While many truckers will keep a close eye on their engine coolants, adding supplemental coolant additives with the precision of chemists, the condition of fan clutches is often overlooked until problems emerge.

But a working fan clutch can control cooling system temperatures accurately, and will ensure that your fan is only engaged when it’s needed – activating the fan with a temperature-controlled thermostat that triggers a solenoid to apply or release compressed air.

The end result is lower fuel consumption, faster response at upper and lower temperature thresholds, and lower exhaust emissions with reduced white smoke and less internal engine deposits.

Various types of thermatic fans are incorporated into today’s cooling systems, depending upon the classification of your truck. On heavy-duty rigs with high-horsepower engines, spring-engaged and compressed-air-disengaged models are popular choices.

Fans are only necessary five to 10 per cent of the time, based upon the speed and load characteristics of your particular engine. But when they do run, there is an associated cost. A typical 32-inch multi-blade fan on a high-horsepower diesel truck can draw 25 to 30 hp. (The fan’s diameter, speed, number of blades and pitch all affect the actual power draw.)

All spring-engaged fans – even when they are not running – have a belt drive that’s spinning the drive hub, even though the fan blades will not turn. This creates a small parasitic power loss, typically in the range of 0.43 to 0.5 hp, although some of the newer models, such as Horton’s DriveMaster model, only draw 0.07 hp in this non-operating mode.

Consider how the thermatic fan works:

Assume that an engine thermostat is set to start to open at 82 degrees Celsius (180 Fahrenheit). It may not be fully open until 93C (200F) since the action is modulated until that point. The cooling sensor’s shutter control may be set to start opening at the same temperature as the engine stat, but could invariably go through its modulation procedure in about a 5 C temperature range. In this example, it wouldn’t be fully open until you’re at about 85C (185 F).

A thermatic fan should never come on or engage before the shutter thermostats are open since the fan cannot draw any air through the radiator core until then.

When the engine’s coolant reaches the fan-on coolant temperature, compressed air inside the fan hub is exhausted to let the internal fan clutch spring apply pressure on a friction disc that, when engaged, drives the fan blades.

Horton’s fan clutch systems are equipped with a patented System Sentry, which acts as an early warning fuse to alert the operator of potentially damaging effects to the fan clutch system. When the system’s fuse blows, it indicates an overload condition. Don’t simply replace the fuse on a continual basis, since something must be causing this problem.

One of the main reasons that a fuse blows can be traced to overheating due to a slipping fan clutch hub.

Check for:

1. Fan clutches that have been installed and used beyond their load capacity. Manufacturers set the spec’s for engine cooling fans to meet the engine’s cooling requirements.

2. Incorrect air presssure to the fan clutch. Air supplies of less than 90 psi (621 KPa) can cause the fan clutch to slip as it attempts to disengage the clutch. This can be traced to leaks in the air line or air pressure that is sourced (drawn off) for other air operated accessories. One good example would be using the compressed air on a cement mixer to pressurize the on-board water supply tank, or when pumping off the load from a tanker. Keep in mind that heavy-duty fan clutches require between 90-120 psi to operate correctly.

3. The improper conditioning of a new fan clutch. A new fan clutch friction disc lining is somewhat like a new brake lining- it needs to be broken in. As the rough surface of a new fan clutch disc is engaged and disengaged a number of times, the surface will become smoother, providing more surface contact and the capability of handling higher torques. You can burnish the new disc by setting the truck idle to approximately 1,500 rpm, then manually engaging and disengaging the fan clutch between 15 to 20 times, allowing 10-15 seconds for each application of the disc.

To manually activate the clutch, you simply need to plumb a compressed air line directly to the fan hub, and use a toggle switch to apply and release the air so that the internal spring can be compressed to release the clutch, with air vented to reapply it.

Fan clutch makers supply operating manuals with a series of excellent tips for preventive maintenance. Examples of these tips to ensure long fan clutch life include ensuring that there are no air leaks in the fan clutch plumbing, ensuring that fan belts are correctly adjusted (using a fan belt gauge), checking the fan clutch solenoid valve for correct operation, and checking the fan blades for damage. Also be sure to check for any signs of dirt accumulated at the splined area on the PFD (Piston Friction Disc), and the air chamber. If there is a sign of dirt, the fan might not completely engage, causing the clutch friction disc to slip and blow the safety fuse.

On today’s electronically controlled engines, the fan clutch can be controlled by the ECM (Electronic Control Module), which can be programmed to turn the fan clutch on and off. The ECT (engine coolant temperature) sensor monitors the jacket water temperature and sends a signal to the ECM, which uses this signal to control injection timing and fan clutch operation. During an overheating situation, it is also used to protect the engine.

The CLS (coolant level sensor) detects the presence or absence of coolant at the probe located in the radiator’s baffled top tank. The ECM uses this signal to protect the engine when coolant is lost. When high coolant temperatures are detected, perhaps due to a faulty fan clutch, the ECM engine protection system will de-rate the engine speed and power, and if programmed to do so, will automatically shut the engine down at a predetermined coolant temperature.

Some newer engines use two coolant level sensors; one warns the operator that engine coolant is low; the second one, which is a very low coolant level sensor, will begin to power down the engine. On trucks with air conditioning, if you do not turn off the A/C control switch, the fan clutch will continually operate. In addition, ECM-controlled fan clutches can be programmed to engage while the vehicle is in the cruise control mode. This will normally occur after the engine compression brakes have cycled through all six cylinders in an inline-six engine. The purpose is to automatically slow the vehicle on a down slope while you’re in cruise.

On electronic engines during normal fan operation, the fan clutch is set to disengage before the coolant temperature drops to 93 C (200 F), or within 3 C (6 F) below the temperature stamped on the temperature sensor.

If you take care of it, you can count on your fan clutch to be with you in a clutch. n

– Bob Brady is the president of Hitech Consulting in Burnaby, B.C.