WD-40 Specialist Electrical Contact Cleaner Spray - Electronic & Electrical Equipment Cleaner. 11 oz. (Pack of 1) - 300554-E
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The sense is from the electric brake circuit. If you put one of those 7-way LED light circuit checkers in the truck connector, it will flash every few seconds even when the brakes aren't applied. Once it senses the resistance of the magnet or hydraulic controller, it knows it has a trailer plugged in. This doesn't apply to non-brake 4-way type trailers.
ONLINE LED STORE 7-Way Blade Trailer Wiring Tester [Nickel-Plated Copper Terminals] [Continuity Test] [Rugged Material] 7-Pin Trailer Light Wire Circuit Tester
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