Last weekend I was towing my 20 foot bass boat on I20. We were running 80-85 and I was keeping an eye on my Attitude (EGT & Engine Temp). Pulling a slight grade I saw the Attitude engine temp hit 203 degrees. Right at 203 the engine fan came on and man it sounded like a jet! We hit 203 probably 3-4 times during the trip and each time the fan would bring it down to 192 (Fast). The Allison never even came close to 200 http://www.dieselplace.com/forum/smileys/Approve.gif You would never see a temp change on the dash only on the Attitude.


I cant see how a duramax could ever overheat with a fan that blows like a hurricane. Did I mention I have a bug screen over the grill...

I can't say that I have ever heard my fan kick in like that...Maybe it never has. It's nice to know what the sound will likely be if it ever does though. It may save me from having my heart jump out of my chest!http://www.dieselplace.com/forum/smileys/HiHi.gif














Kevinhttp://dieselplace.com/forum/smileys/Big%20Smile.gif

When that baby kicks on it sounds like a tractor trailer!

I've had the fan come on once and her hit 210 acording to the attitude and then I decellerated from 95 to about 85. This was a 60 degree day with 1000#'s of brick in the bed and my plow up front. 9'-6"s of steel blocks some air leme tell ya but I was in a hurryhttp://www.dieselplace.com/forum/smileys/HiHi.gif


Scared the bejesus outa me for a sec till I relized what it was

If you pull a long enough hill, hard enough, heavy enough, especially at higher altitudes where the EGT is in the low 1200 degrees, you can get it pretty hot. At some point in the heating up cycle, a second thermostat opens, and things cool down in a hurry. Even with that big honking fan roaring.

<DIV>First time mine did this I was pulling my 5th wheel, I thought the trans was slipping. </DIV>
<DIV>Scared the hell out of me.</DIV>

Do they sell electric fan replacements to help reduce the HP loss when the fan kicks in?

Both the wife and I thought there was something really wrong, but I remebered a post on the subject and placated her. I looked it up again just to make sure. Now it happens frequently when climbing 6% grades with 5vr with the warmer weather.

An electric fan is no more efficient than the stock fan. It loads the alternator as much if not harder than the mechanical unit (Enstein first rule of physicshttp://www.dieselplace.com/forum/smileys/Approve.gif). The only time an electric fan is more efficient is when you turn it off, but a properly working mechanical will also free wheel when not needed.





Hosre Trainer, I have the service manual here, and can't find a second thermostat you mentioned. Where is it?

I believe there are two inside the housing. One opens earlier than the other (if I remember correctly - it has been a while). I got curious when I would see my temp climb way up (even with fan engaged), and at a certain point, really drop back to normal. I'll see if I can find a page in the fully illustrated service manual.

Thermostats (6.6L Diesel Engine)


The thermostats are coolant flow control components. The purpose of the thermostats are to regulate the correct operating temperature of the engine. The thermostats utilizes a temperature sensitive wax-pellet element. The element connects to a valve through a piston. When the element is heated, it expands and exerts pressure against a rubber piston. This pressure forces the valve to open. As the element is cooled, it contracts. This contraction allows a spring to push the valve closed.


The 6.6L diesel engine requires two thermostats for correct coolant flow. The front thermostat is a dual purpose thermostat. The front thermostat controls the coolant flow to the bypass port and to the water outlet. The rear thermostat only controls the coolant flow to the water outlet.


When the coolant temperature is below the rated thermostat opening temperature, the front thermostat valve remains closed to the water outlet and is opened to the bypass port. The bottom portion of the thermostat is raised off of the bypass port while at the same time the top portion closes the coolant flow to the water outlet. The rear thermostat also is closed to the water outlet during engine warm-up. This prevents circulation of the coolant to the radiator and allows the engine to warm up quickly. After the coolant temperature reaches 82°C (180°F) the front thermostat primary valve opening temperature, the front thermostat primary valve will start to open. The coolant is then allowed to circulate through the thermostat to the radiator where the engine heat is dissipated to the atmosphere. As the engine coolant reaches 85°C (185°F) and more coolant demand is required the front thermostat secondary valve begins to close the bypass port and the rear thermostat begins to open coolant flow to the water outlet. The thermostats will continue to control the coolant flow by opening and closing. The front thermostat will be fully open when the coolant temperature reaches 95°C (203°F) the rear thermostat will be fully open when the coolant temperature reaches 100°C (212°F). The thermostat also provides a restriction in the cooling system, even after the it has opened. This restriction creates a pressure difference which prevents cavitation at the water pump and forces coolant to circulate through the engine block.

In some circumstances, when I tow very heavy in the 15s,16s my fan doesn't come on ntil the temp on the attitude is around 225F-226F. Most of the time it's in the 205F region. Should I be worried? Is the attitude temps the same as the one used by the fan?

Perform a fan clutch engagement test.

<LI =1>Ensure the engine coolant level is full.
<LI =1>Ensure the cooling fan drive belt tension is correct and not slipping.
<LI =1>Position and secure a thermometer between the fan clutch and the radiator.
<LI =1>Ensure the cooling fan is disengaged before starting this test.
<LI =1>Sufficiently cover the radiator grille to restrict the air flow.



<H4>Important</H4>


Do not allow engine temperature to exceed 121°C (250°F).



<LI =1>Start the engine.
<LI =1>Turn the A/C ON, if equipped.
<LI =1>Operate the engine at approximately 2,000 RPM.
<LI =1>Inspect the thermometer reading when the fan clutch engages.
<LI =1>Do not continue this test if the fan clutch does not engage between 85-96°C (185-205°F). </LI>


Fan clutch engagement will be indicated by an increase in fan air noise, fan speed, and a drop of about 3-10°C (5-15°F) on the thermometer reading.


Did the fan clutch engage between 85-96°C (185-205°F)?


if it does then you good to go.......

I would like to have the electric fan just to get rid of the noise when it comes on.


Uni Cop

Thanks GM - didn't get to the book last night.

I wonder if adding some vents i the hood (like the ricers) would allow my truck to pull mor air through the radiator and help it not need the fan so much. Mine comes on around the 200 to 205 mark and it does it when I am pulling a 7k lb tt aroudn 70 miles per hour. It is hard to see how I can be pusing 70 and it isn't keeping it cool enough not to engage. My next oil change I am going to synthetic and I will see if that reduces my tems some when pulling. Perhaps that is all I need. If not I guess I will get out the hack saw and stretch some screen on the underside of my hood http://www.dieselplace.com/forum/smileys/Big Smile.gif

my fan will come on without towing, usually for a couple of minutes every so often on the highway (running around 85). In town, It will come on regularly as the temps around here are hovering at 90*.

ok, someone enlighten me. If the fan comes on due to high temp, then just how the hell is covering something with screen gonna help this?? Constricting air flow, last time i was in school, would NOT cool things down more.

The size of the holes in the screen is really important - too small, and the airflow is blocked. We have had 50mph gusts the last couple of days, and a screen door on the house keeps that wind out - not one I would use as a bug screen or outlet screen.


As to fans, I can be pulling a load hay up a grade that is just slightly uphill, and if we have even a bit of a cross wind, there is enough airflow to keep the fan off most of the time - guess there are as many variations as there are trucks.

Hey GM, I have seen on the RV forum that I frequent at least one person saying the fan will kick in during grade braking to create additional drag. Can you confirm if this is true?

Thanks!http://www.dieselplace.com/forum/smileys/Question.gif

I've never seen this behavior so far. Even during a 12% grade braking with a 16,000 lbs load. But if the fan is already working lets say when climbing the other side of that hill; it will continue working until the temps is ok.

The only thing that still concerns me is the 225F startup temp in 'some' occasion. Can somebody comment on that?

Cooling Fan and Clutch


The engine cooling fan and clutch are driven by the crankshaft via the drive belt. The cooling fan draws air through the radiator to improve the transfer of heat from the coolant to the atmosphere. As the fan blades spin, they pull cool, outside air past the radiator core. The fan clutch drives the cooling fan. The fan clutch controls the amount of torque that is transmitted from the crankshaft to the fan blades. The clutch allows more torque to engage on the fan when the engine operating temperature increases and/or the vehicle speed is low. As the torque increases, the fan turns more quickly. The fan clutch decreases the torque applied to the cooling fan when the engine temperature decreases and/or the vehicle speed is high. As the torque decreases, the fan speed decreases.

The past two mornings, my fan has kicked on almost right out of the driveway. Runs for about 1/2 mile then shuts off the rest of the day. Doesn't even come on in the evening after sitting in a hot parking lot all day. I asked the local service dept about it when the truck was in for noisy door stops and he seemed to indicate it was normal. Seems strange to me. I'll have to keep an eye on it - still lots of warranty left.

That fan behavior is normal - the fluid in the fan clutch is thick and acts as if it is being asked to run the fan. In fact, when I worked on M-B's, the unsophisticated test of the fan clutch was to turn the fan with everything cold and if there was no resistance, the clutch was bad.

Thanks for the information - that's why I love this forum!! http://www.dieselplace.com/forum/smileys/Big Smile.gif