Viscous Fan Clutch and Fuel Consumption

[NICAT]

Hi friends,
My 2002 ML320 comes with a viscous fan clutch.
Several driver friends have reported that replacing it with an electric fan from the ML500 reduced fuel consumption and minor improvement engine response.
However, I find it hard to believe that fuel consumption most of which comes from moving a 2.2-ton vehicle, could be noticeably affected by just a fan that only engages when needed. Moreover, an electric fan still draws power from the alternator, which in turn adds drag to the engine when electrical demand is high.
My mechanic is offering to replace it with an electric fan at a discounted price.
Would it really worth?
Regards.

 

[RcClockman]

I would leave it stock, more reliable...

 

[Tman220]

Even a "disengaged" viscous clutch fan will turn about 20-35% of the input shaft speed depending on temperature. If you spend a lot of time on the highway that is a lot of wasted energy when you don't need to be expending extra power turning a fan when vehicle speed is high enough for adequate cooling.

An electric fan will increase power available to propel your vehicle when the additional cooling from the cooling fan is not needed. Whether you will notice or not is up for debate.

What may be a downfall for adding an electric fan is when you need air conditioning. Your clutch fan even when disengaged will draw air through the ac condenser when you are stopped. A vehicle which comes stock with electric fan and AC will usually trigger the cooling fan depending on pressures on the high side of the system. Adding a electric fan you may notice less ac performance when stopped until the cooling fan come on, unless you trigger the fan manually and then the benefits become less great.

Another potential issue is that vehicles that come factory with electric cooling fans tend to have larger capacity alternators. The draw from a high power electric fan setup can easily be 20-30 amps. Some vehicles may run into dimming issues at idle at night when the cooling fan kicks on.

 

[NICAT]

Even a "disengaged" viscous clutch fan will turn about 20-35% of the input shaft speed depending on temperature. If you spend a lot of time on the highway that is a lot of wasted energy when you don't need to be expending extra power turning a fan when vehicle speed is high enough for adequate cooling.

A vehicle which comes stock with electric fan and AC will usually trigger the cooling fan depending on pressures on the high side of the system. Adding a electric fan you may notice less ac performance when stopped until the cooling fan come on, unless you trigger the fan manually and then the benefits become less great.

Thanks for the very detailed explanation.
I think the A/C concern can be ruled out . I’ve made some major modifications, and now it’s extremely cold inside even during the hottest summer days.

 

[ka9mnx]

Your friends are trying to justify their mistake.

 

[SubieRubyRoo]

Replacing belt-driven fans back in my Mustang days was usually good for 2 tenths (or slightly more) and nearly 2mph. Back in those days, ($1.0X gas) I didn’t care about mileage. But drag racing rules-of-thumb means that fan equated to roughly 20HP to turn. 20HP can be a significant MPG gain depending on what % of total output it is.

If you go electric fan, be sure to get one that you can control the on/off temps at, because as some mentioned, even a “bypassed” viscous clutch is moving some air. I’d say you’d need the electric to turn on directly at or just a degree or two before the thermostat opened to keep similar performance.

 

[NICAT]

Replacing belt-driven fans back in my Mustang days was usually good for 2 tenths (or slightly more) and nearly 2mph. Back in those days, ($1.0X gas) I didn’t care about mileage. But drag racing rules-of-thumb means that fan equated to roughly 20HP to turn. 20HP can be a significant MPG gain depending on what % of total output it is.

If you go electric fan, be sure to get one that you can control the on/off temps at, because as some mentioned, even a “bypassed” viscous clutch is moving some air. I’d say you’d need the electric to turn on directly at or just a degree or two before the thermostat opened to keep similar performance.

But this would only be true when fan is fully actuated, and at higher RPM's right ?
Because air drag (aerodynamic resistance), power loss increases with the cube of speed.
If Power loss at 2000RM is 1HP at 6000 it would be 27HP ?

 

[kschachn]

Hi friends,
My 2002 ML320 comes with a viscous fan clutch.
Several driver friends have reported that replacing it with an electric fan from the ML500 reduced fuel consumption and minor improvement engine response.
However, I find it hard to believe that fuel consumption most of which comes from moving a 2.2-ton vehicle, could be noticeably affected by just a fan that only engages when needed. Moreover, an electric fan still draws power from the alternator, which in turn adds drag to the engine when electrical demand is high.
My mechanic is offering to replace it with an electric fan at a discounted price.
Would it really worth?
Regards.

There is no way you will recoup the cost, especially if you're not doing the work yourself. Even then I doubt it will be profitable. I looked into it for my old BMW but it quickly became a silly exercise.

The BMW was a bit different in that it already had both the mechanical and an electric fan. Your Mercedes-Benz has no electric fan at all?

 

[NICAT]

There is no way you will recoup the cost, especially if you're not doing the work yourself.

İn My Country brake pad replacement is as low as 7 usd for my car. imagine the rest
AC fans are electric.

 

[Fabulous50s]

I used to remove my viscous fan altogether on a Ford Ranger in the winter months. No cooling fan at all. I saw about 1mpg increase even with more idle time for warm up. The heater run on high would keep the coolant temp from climbing if need be. It was a 5 min job on that vehicle and I was trying to save as much fuel as possible as it was very expensive ($5 USD/gal) and I was nearly broke.

That vehicle is long gone, and ive converted several older vehicles with mechanical fans to electric since then, after electrical upgrades to support such things. Ive done all the work myself so the costs have been parts only.

 

[Snagglefoot]

How long do you plan to live? I’m always leery of the capital cost of improvements and the payout time. I have a viscous clutch on my old Suburban even though they went to an electric fan on subsequent models. Pick a gas mileage improvement, figure out how many more miles you’ll drive and I can almost guarantee you’ll never pay it out.