Quick clarification first: the
EW-SW310 isn't your motor — it's the wiring junction/sub-harness in the STEPS system. So what you're describing is a
power/comms fault somewhere in that loop, not a motor failure as such. That's actually good news, because harness and switch faults are usually cheaper to chase than a dead drive unit.
Here's what your symptom is telling us. Back-pedalling spins the motor's generator enough to briefly wake the system, but the moment you stop, it dies — which means
the bike isn't getting a stable power feed from the battery through to the controls. Classic culprits, roughly in order of likelihood:
1. Dead or flat switch/display, or its feed
On STEPS, if the switch unit has no power, the whole bike won't boot.
@philippe1 has flagged the back-pedal jump-start trick before — spinning the cranks backwards firmly can force a start — but if it instantly dies, the switch isn't holding the system awake. Worth checking the switch is actually powering up at all.
2. Battery-to-harness connection
A poor seat between the battery and the main harness is the most common cause of exactly this "no power / intermittent" behaviour.
@Stihldog found a bad battery-to-harness connection threw intermittent power loss and error codes;
@digitale traced a similar mid-ride shutoff on a Spectral:ON to a power cable that needed reseating onto the motor. Pull the battery, check the contacts for corrosion or a bent pin, reseat it firmly.
3. The SW310 sub-harness / connectors themselves
Canyon had a known batch of
bad EP801 wiring harnesses that Shimano quietly replaced (per
@digitale). If a wire or connector in your EW-SW310 run is broken or water-ingressed, you get precisely this: momentary life, then nothing. Inspect every connector along that harness for water, green crud, or a loose latch.
What I'd do, in order:
• Disconnect the battery for a few minutes, then reseat it — also acts as a factory reset (per
@RustyMTB).
• Inspect and reseat every connector: battery, switch, and the SW310 junctions.
• Charge the battery fully — rule out a genuinely flat pack hiding behind this.
• If it persists, get it on Shimano's
E-Tube diagnostic — that'll read which part of the loop has dropped out, and flags if you're owed a harness replacement under that known fault.
If you can tell me the host bike (frame/model) and whether the display/switch lights up at all when you try a normal start, I can narrow it down further.
