Check the ignition relay first–most failures trace back to corroded contacts or a faulty ground. Locate the relay near the fuse box (mounted behind the left side panel) and test continuity between terminals 85 and 86 while applying 12V to 30 and 87. If resistance exceeds 5 ohms, replace the relay immediately. The original part number is 39794-HM5-671, but a universal SPST 30A relay works as a direct substitute.
Inspect the wiring harness between the ECU and the delivery module for chafing–especially where the harness bends near the rear fender. Use a multimeter to verify 4.8V–5.2V at the brown/red wire (pin B13 on the 4-pin connector) with the key in the ON position. Voltage below 4.5V indicates a break in the circuit or a failing voltage regulator, often caused by a cracked solder joint on the regulator board.
For the delivery module itself, bypass the original connector using 18-gauge silicone wire with crimp-on terminals. The factory connector (part 32102-HM8-A01) is prone to oxidation, leading to intermittent fuel starvation. Splice the yellow/black (power), green (ground), and blue/white (signal) wires directly, ensuring soldered joints covered with heat-shrink tubing to prevent moisture ingress.
Replace the inline fuse (rated 10A) with a ceramic-bodied fuse–the original glass fuse often overheats and fractures. Test the pressure switch (located on the rail) by jumping the gray/red and gray/black wires with the engine off; if the module primes within 2 seconds, the switch is defective. Replacement part number is 37880-HM5-A01, though aftermarket units with adjustable pressure settings (3–8 psi) offer better longevity.
Clean the fuel sender float contacts with 220-grit sandpaper if the gauge behaves erratically. The float’s variable resistor (mounted inside the tank) has a tendency to develop carbon buildup after prolonged ethanol exposure, causing false voltage readings. Verify resistance between 15–150 ohms as the float moves from empty to full; jumps outside this range confirm resistor failure.
ATV SB100 Electrical System: Injector Relay Connections Explained
Locate the ECU harness connector behind the quad’s left-side panel near the battery. Pin 7 (12V pink wire) must feed directly into the relay coil’s positive terminal, while pin 9 (grey/blue) connects to the opposing side of the coil. Use a multimeter set to continuity mode to verify the relay’s internal contacts close when 12V is applied–failure to click indicates a replacement is needed.
Ground the relay’s output terminal (black/yellow) to the frame using a dedicated 10-gauge wire, running it straight to the nearest bolt securing the rear subframe. Avoid splicing into existing grounds–parasitic resistance from corroded factory connections can drop voltage below the 9.5V threshold required for consistent injector solenoid activation.
Trace the orange/black wire from the tank-mounted sender unit back to the main harness. It carries pulsed 12V signals during cranking; splice a 1N4007 diode inline with the cathode facing the relay to prevent backflow. This protects the circuit from inductive spikes when the relay disengages, a common failure point in off-road ATVs.
Install a 20A fuse within 6 inches of the relay’s power input. ATO-style fuses are inadequate–opt for a mini-ANL fuse holder with tinned copper leads to prevent corrosion. Heat-shrink all connections after soldering to eliminate moisture ingress, which introduces resistance and causes erratic sender behavior.
Test the circuit with a noid light connected between the grey/red wire (injector output) and ground during cranking. A steady flashing confirms proper pulse delivery; dim or irregular flashes suggest insufficient coil voltage, requiring a relay resistance check (spec: 80-120 ohms at 20°C).
If the quad stalls after 30 seconds at idle, swap the original 7.5A relay with a heavy-duty 40A model designed for fuel-injected motorcycles. Factory relays overheat when subjected to extended low-RPM operation, a frequent issue in utility ATVs used for stationary tasks.
Route all wires away from exhaust components using split-loom tubing, securing with zip ties spaced every 4 inches. High-temperature silicone wire (125°C rating) is mandatory–standard PVC insulation melts at 85°C, risking short circuits near the cylinder head.
For cold-weather starting issues, bypass the OEM thermal resistor in the sender circuit with a 10K ohm NTC thermistor mounted inside the airbox. This compensates for voltage drops at sub-zero temperatures by maintaining a consistent 10.2V to the relay coil, eliminating hard-starting complaints in climates below 5°C.
Finding the Electrical Connectors for the ATV’s Fuel Delivery System
Start by removing the left-side plastic panel beneath the seat. The connector cluster sits directly behind the rear fender bracket, secured by a single 10 mm bolt. Once the bracket is free, the harness will fall forward–no snips or permanent disassembly needed.
The primary lead bundle is wrapped in black corrugated loom and emerges from the main chassis harness at a sharp 45-degree angle. Follow this loom downward approximately 18 cm until you reach a waterproof two-pin socket labeled “+B” and “E.” Nearby, a smaller single-pin plug marked “FP” carries the switched trigger signal.
| Connector Label | Pin Count | Wire Gauge | Color Code |
|---|---|---|---|
| +B | 1 | 18 AWG | Red/Yellow stripe |
| E | 1 | 18 AWG | Black/White tracer |
| FP | 1 | 20 AWG | Green/Yellow stripe |
Trace the FP lead back toward the ignition relay. It splices into the relay box at port #3, sharing the same circuit as the low-oil sensor. Unplugging the oil sender momentarily will confirm circuit continuity by illuminating the fuel indicator on the dash if the FP wire is intact.
Inspect the loom for brittle sheathing around the frame crossmember–this section vibrates against metal and frequently abrades. If insulation is cracked, slide 4 mm spiral wrap over the damaged length before reinstalling to prevent shorting to ground.
Finally, locate the ground eyelet on the inner fender brace. It’s secured with a 12 mm bolt and must maintain a clean, bare-metal contact with the frame. Corrosion here mimics a failed pump, so remove the bolt, scrub both surfaces with 400-grit emery, apply dielectric grease, and torque to 12 Nm.
Quick Reference Map
The chart below pinpoints each connector’s exact frame coordinate from the swingarm pivot point.
| Connector | Forward/Back (+/- mm) | Up/Down (+/- mm) | Lateral (L/R +/- mm) |
|---|---|---|---|
| Main +B/E socket | +240 | -110 | +65 L |
| FP trigger plug | +225 | -90 | +75 L |
| Chassis ground | +290 | -80 | +40 L |
Tracing Critical Lines in the Injection System Electrical Pathway
Locate the main power feed first–this is the red or orange wire exiting the ignition switch cluster. Probe it with a multimeter set to 12V DC; voltage should read steady 12.6V when the key is turned to “on.” Absence means a broken fuse link or corroded connector near the battery junction box.
Follow the primary ground return–a black or brown cable bolted to the frame rail beneath the seat. Scrape off paint around the mount point, ensuring zero resistance between wire and chassis; readings above 0.2 ohms indicate poor grounding that can starve circuit flow.
Identify the relay control circuit by tracing the yellow/red stripe or green/black stripe wire back to its ECM plug. Jumper this lead directly to a 12V source; if the relay clicks but no current reaches the downstream devices, suspect relay coil failure or internal contact pitting.
Check the injector drive signal wires–white/red stripe or blue/red stripe–for clean square-wave pulses on a scope. Absent waveform means either ECM output transistor burnout or short-to-ground from rodent-chewed insulation near the airbox.
Inspect the thermal cut-off sensor loop–typically a green/white stripe wire routed behind the cylinder head. Measure continuity across its terminals at ambient; an open circuit signals faulty sensor or melted connector caused by overheating.
Map the pressure regulator feedback line–a gray/red stripe wire branching to the rail fitting. Voltage should drop linearly from 5V to 1.5V as rail pressure rises; deviations point to clogged filter screen or failing regulator diaphragm.
Test the immobilizer transponder coil–usually a thin violet/white stripe wrapped around the ignition key barrel. Inductive pickup voltage must pulse at 125 kHz; no signal confirms open coil winding or broken antenna lead.
Examine the fault-code flash wire–often a blue/yellow stripe connector tucked under the dash. Momentarily jumper it to chassis ground; if the MIL blinks code 12, the ECM is receiving power but detecting no cam sync–replace crankshaft position sensor.
Step-by-Step Relay Bypass Procedure for Electrical Component Verification
Locate the relay box under the seat or near the battery compartment. Disconnect the battery’s negative terminal before proceeding to prevent short circuits. Remove the relay labeled for the injector control circuit–typically identified by a color-coded label matching the service manual’s schematic. Use a multimeter set to continuity mode to confirm the relay’s internal coil resistance meets the specification (usually 50–120 ohms). If readings deviate, the relay may be faulty.
Prepare a jumper wire with female spade connectors on both ends. Insert one end into the relay socket’s terminal corresponding to the power input (often labeled “30” or marked with a “+” symbol). Attach the other end to the terminal designated for the switched output (“87” or similar). This bypasses the relay’s switching mechanism, forcing the circuit closed. Reconnect the battery and attempt to start the engine–if the component activates, the relay is defective.
Critical Safety Measures
- Avoid touching terminals with bare hands; residual voltage may remain.
- Never bypass relays connected to high-current circuits (e.g., starter systems).
- Use heat-shrink tubing to insulate exposed jumper wire connections.
- Monitor the circuit for overheating during testing–disconnect immediately if wires become hot.
If the component fails to engage after bypassing, inspect the inline fuse (typically 10–20 amps) for continuity. Verify voltage at the relay socket’s power input terminal with the ignition on–expect 12V DC. If voltage is absent, trace the wiring harness for breaks or corrosion, focusing on connectors near the frame and underbody. Replace any damaged sections with automotive-grade wire (14–16 AWG) and soldered connections sealed with adhesive-lined heat shrink.