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Old 07-30-2018, 09:38 PM   #1
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How to test a fan clutch?

My bus has a Cummins 5.9 engine, which recently threw an over-temp alarm on a fairly short hill. I pulled over and idled for awhile, which brought the temp down to 240 degrees (apparently its normal operating temperature). I wonder if my fan clutch is working properly. This is important to me because I want to cross the Rockies in a couple of weeks.

How does one test one's fan clutch? Gently try to jam a rolled-up magazine in the blades of the fan when the engine is up to full temperature?

My thought is that if one can stop the fan then the fan clutch is not working properly. Thus the clutch or the sensor or the wires or something is bad in that system. If one gets confetti from the end of the rolled-up magazine and the fan keeps spinning with enthusiasm, then that isn't the problem. Might be a plugged radiator or something. Right?

Any other thoughts would be welcome. There are no leaks and the fluid is green. I have an IR thermometer if that would assist in the diagnosis.
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Old 07-30-2018, 10:30 PM   #2
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Quote:
Originally Posted by Dapplecreek View Post
My bus has a Cummins 5.9 engine, which recently threw an over-temp alarm on a fairly short hill. I pulled over and idled for awhile, which brought the temp down to 240 degrees (apparently its normal operating temperature). I wonder if my fan clutch is working properly. This is important to me because I want to cross the Rockies in a couple of weeks.

How does one test one's fan clutch? Gently try to jam a rolled-up magazine in the blades of the fan when the engine is up to full temperature?

My thought is that if one can stop the fan then the fan clutch is not working properly. Thus the clutch or the sensor or the wires or something is bad in that system. If one gets confetti from the end of the rolled-up magazine and the fan keeps spinning with enthusiasm, then that isn't the problem. Might be a plugged radiator or something. Right?

Any other thoughts would be welcome. There are no leaks and the fluid is green. I have an IR thermometer if that would assist in the diagnosis.
There are two type of fan clutch systems. One has a bi-metal temperature sensor on the clutch housing, the other is externally controlled. Both systems need to get your attention when the fan kicks in. A clutch with the built in bi-metal thermostat cycles a little softer but you may have heard the fan sound big rig trucks make when they pull into a truck stop. The HMMWV for example has a clutch that is hydraulically actuated and controlled by an external valve and thermostat. When that fan kicks in for the first time you hear it you'll think the engine is just coming apart on you. Long story short, a well working engine-driven cooling fan is making substantial noise in a truck, bus, etc. If it doesn't make noise, it's not working.
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Old 07-31-2018, 05:36 AM   #3
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Donít do the magazine thing

Can you spin the fan by hand with the engine off and cold?
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Old 07-31-2018, 07:12 AM   #4
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alpine has it right... there are 2 types..



the viscous fan clutch has a minimum speed.. when the engine is running your fan will be turning at a nominal speed.. say 300-400 RPM and will be moving some air but not a ton.. when thje engine is dead cold, your viscous clutch should engage fully wit ha good fan roar foe the first minute or so you operate.. then the fan sound will slowly fade over a few second time frame and the fan disengages to its minimum speed. these clutches will have varying amounts of resistance when the engine is turned off.. when running they are controlled by thermostat that measures the air temp from the radiator.. low coolant will make these malfunction, as will defective clutch... sometimes you can adjust them.. here is the pic of the nose of a viscous clutch showing that bi-metal thermostat.




IMG_1475.jpg


an electric or air fan clutch will hace a very slow off speed.. and will likely have a wire harness or air line going to the hub on the engine near the fan..



this is what an electric clutch looks like. when the engine is off that fan has zero resistance and is completely free..



IMG_1677.jpg




-Christopher
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Old 07-31-2018, 07:17 AM   #5
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I could be wrong

But my understanding of my air fan clutch is with no air pressure it is fully engaged

Meaning

With engine off it shouldnít spin freely - this would be evidence of a worn lining

APC_0004.jpg
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Old 07-31-2018, 08:29 AM   #6
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Quote:
Originally Posted by Dapplecreek View Post
My bus has a Cummins 5.9 engine, which recently threw an over-temp alarm on a fairly short hill. I pulled over and idled for awhile, which brought the temp down to 240 degrees (apparently its normal operating temperature).

As a general rule, 240 degrees is *WAY* overheated/ruined engine territory. However, since (I am assuming) yours has been running for considerable time at that reading, I will assume your engine is not blown. I would suspect, however, you may not be getting an accurate reading. I would check the actual temp when it's warmed up, which should be in the 180-195 range. A failed sensor could be the culprit, or the gauge or wiring.


Given that you had an over-temp alarm which didn't sound until you went up a hill, that supports my thought that your engine is not running excessively hot. I would still check it's actual temp and make sure the gauge reflects correctly. There's a few likely causes for the over-temp event:


1. As noted by others, the fan isn't engaging to full speed when needed.
2. The radiator fins could be clogged with dirt and debris. Carefully blow out from behind (can also use pressure washer, be *VERY CAREFUL* not to bend the fins)
3. Low coolant/poor circulation/failing water pump (unlikely in your case, but included for completeness)
4. Inadequate cooling system (highly unlikely in your case, this would apply if the engine has been modified for much higher power without regards to the extra heat it will produce)
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Old 07-31-2018, 10:47 AM   #7
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Update:

I didn't to the magazine trick, but instead used a half-used roll of paper towels (didn't see the replies above until afterwards, sigh).

The engine went to 240 degrees while idling (clue #1 that we have a problem).

The fan could be stopped by gently letting the fan blades whack the edge of the paper towel roll. It tore the corner off the roll, but stopped the blade without much difficulty. No roar of fan could be heard, either - though the temp and the ability to stop the fan kinda tells the story.

I am sourcing the clutch right now and I have a truck shop that can see me tomorrow at noon. Yay!

Thanks, all!
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Old 07-31-2018, 11:12 AM   #8
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so which clutch do you have? the kind with the coil in front or the electric type? if you have the electric type and get no engagem,ent, its worth checking the wiring to make sure its getting power applied to it..



if its the viscous type then check with Kit-masters, thats who i source all things viscous clutch from and they will likely have one to fit your freightliner..
-Christopher
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Old 07-31-2018, 11:19 AM   #9
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overheating at idle sounds like a tstat or water pump to me.

either will be cheaper than the fanclutch
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Old 07-31-2018, 11:23 AM   #10
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Quote:
Originally Posted by Dapplecreek View Post
Update:

The engine went to 240 degrees while idling (clue #1 that we have a problem).

Not necessarily, there is a thermostat for a reason, it should warm up even when idling.


Quote:
Originally Posted by Dapplecreek View Post
The fan could be stopped by gently letting the fan blades whack the edge of the paper towel roll. It tore the corner off the roll, but stopped the blade without much difficulty. No roar of fan could be heard, either - though the temp and the ability to stop the fan kinda tells the story.

The fan should still be free even when warmed up. Usually there is a threshold temperature at which it should engage fully, and just idling with the fan turning a little, the engine may not reach this threshold. My bus has the viscous coupling type fan clutch and when I'm going down the road, it occasionally kicks in, and there is a very obvious rush of air that goes with it. As it cools, the fan disengages and goes back to turning just a bit. I've driven trucks with the electric clutch and there is sometimes a belt chirp as the fan engages (fairly common when idling with the A/C on, it needs to cool the condenser moreso than the coolant).


A viscous fan clutch can (and should) be checked with the engine off - it should turn, but with some resistance. The warmer the clutch (or, more correctly, the bimetal spring), the more resistance it should have. If it turns very freely, check to see if there's signs the viscous fluid has leaked out. An electric clutch can be checked with the engine off as well, just put 12 volts across it and the fan should turn with the belt (IE, with the engine off, it should not turn at all).


I *DO NOT* recommend fan testing with the engine running. Be safe out there.
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