Forklift Engine Oil Water Separator Sensor YM129901-55820 – 4D94E/S4S

Forklift Engine Oil Water Separator Sensor YM129901-55820 — 4D94E/S4S, Kingstream, 0.35kg

This is the oil water separator sensor (part number YM129901-55820) for 4D94E and S4S diesel engines — the electronic sensor that detects when water has accumulated in the separator bowl and triggers the dashboard water-in-fuel warning light. The sensor is typically mounted at the bottom of the water separator housing and uses a conductivity probe — water conducts electricity (it has dissolved minerals and ions), while diesel fuel is an electrical insulator. When water accumulates to the level of the probe tips, the circuit closes and the warning light illuminates on the dashboard, alerting the operator to drain the water separator. A failed sensor — either stuck in the “on” position (constant warning light even when the separator is drained) or failed in the “off” position (never warns of water accumulation) — defeats the purpose of the water separator entirely. A sensor that never warns allows water to accumulate until it overflows into the fuel system, destroying the injection pump. A sensor that falsely warns constantly trains the operator to ignore the warning light — which means the one time there is actually water, it will also be ignored.

Kingstream brand, manufactured in China. New condition (brand-new 100%), 100% professional tested. 0.35 kg weight (0.5 kg gross with 20×15×15 cm package). Milky color, plastic and metal construction, standard size. For 4D94E and S4S diesel engine applications. When replacing the sensor, check for corrosion on the electrical connector pins — corroded pins create high resistance that can cause a false water warning. Apply dielectric grease to the connector after installation to prevent future corrosion. Test the new sensor by momentarily grounding the probe tips with a wet finger — the warning light should illuminate — then dry the tips and confirm the light extinguishes. Do not test by filling the separator with water — this risks water entering the fuel system.

Compatible with 4D94E and S4S diesel engines. Browse our water oil separator selection for FS1242 and XR708. Visit Kingstream official website.

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