U0103 - Lost communication with Gear Shift Control Module (GSM) 'A' (GSM)
System
Network & Communication (CAN-bus)
Technical Description
The control module has detected a fault in the Data Bus Communication (Gear Shift Control Module (GSM) 'A' (GSM)) (U0103). Node involved: Transmission Control Module (TCM) ’A’. Carried over the vehicle data bus. The "A" designation identifies one specific module or bus branch among several identical ones — verify the correct connector against the wiring diagram. Communication with the module has dropped out entirely — the controller no longer receives any frames from it on the data bus.
Multimeter Test Points & Nominal Values
| Test Point / Component | Measurement | Nominal Target Range | Test Condition |
|---|---|---|---|
| Check for "Transmission Control Module (TCM) ’A’" (U0103) (Action ()) | Mätning | Measure ignition supply and ground right at the module connector, then check CAN continuity from the module pins back to the gateway. | Vehicle data bus |
| CAN Termination (OBD pin 6 ↔ 14) | Resistance (Ω) | 60 Ω (two 120 Ω in parallel) | Battery disconnected, ignition OFF |
| CAN-H / CAN-L Bias Voltage | Voltage (V) | CAN-H ≈ 2.5 - 3.5 V, CAN-L ≈ 1.5 - 2.5 V (recessive ≈ 2.5 V both) | Ignition ON, bus active |
| Gear Shift Control Module (GSM) 'A' (GSM) – Power and Ground | Voltage (V) | Supply 11.8 - 14.4 V / ground drop < 0.1 V | At module connector under load |
Vehicle data bus
Battery disconnected, ignition OFF
Ignition ON, bus active
At module connector under load
Oscilloscope Waveform Analysis
10 µs/div, 1 V/divPossible Causes
The module has lost power or ground, or the bus stub feeding it is broken
Module location:Transmission Control Module (TCM) ’A’ — on or inside the transmission, or engine-bay adjacent — high heat and oil exposure. Check connector, power and ground right there.
Bus segment:look up topology, speed and termination in the wiring diagram.
Loose or corroded splice where the bus stub to the Transmission Control Module (TCM) ’A’ branches off the backbone.
An internal fault in the Transmission Control Module (TCM) ’A’ or its bus transceiver — confirm with a known-good module only after wiring and power are ruled out.
Symptoms
Recommended Solution
Measure ignition supply and ground right at the module connector, then check CAN continuity from the module pins back to the gateway.
Physically get to the Transmission Control Module (TCM) ’A’ (on or inside the transmission, or engine-bay adjacent — high heat and oil exposure), unplug the connector and inspect the pins for corrosion, spreading and moisture.
Measure ignition supply and ground at the Transmission Control Module (TCM) ’A’ under load (voltage drop < 0.1 V) and check the module dedicated fuse.
Measure bus resistance across CAN-H/CAN-L, ignition off: 60 Ω normal, ~120 Ω = a terminator/branch missing, <10 Ω = conductor-to-conductor short.
Read DTCs in EVERY module:a single dead node points at this module or its stub, while U-codes in several modules point at the backbone or gateway.
Confirm from the wiring diagram which physical module/branch is designated "A" — mixing it up with the sister module is a common error.
Mechanic's Tip
If several modules drop together the fault is on the backbone or gateway; if only this one drops, the fault is local to the module.