connector path to the solenoid, or a bad solenoid coil, rather than a hydraulic or mechanical transmission fault.
Do I need a scan tool to diagnose fault code 9E?
A scan tool is useful for confirming the code and reading TCU output pin status, but the actual repair work involves physically checking the cable, connectors, and solenoid resistance with basic electrical test tools.
Can corrosion cause this fault code?
Yes. Corroded or loose connectors between the TCU and the retarder solenoid can create the same wrong voltage reading as a broken cable, so checking connectors for corrosion is one of the first steps.