Trailer Talk

Trace Down Circuits Before Replacing Liftgate Parts

April 14, 2014

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Lower green cord is the “stinger” that carries power and a return ground line to trailer batteries that operate the liftgate. Connector cord should remain plugged in to keep a load on the line while its circuit is tested. Photo: Con-Way Freight
Lower green cord is the “stinger” that carries power and a return ground line to trailer batteries that operate the liftgate. Connector cord should remain plugged in to keep a load on the line while its circuit is tested. Photo: Con-Way Freight

The driver’s come back from his daily peddle run a couple of hours early because of problems with the trailer’s liftgate. He says it’s operating very slowly, and at his last stop he barely got the platform off the ground and stowed.

Right away the trailer mechanic allows as how it’s probably the hydraulics or maybe the motor that spins the pump. He pokes around underneath and then tells the driver to pull the trailer into the third bay over there in the shop. But wait. Is the mechanic on the right track?

There’s a 70% chance that he’s wrong, and the problem is actually electrical. So says Bruce Purkey, founder and president of Purkey’s Fleet Electric and long-time activist at the Technology & Maintenance Council of the American Trucking Associations. He led a discussion on proper methods of testing for voltage drop on the circuits that power trailer lift gates at TMC’s recent annual meeting in Nashville, Tenn.

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Most liftgates these days are powered by a dedicated line from the tractor to the trailer, and thence down the trailer’s frame to the batteries. Also called “stingers,” these lines are usually two-pole, meaning there’s a hot wire and a ground wire, the latter leading back to the tractor. Single-pole circuits are also used, and they rely on the trailer’s frame and its physical connection to the tractor’s fifth wheel to supply a ground circuit. Too often it doesn’t, which is why the “full-circle” two-pole lines are favored. The batteries in turn connect to the motor that runs the hydraulic pump.

All through the stinger’s length there may be fuses and switches that can cause problems. Dirty or corroded connectors can reduce voltage along that line, which on the trailer alone is a long one – 40 to 50 feet or more, which in itself can lead to a voltage drop, especially if the cable is of insufficient gauge. It should be at least 4 gauge, Purkey and others say.

The output circuit – the cable from the trailer batteries to the motor – is very important because it functions like the cable leading to an engine’s cranking motor, Purkey says. For example, a bad magnetic switch that’s supposed to send maximum voltage to the motor can instead send lower voltage, causing the motor to turn slowly. So the pump puts out insufficient hydraulic pressure and the gate also moves slowly.

Left alone, the low voltage can damage the motor. If attended to promptly, such symptoms can mislead a mechanic into suspecting the motor, pump or cylinders, and can begin replacing these components. It’s a waste of time and money if the problem really is in that switch, or in corroded terminals at the batteries. Those are the first things the mechanic should look at when the trailer’s in the maintenance bay.

He should also check the voltage at the batteries to see if it’s at least 12 volts and preferably 14. If not, it’s time to check the health of the stinger line all the way to the trailer’s front, and then through the connector plugs to the tractor. If voltage is low way up there, then the problem could be in the tractor. The tractor’s stinger plug essentially connects to the tractor’s batteries, so the lines and fuses along that circuit should then be tested. For accuracy there needs to be a load on that circuit, which the stinger cable provides if the mechanic leaves it plugged in and connected to the trailer, Purkey says.

What can a driver do if he finds himself on a street with the lift gate’s platform on the pavement and the batteries run down? Some hydraulic systems might have enough pressure left to raise the platform, Purkey says. Or if the maintenance chief spec’d the trailer wisely, there’s a switch that sets the tractor’s batteries in parallel to those on the trailer, in effect jumping them so they’ll send juice to the ‘gate’s motor.

But yes, it’s happened that jumper cables from a service truck have revived those batteries and dead lift gates. And we can hope mechanics back at the terminal fixed the systems properly by looking first at the electricals.

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Author Bio

Tom Berg

Senior Editor

Journalist since 1965, truck writer and editor since 1978. CDL-qualified; conducts road tests on new heavy-, medium- and light-duty tractors and trucks. Specializes in vocational and hybrid vehicles.

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