Why Steve's winch BURNED

[WinchMan]

Thankfully, a passing motorist with a fire extinguisher and some quick thinking prevented a serious disaster.

The really important issues are, no injuries and minimal property damage. However, I've a problem with the failure analysis.

Reversed polarity of the quick-disconnect plugs and jacks is a contributing factor to the mishap, but the explanation of the short-circuit seems questionable, because--

Warn winch motor terminals are ISOLATED electrically from chassis ground; that is, neither winch motor terminal is permanently connected to "ground," or to the negative battery terminal.

This electrical isolation of the winch motor terminals can be tested; first, DISCONNECT both winch motor terminals; then, check continuity between chassis ground and each winch motor terminal with an ohm-meter; I think a proper system will show an open circuit. Or, with winch motor terminals disconnected, one may "jump" each winch motor terminal to battery positive voltage; no sparks, no motor rotation, nothing should happen.

The winch motor termninals must be isolated electrically from the chassis ground to permit the winch motor to reverse its direction of rotation when commanded "OUT" or "IN."

Thus, the explanation, "He [the previous owner] wired the winch polarity backwards and positive juice was flowing right into the ground/carriage of the winch," doesn't fit, entirely, as the cause of the short-circuit.

At rest, with the winch control switch in its neutral position, both winch motor terminals are shunted, connected together by the solenoid relays in the control box; the shunt provides "dynamic braking," applying an electrical load to a spinning winch motor when power is disconnected (the motor acts as an electrical generator when in this transitory state), reducing "coasting" of the moving system.

When connected properly, the winch motor terminals are shunted to the battery negative connection. If polarity of the power cables is reversed, both winch motor terminals would be connected to the positive battery connection when at rest. No problem, as long as the electrical isolation of the battery terminals from chassis ground does not break down.

The failure mode I suspect is: the quick-disconnect polarity was reversed, the winch motor terminal shunt connected both winch motor terminals to the positive battery terminals when at rest, and the electrical isolation of the winch motor terminals broke down, possibly as a result of mechanical shock from the dynamic G-loads the winch experienced when mounted to the moving vehicle.

When the electrical isolation of the winch motor terminals from chassis ground failed, a path for current from the winch motor terminals (both connected to battery positive voltage from the "backwards" quick-disconnect) to ground resulted in the short-circuit and the consequent fire.

The failure appears dynamic (breakdown of winch motor terminal electrical isolation), because otherwise, the short-circuit would have been constant; i.e., would have existed from the moment the winch was connected.

The, ". . . paint on both the carriage and H2 recepticle had kept things insulated well, but finally it contacted," hypothesis seems unlikely, because--numerous fasteners (e.g., bolts) have "naked" threads providing electrical continuity between the winch motor chassis and vehicle ground (the winch carriage is connected both physically and electrically to the vehicle frame through the receiver hitch and hitch pin, when mounted), these electrical paths would trump any paint-as-insulation, seems to me.

So, reversed polarity of the quick-disconnect plugs and jacks is a contributing factor to the malfunction; but, the immediate cause of the short-circuit appears to be a breakdown in winch motor terminal electrical isolation from chassis ground.

That's the way it looks to me; I welcome correction.