Applicable Systems
PowerCommand
Symptoms
The PowerCommand controller displays fault code 639 indicating the SAE J1939 datalink cannot transmit. This is a Level 2 (Repair Engine Same Day) severity. Communication between the ECM and other controllers on the J1939 network is interrupted, which may disable remote monitoring and coordinated control.
Possible Causes
- SAE J1939 datalink wiring open or shorted
- Terminating resistor missing or failed (should be 120 ohms at each end)
- Faulty controller on the J1939 bus causing network collision
- ECM J1939 port internal failure
Troubleshooting Steps
- Step 1: Read fault code 639 on PowerCommand display
Code 639 displayed as active, Level 2 Repair Same Day - Step 2: Inspect J1939 datalink wiring for opens, shorts, or damage
Wiring continuous from ECM to all network nodes - Step 3: Measure terminating resistance across J1939+ and J1939- at diagnostic connector
60 ohms indicating both 120-ohm terminators present - Step 4: Disconnect controllers one by one to isolate faulted node
Fault clears when faulty controller is disconnected
How to Reset
To reset: 1) Repair the J1939 datalink wiring or replace faulty controller. 2) Clear fault with INSITE tool or key cycle. 3) Press RUN on PowerCommand.
Common Replacement Parts
- 3919430: Cummins J1939 Terminating Resistor 120 ohm (Install at each end of J1939 bus)
Frequently Asked Questions
- Q: Why is the terminating resistor important?
The J1939 CAN bus requires 120-ohm terminators at both physical ends to prevent signal reflection. Missing or failed resistors cause communication errors.
- Q: Can multiple fault codes appear with 639?
Yes. If the J1939 bus is down, all controllers on the network may report communication faults. Fixing the bus will typically clear all related codes.
Source: Cummins Diagnostic Fault Codes Manual, Bulletin 80-1044 (alliedsystems.com), Cummins Tier 4 Fault Codes, Bulletin 80-1235 (dieselecmtech.com)
Category: Generator