Saturday Mechanic - Fan Clutch Replacement

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KEYWORDS

• maintenance
• Saturday Mechanic
• transmission

NONE LIKE IT THAT HOT
The first signs that a clutch fan is failing are obvious: The engine starts running hotter and a/c performance drops. To confirm the diagnosis, start with this simple test: Spin the fan as hard as you can on an engine that has not been started that day. If the fan rotates more than five times, you can bet the clutch is bad. You should feel some resistance and the fan may spin up to three times, depending on the ambient temperature. But even if it rotates three or fewer times the clutch could still be bad. You need to do more tests. A few late-model pickups and SUVs, such as Ford diesels and the Chevy TrailBlazer and others with the 4.2-liter inline Six, have an electronically controlled valve for the fan clutch. In these vehicles, the silicone fluid doesn't drain back overnight, so the fan may barely spin on a cold engine. It could take a brief drive to get it to declutch.

BAD CLUTCH
If your fan clutch is not operating the way it should, it's likely due to a fluid leak or a bad thermostatic spring or valve.

When a leak occurs, it's at the bearing seal, at the center rear of the clutch. Run your finger around the joint and if you get a big dollop of black goo, that's silicone fluid that has leaked from the housing. Replace the fan clutch. A light smear of silicone fluid could be normal seepage--no seal is perfect. Obviously, if a substantial amount of fluid is missing, the clutch won't spin as fast as it should.

As for the other potential culprit, most failing thermostatic springs open the valve too soon. This premature clutch engagement means the fan spins faster than it should, but that doesn't make the engine or a/c run hot. On the other hand, failed valves that stick closed result in low fan speeds and little cooling.

HOW HOT WAS IT?
Let's say spinning the fan didn't reveal a problem, there's no leak and you think the spring is okay. You could still have a bad valve. Here's how to find out. You'll need to test the temperature of the air moved by the fan using a probe-type thermometer that reads to at least 220° F, preferably to about 250° F. You can't use an infrared thermometer because it will read the temperature of a nearby surface, which could be very different from the air temperature. Find a joint in the fan shroud with enough flex to let you insert the probe without getting it in the path of the fan. If necessary drill a small hole in the shroud. Run the engine at fast idle. Check the engine temperature gauge or your scan tool. If it's a hot day, the coolant temp will rise. As it approaches 200° F, you should hear the fan noise rise to a mild roar and see the fan spinning a lot faster. Air temperature in the shroud should remain in the 150° F to 190° F range. There are some exceptions--for instance, there are Chrysler products that may have a clutch fan rated at up to 205° F. Note: Don't try to take an instant reading. A conventional thermometer, even the probe type, takes at least a couple of minutes to stabilize.

If you don't have a thermometer, you can hear what you need to know. Listen to the fan noise with the engine cold, then warm up the engine. Watch the temp gauge. As the coolant nears 200° F, you should hear the noise increase as the fan engages and starts pulling a lot of airflow. If ambient temperatures are cool, cover the front of the grille with cardboard to block the flow of air through the radiator and to help raise the coolant temperature (leave the a/c off). As the coolant gets hot, fan noise should increase to a roar and the fan should spin a lot faster. If not, you'll need a new clutch.

Don't let the vehicle overheat. Remove all or part of the cardboard before the temperature goes over 210° F.

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Use a probe-type thermometer to check the temperature of the air moving through the radiator.

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You can check the fan's speed against the manufacturer's specs by using a photo-tachometer.