JCB Excavator 651 Fault Code: Machine Tracks Off Line During Straight Travel
Machine Tracks Off Line During Straight Travel · ai-assisted, editor-reviewed · Last updated 2026-07-13
TL;DR
Code 651 means the excavator pulls to one side instead of tracking straight. It is not an electronic sensor fault with a stored voltage or numeric threshold, it is a mechanical/hydraulic performance issue that JCB diagnoses through a series of pressure, flow, and speed checks on the travel circuit.
Medium severity. Tracking off line will not stop the machine from working, but it points to a hydraulic imbalance that can wear track motors, hoses, or the main pump unevenly if left unaddressed. Diagnose within the shift, especially if the deviation is severe or worsening.
What does JCB Excavator error code 651 mean?
Fault code 651 is logged when an excavator does not travel in a straight line under straight travel command, instead drifting or curving to one side. This is a mechanical and hydraulic symptom rather than a single sensor failure, so there is no stored voltage, temperature, or timer value tied to this code.
The undercarriage on a tracked excavator relies on two independent hydraulic track motors, each fed by its own side of the main pump and controlled through negative control signals. If one side receives less flow, less pressure, or has a different relief valve setting than the other, the two tracks turn at different speeds and the machine pulls toward the slower side.
JCB's guidance for this code is a structured elimination process: check the control signals first, then pressures, then relief valve settings, then drain line flow, then motor speeds, working from the electrical/hydraulic command side down to the mechanical output side of each track motor.
Common causes of 651
- Unequal negative control signals reaching the left and right track motor control valves
- Difference in main pump pressure between the two circuits feeding each track motor
- Track motor relief valve settings out of balance side to side
- Uneven track motor drain line flow rates, indicating internal motor wear or leakage on one side
- Mismatched track motor speeds between the two sides under the same command input
- A faulty or incorrectly routed priority valve, if fitted, feeding the low flow or weedcutter circuit and robbing flow unevenly from one track circuit
How to troubleshoot JCB Excavator 651: first checks
- Run the machine in a straight line over a measured 20 meter distance and record how far it deviates, along with the ground conditions at the time, since soft or uneven ground can mimic a hydraulic fault
- Test the negative control signals to both track motor control valves to confirm they match side to side
- Check main pump pressures on both circuits feeding the track motors and compare readings
- Check each track motor relief valve setting and compare against its opposite side
- Measure track motor drain line flow rates on both sides to look for excess internal leakage on one motor
- Compare track motor speeds side to side under identical travel commands
- If the machine has a priority valve in the pump line feeding a low flow or weedcutter circuit, connect the two main service hoses at that valve together and see if the deviation changes
- Swap the two main hoses on the hydraulic pump and check whether the direction of the pull changes, which helps isolate whether the fault is in the pump, the hoses/valve, or the track motors themselves
How the code clears
No reset procedure is listed for this code. This is a mechanical/hydraulic diagnostic fault, not a sensor code that clears once a threshold is no longer exceeded. Resolution comes from correcting the pressure, flow, or relief valve imbalance found during testing, after which the tracking issue itself goes away since there is no separate stored fault to clear.
Frequently asked questions
What does JCB fault code 651 mean?
It means the excavator's tracks are not staying in a straight line during straight travel, it drifts or pulls to one side. It is a hydraulic/mechanical performance fault in the travel circuit, not a simple sensor failure.
Is fault code 651 dangerous to keep working with?
It will not shut the machine down, but an unbalanced track circuit can accelerate wear on one track motor, hose, or the main pump. It is best diagnosed within the same shift rather than ignored long term.
How do I measure how bad the tracking deviation is?
JCB's own test method is to run the machine straight over a measured 20 meter distance, record how far off line it travels, and note the ground conditions, since soft or uneven ground can produce a similar drift without any hydraulic fault present.
What parts should be checked first for code 651?
Start with the negative control signals to each track motor control valve, then check main pump pressures, track motor relief valve settings, drain line flow rates, and finally track motor speeds, comparing left side against right side at each step.
My machine has a priority valve for a weedcutter or low flow circuit, does that matter?
Yes. If fitted, try connecting the two main service hoses at that valve together and see if the deviation changes. This helps determine whether the priority valve itself is contributing to the uneven flow.
How do I tell if the problem is in the pump or in the track motors?
Swap the two main hoses on the hydraulic pump. If the direction the machine pulls also swaps, the fault is likely upstream in the pump or hose routing. If the pull stays on the same physical side of the machine, the fault is more likely in that side's track motor.
Is there a reset procedure for fault code 651?
No reset procedure is listed for this code. It clears on its own once the underlying pressure, flow, or valve imbalance causing the tracking drift is corrected.