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Explanation needed of Mercury Marquis cooling fan clutch operation

My Mom has a 1995 Mercury Grand Marquis with a radiator/engine overheating problem. The temperature skyrockets on occasion during slow, stop/go driving. Upon stopping and opening the hood with the engine still running, I see that the water pump belt-driven cooling fan is not turning(the smaller aux electric fan is running[while AC is on]) and the belt is not slipping.

I've been told my various shade tree mechanics that there is a clutch that gets the fan blade turning, but as to how the clutch is activated, I get varying stories. Some say the clutch is centrifugally activated due to the RPM of the drive belt. Some say the clutch is electrically activated by a fan clutch switch but the location of said switch is not known...some say in the radiator, some say in the thermostat, some say in the engine block.

If there is a switch, I'm thinking it's thermostatically activated, but that's a guess on my part. That stuff about centrifugal activation kind of has me puzzled.

I'm used to GM vehicles' electric cooling fans, having had to replace one on a Celebrity, and two on a Century. Voltage present to fan, but fan doesn't work...bad motor. No voltage to fan motor, switch problem. Simple.

I'd appreciate a "How it Works" tutorial on a, for lack of a better name, mechanical cooling fan. Thanks for any information or insight you can provide. Also, if you know whether it's the clutch or switch that is the more common culprit.

Danny getting hot in Yorktown, VA

Reply to
Danny
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The engine driven fan is equipped with a viscous clutch. It operates by allowing the fan to spin more freely( slip) when the clutch hub is cool, but as the hub gets heated up by the hot air flowing through the cooling radiator it begins to grip more completely and does not allow as much slipping of the fan on the hub. Inside the fan clutch are some metal clutch plates immersed in a viscous silicone material. That material gets thicker as it gets hotter and the thicker fluid causes more friction between the fan clutch plates. When it is fully warmed up we say it is engaged because very little slippage is occurring between the engine pulley and the fan blades. Some types of fan clutches also have an internal spring that uncoils as it gets hotter. This spring applies more pressure to the clutch discs and causes them to have more friction. Either way, there is no electrical switch or thermostat so to speak, that is used. The fan clutch is a sealed unit and is replaced whenever it fails to perform as required. To check it for proper operation, you should warm the engine up to normal operating temperature then have an assistant turn off the ignition while you watch the fan. If the fan continues to spin for more than one second after the engine stops then it needs to be replaced.

Some more information on cooling fans here:

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hope this helps.

Reply to
Kevin

As Kevin has posted, those hubs work to increase viscoelastic coupling as the temperature increases so that the fan rotates faster at higher temperatures than they do when the engine is cold. If you had a direct drive fan (which cars used to have) they would always run at full speed, increasing noise and draining horsepower. (A fan can easily consume 2-3 horsepower)

If this fan isn't turning adequately with the engine hot, then the engine is likely to overheat when you are driving at low speed. At highway speed you may get enough air flow that it will not overheat, but the problem will be back again as soon as you slow down.

Now, what do you do about it: (1) Replace the viscous hub with an aftermarket or dealership one (2) Replace the whole mess with a 'flex fan'. (I prefer the stainless steel ones)

Both, IMHO, are good options. One may be substantially cheaper than the other.

Reply to
<HLS

Just one other little word of counsel...You will hear a lot of things from people, many of which seem to make sense, but which may not be totally correct.

Automobile systems are more complicated now than they have ever been before, and a good mechanic needs to be current with the actual theory and facts about how systems work before he (or she) can troubleshoot them properly.

The more you learn about how the car works, the more you can do for yourself, and the more you can avoid bad advice. Dealership mechanics can be just as guilty at putting out misinformation as others.

Learn how things work, and you will save yourself time, heartache, and LOTS of money.

Reply to
<HLS

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