Turn Signal Intermittent or Complete Failure

Symptoms

• Turn signals activate erratically, flashing several times before stopping — a pattern that worsens progressively over days or weeks [0].
• Signals flash only once or twice before going completely dead; emergency flashers exhibit the same behavior [2].
• A rapid 'tick-tick-tick' double-click sound precedes the failure, often serving as an early warning [7, 8].
• Signals go completely dead mid-drive, then spontaneously return after some time or after a sharp turn [8].
• Wiggling the relay or flexing the wiring harness under the dash restores turn signal operation temporarily [2, 4].
• With headlights on, both turn signal lights come on steady; with headlights off, both flash together as if hazards are active [9].
• Turn signals stop canceling automatically after a turn — a separate but related mechanical issue involving the cancel cam under the clockspring [13].
• Left turn signal flashes fast, possibly related to a burned-out or incorrectly seated bulb or a wiring splice issue at common splice S351 [3, 6].

Causes

• The turn signal relay in Fuse Block #1 (under the steering column) loses proper contact due to age, oxidation, or a loose fit in its socket [0, 4].
• A cracked or broken solder connection on the FB #1 circuit board — typically in the lower-right corner near the blinker relay socket — interrupts the flasher circuit [13].
• A chafed or loose wiring harness under the dash causes an intermittent open circuit that can be temporarily restored by wiggling the harness [2].
• A failed trailer light electronic module (at the end of the trailer wiring harness) can disrupt the blinker circuit [1].
• A burned-out or incorrectly seated bulb causes rapid (fast) flashing, as the Sprinter uses load-sensing flash rate as a bulb-out indicator [6].
• Corroded lamp sockets or cracked lens seals allow moisture in, causing back-feeding or ground issues that produce erratic behavior [11].
• The turn signal cancel cam (a plastic part under the clockspring) can break, preventing automatic signal cancellation after a turn [13].

Diagnosis

• Wiggle the turn signal relay at the bottom of Fuse Block #1 under the steering column while someone operates the turn signal lever — if signals restore, the relay or its socket contacts are the root cause [0, 4].
• Remove the panel under the dash and flex the wiring harness bundles; if the signals start or stop, a chafed wire or loose connector is the culprit [2].
• Pull each fuse in FB #1 individually and inspect closely — blown fuses may look intact until physically removed [11].
• Disconnect the trailer connector and/or pull the dedicated trailer module fuse; if signals immediately improve, the trailer light module is causing the fault [1, 2].
• Walk around the vehicle and check all turn signal bulbs for burnout or improper seating — a single bad bulb will cause fast flashing on that side [6].
• Remove rear turn signal lenses and inspect for cracks, moisture, and corroded contacts; ground issues from corroded sockets can produce erratic multi-bulb behavior [11].
• If signals fail to auto-cancel after turns (and a steering wheel has recently been removed for service), inspect the cancel cam under the clockspring for breakage [13].
• Remove and disassemble FB #1; inspect the solder joint in the lower-right corner of the board (near the blinker relay) under good lighting for cracks or cold joints [13].

Repair

The most common repair path is cleaning or replacing the turn signal relay and burnishing the relay socket clips inside FB #1. If relay service doesn't hold, the FB #1 board itself likely has a cracked solder joint that can be re-flowed, or the entire fuse block can be replaced as a unit. Both approaches are within DIY reach. Trailer module faults and bulb/socket issues are straightforward fixes. A broken turn signal cancel cam requires steering column disassembly and is more involved.

Parts

Plain part names — affiliate links and pricing are coming in a later update.

• Turn signal relay (fits FB #1 socket under steering column)
• Fuse Block #1 (complete replacement unit with new fuses and relays, ~$100–$125) [10, 13]
• Trailer light electronic module (if trailer-equipped and module is faulty) [1]
• Turn signal bulbs (as needed) [6]
• Rosin-core solder (if re-flowing FB #1 board joint) [13]

Related forum threads

• 19 ↳Turn signal intermittent Repaired
• 19 ↳Help debugging a failing left turn signal?
• 19 ↳Left turn indicators flash very fast
• 19 ↳Turn Signals fail to work intermitently

From the manuals

• Mercedes fault-code reference

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Sources