SIDEBAR: Fan Drives

Cold weather’s here, and one would think that the engine’s fan drive should be the least of the trucker’s worries. And it should be, for the fan-drive mechanism ought to cycle little in low temperatures, especially if the truck’s cruising down the road into the face of an icy blast of air. Yet the fan must come on if the engine gets hot, no matter what the weather.

As beneficial as this device is in terms of fuel economy, there’s no lack of things to go wrong with it. For example, let’s say the engine overheats while climbing a long mountain grade and the driver determines that the fan has not come on.

A number of things could’ve caused it: a faulty temperature switch, temp gauge, or coolant sensor can be telling the fan drive that everything’s cool when it’s not. A short, break, or bad connector in wires leading to those parts could be blocking signals. Air pressure can be too low to run the fan drive. Or the clutch facing is too worn to grab.

A little experience with fan drives used by the carrier or truck maker will usually lead the technician right to the problem. There are, of course, various makes and types of fan drives. Their electric control systems run solenoids with various functions. The systems can be “normally open,” where all switches are usually open and are wired in parallel, and “normally closed,” where switches are usually closed and wired in series.

If the fan drive is the air-actuated “on” type and its controls are “normally open,” the solenoid valve would be plumbed so it’s normally closed. When the control system is activated by pressure or temperature, the solenoid valve is energized, allowing air pressure to engage the fan.

Fan-drive maker Horton Inc. warns that the solenoid valve can fail at a relatively young age through its exposure to engine heat. So engineers recommend mounting the solenoid somewhere off the engine-on the firewall, for instance. You might consider changing the solenoid’s location if it’s now on the engine. This is a relatively easy job because of course a solenoid’s electrical and air lines can be routed to the remote mounting location.

Heat sensors perform various functions. Two of them-on the radiator’s outflow and on the engine block near the water pump-tell the fan drive when to switch on or speed up. A heat/pressure sensor on the A/C compressor will pick up high temperature or pressure in the A/C’s refrigerant. The A/C sensor, in fact, may switch on the fan much more than anything in the engine’s cooling system.

Heat sensors may also be placed in the air intake manifold and the transmission. In spite of the aftercooler, inlet air may become hot and the cooling system may have to compensate. And the transmission may begin running hot and its remote-mounted oil cooler may need more air flow. In any of these instances, the sensors will alert the control system, which will switch on the fan.

At least two vendors now make a combination sensor that eliminates the separate units, and it may be optional from truck builders.

One design, which incorporates a miniaturized circuit board inside its brass housing, also controls the temperature gauge and warning lights, and limits the fan drive’s on-off cycling in response to A/C refrigerant pressure.

Another product, from Index Sensors and Controls, electronically monitors the air-conditioning compressor and other parts of the A/C system. It warns the driver and maintenance people when something is wrong in the system, probably before anybody notices.

This allows mechanics to fix the fault before hot weather sets in. The sensor also works more precisely with the fan drive, preventing needless cycling and wear-and-tear.

Several truck builders have made Index’s “APADS” system standard on certain models, and fleets who use it testify that it saves big dollars in A/C and fan clutch maintenance. It’s something to consider if A/C-related problems are plaguing your equipment-or will be when warm weather returns.

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