JCB Dump Truck 91 Fault Code: Short Circuit To Earth At The Reverse Warning Alarm Relay
Short Circuit To Earth At The Reverse Warning Alarm Relay · ai-assisted, editor-reviewed · Last updated 2026-07-13
TL;DR
Fault code 91 means the TCU (Transmission Control Unit) detected a wrong voltage at the output pin feeding the reverse warning alarm relay, consistent with a short circuit to vehicle earth. This is an electrical fault in the reverse alarm circuit, not the transmission itself, but it can leave the truck without a working reverse warning signal.
Medium severity. This code points to an electrical short in the reverse alarm circuit. It will not stop the truck from driving, but a dead reverse alarm is a real safety issue around people and other equipment, so it should be diagnosed the same shift it appears rather than left for later.
What does JCB Dump Truck error code 91 mean?
Fault code 91 is set when the TCU detects a wrong voltage at the output pin that drives the reverse warning alarm relay. The pattern of voltage the TCU sees is consistent with the circuit being shorted to vehicle earth (ground) rather than working normally.
The reverse warning alarm is there to warn people behind or around the truck when it is backing up. When this circuit shorts to earth, the TCU can no longer control the relay properly, which usually means the alarm stops sounding as it should, or behaves erratically.
This is purely a wiring and connector issue tied to the alarm circuit. It does not indicate a transmission problem on its own, but because it affects a safety warning device, it should not be ignored.
Common causes of 91
- The cable running from the TCU to the reverse alarm device is defective and is in contact with vehicle earth.
- The reverse alarm device itself has an internal defect that creates a path to earth.
- A connector pin in the circuit is connected to vehicle earth, either from corrosion, chafing, or a wiring fault.
How to troubleshoot JCB Dump Truck 91: first checks
- Inspect the cable from the TCU to the reverse alarm device for chafed insulation, pinched sections, or bare wire touching metal chassis or brackets.
- Check all connectors between the reverse alarm device and the TCU for corrosion, moisture intrusion, or pins pushed out of place that could be bridging to ground.
- Check the resistance of the reverse alarm device itself to see if the internal defect is inside the alarm unit rather than in the wiring.
- Trace the full harness run for any spot where the cable may be rubbing against the frame or other grounded metal, especially near mounting points and cable ties.
How the code clears
No reset procedure is listed for this code. Once the short to earth is found and repaired, whether in the cable, connector, or the alarm device itself, the code should clear on its own once the TCU sees normal voltage at the output pin again. No separate clearing step is listed.
Frequently asked questions
What does fault code 91 mean on a JCB dump truck?
It means the TCU detected a wrong voltage at the output pin for the reverse warning alarm relay, a pattern consistent with a short circuit to vehicle earth in that circuit.
Will fault code 91 stop the truck from driving or reversing?
No. This fault is limited to the reverse warning alarm circuit. The truck should still drive and reverse normally, but the audible reverse alarm may not work correctly.
Is it safe to keep operating with code 91 active?
You can generally keep working, but since the reverse alarm may not be sounding properly, extra caution is needed when backing up around people or other equipment until the short is fixed.
What usually causes this short to earth on the reverse alarm circuit?
JCB lists three possible causes: a defective cable contacting vehicle earth, an internal defect in the reverse alarm device, or a connector pin that has become connected to vehicle earth.
How do I start diagnosing fault code 91?
Start by checking the cable from the TCU to the reverse alarm device for damage, then inspect the connectors for corrosion or grounding, then check the resistance of the reverse alarm device itself.
Does fixing the wiring clear the fault code automatically?
No separate reset procedure is listed. Once the short to earth is repaired, the code should clear on its own as the TCU reads normal voltage again.