J1939 Data Links Fault Tracing
Initial Setup & Safety
- Battery Check: Ensure the vehicle has a good batteries.
- Multimeter: Ensure your multimeter is set correctly and the internal fuse is good.
- Analyze DTCs: If you have communication loss with multiple ECUs, it’s likely a J1939 network issue. This guide covers FMI 2 and FMI 9 “Global” errors.
- Locations: Locate Connector PT (Right side/TECU) and Connector MA (Left side/EECU) behind the front grill.
SECTION 1: RESISTANCE TESTING
Condition: Ignition Key OFF. Multimeter set to Ohms (Ω).
Locate the J1939 wires (green and yellow twisted pair) and using your multimeter measure the resistance between the 2 wires at Connector MA or PT.
Expected Value ≈ 60 Ω
Do you get 60 Ω?
Remove Connector MA and measure toward the EECU (pins in the chassis harness going down).
Expected Value ≈ 120 Ω
Do you get 120 Ω?
Remove Connector EI (D13) / EM (D9, 12, 16) and measure toward the engine.
Expected Value ≈ 120 Ω
Do you get 120 Ω?
Disconnect EECU Connector EB and measure directly on the EECU pins.
Expected Value ≈ 120 Ω
Do you get 120 Ω?
Measure resistance between the 2 pins on the cab side of connector MA (Pins 3 & 4).
Expected Value ≠ (Not 120 Ω)
Do you get 120 Ω?
Disconnect PT and measure resistance between the 2 pins on the cab side.
Expected Value ≈ 120 Ω
Do you get 120 Ω?
Reconnect connector PT and measure again.
Expected Value ≈ The same as before disconnecting PT in the previous step ≠ (Not 120 Ω)
Did the 120 Ω reading disappear?
Disconnect Transmission ECU connector AA and return to PT to measure.
Expected Value ≈ 120 Ω
Do you get 120 Ω?
SECTION 2: VOLTAGE TESTING
Condition: Ignition Key ON. Multimeter set to DC Volts. Ensure 60 Ω resistance was confirmed first.
Measure Yellow (Pin 3) and Green (Pin 4) at Connector MA to ground.
Expected value : Yellow (H) ≈ 2.7V
Green (L) ≈ 2.5V
Are voltages correct?
Disconnect Connector MA and measure toward the Cab.
Expected value : Yellow (H) ≈ 2.7V
Green (L) ≈ 2.5V
Are Cab voltages correct?
Reconnect MA. Disconnect EI/EM. Measure at MA again.
Expected value : Yellow (H) ≈ 2.7V
Green (L) ≈ 2.5V
Are voltages now correct?
Reconnect interface. Disconnect EECU Connector EB. Measure at MA again.
Expected value : Yellow (H) ≈ 2.7V
Green (L) ≈ 2.5V
Are voltages now correct?
Reconnect MA. Disconnect Connector PT. Measure at MA again.
Expected value : Yellow (H) ≈ 2.7V
Green (L) ≈ 2.5V
Are voltages correct?
Reconnect PT. Disconnect TECU Connector AA. Measure at MA.
Expected value : Yellow (H) ≈ 2.7V
Green (L) ≈ 2.5V
Are voltages now correct?
Disconnect Cab ECUs one by one (Cluster, VECU, etc.) while measuring at MA.
Expected value : Yellow (H) ≈ 2.7V
Green (L) ≈ 2.5V
Did voltage return to normal after a specific ECU?
DIAGNOSTIC RESULT
The problem is the chassis harness between EI/EM and MA. Check the bend below the cab at the metal cradle.
DIAGNOSTIC RESULT
Problem is the engine harness between EECU (EB) and Interface (EI/EM).
DIAGNOSTIC RESULT
Fault lies within the MA connector (check for corrosion or loose terminal connections).
DIAGNOSTIC RESULT
Fault is between PT, BAB, CLF and MA. Check VECU connector PC pins 4 and 5. If 120 ohms is found there, check between CLF and MA. Check passenger side climate unit bracket and floor harness behind driver seat.
DIAGNOSTIC RESULT: INTERMITTENT CHASSIS FAULT
This indicates a high probability of a “near-break” or chafed wiring in the chassis harness bend (usually at the metal cradle below the cab). The physical movement of disconnecting PT likely shifted the wires enough to temporarily bridge or break the connection. Inspect that bend immediately for green crusties or broken copper.
DIAGNOSTIC RESULT
Voltages are currently OK. Shake harnesses while watching Tech Tool Fault Codes to identify intermittent shorts.
DIAGNOSTIC RESULT
Short circuit in the chassis harness. Inspect harness for damage or water ingress.
DIAGNOSTIC RESULT
The EECU is back-feeding incorrect voltage. Replace or repair EECU.
DIAGNOSTIC RESULT
Short circuit in the engine harness between EB and Interface EI/EM.
DIAGNOSTIC RESULT
The TECU is causing a voltage imbalance. Replace or repair TECU.
DIAGNOSTIC RESULT
The last disconnected ECU is faulty and pulling the network down.