The Right Connections

| April 07, 2005

This cross-section of the Sonogrip plug from Tramec Corp. shows how the female connectors are designed, mounted, reinforced and connected to the wiring in the cable.

The vital services your engine provides for the trailer – braking and lighting – depend on air hose and electrical cable connectors that must be kept in tip-top shape.

A tractor’s diesel engine provides power for everything on a truck. That includes all electrical power for lighting and ABS from the alternator and all air pressure for the brakes from the air compressor and reservoirs via the other brake system parts. Since trailers and tractors are interchangeable, every tractor needs to have detachable hoses that supply trailer air brake pressure and a detachable electrical cable that supplies power to the trailer lighting and ABS.

The tractor-to-trailer air hoses conduct air pressure between the two parts of the combination. The hoses have attachment fittings that fit together, turn and lock so that rubber or polyurethane seals butt up against corresponding surfaces and seal in the air pressure. The two parts fit together prior to rotating and locking like two people shaking hands, an action sometimes known as “glad-handing.” Hence the moniker “gladhand.”

The parking or emergency brake hose provides parking brake release pressure to the outer brake diaphragm on each of the trailer brakes. The other air hose, the service brake hose, provides signal pressure from the treadle valve when the operator applies the service brakes. The parking brake hose also supplies pressure to reservoirs that ultimately supply the brake pressure to the trailer brakes.

The electrical cable, or coiled electrical cord, has a seven-pin connector to carry current from the tractor to the trailer for all lighting and the ABS. A standard seven-pin connector provides power for every circuit for both ABS and pre-ABS trailers. The ABS system is designed so that its trailer-mounted microprocessor, and the ABS electrical solenoids in the relay valves that cycle the brakes, are powered through the stop lamp circuit. This is why the same standard connector fits all trailers whether they have ABS or not.

Why the law cares about connectors
The parking brakes are called “spring brakes” because they are under constant spring tension. Those springs are located in the outer brake chambers and constantly oppose the force generated by the diaphragms in those chambers. This way, if there is an air system failure (and whenever the driver pulls out the parking brake button on the dash), the springs automatically apply the four trailer axle brakes used on a five-axle combination as hard as a hard brake application. (The same thing happens with the drive axle brakes.) When the driver depresses the parking brake button, air pressure fills the outer brake chambers, which then compress the springs and retract the brakes.

If the parking brake hose or gladhand develops a significant or partial leak, the result is often dragging brakes. The worst-case scenario is a drag so light that the driver doesn’t notice a loss of power and stop to check for a problem. Even such a light drag can cause severely overheated trailer brakes. This will leave the rig and driver vulnerable to a greatly increased stopping distance, or a runaway on a long downhill grade. The problem can be made worse by reduced pressure in the reservoirs that apply the brakes.

If the service brake hose develops a severe leak, there will be no trailer brakes.
Fortunately, the tractor protection valve will at least preserve pressure in the tractor brake system. A partial leak in the service brake hose would result in a vastly slowed trailer brake application, or even no application at all. Having tractor brakes with no trailer brakes is an ideal scenario for a jackknife. It could also easily cause very long stopping distances or a runaway.

An air leak in either hose or connection would remain undetected unless the driver had someone release the parking brakes and apply the service brakes during a pretrip inspection.
The hoses are constantly flexing as the rig rides along, especially in sharp turns. The gladhand seals are subject to wear every time the gladhands are connected or disconnected, and rubber can lose its elasticity over time. These vulnerable points are primary areas for examination during a roadside inspection.

This gladhand from Phillips shows how one should ideally look in terms of seal condition and the absence of corrosion.

The electrical connectors are subject to wear every time cables are connected or disconnected from the trailer. Like the brake hoses, these cables are under constant stress from bumps in the road and stretching in turns. Loose connectors can result in low voltage and dim lamps or flickering. Edward “Dee” Sell, vice president of engineering at Tramec Corp., says flickering makes short work of lamps because their filaments are repeatedly exposed to the rapid inrush of current that occurs every time electricity starts to flow. Worst of all, bad connectors can mean lights going out entirely, leaving the extreme vulnerability of an invisible trailer. A problem with the stop lamp connector pin can compromise voltage to the ABS system. While the service brakes would continue working, the anti-skid control of the ABS might be reduced or entirely lost.

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