Before I go on and on about this subject, how many peopel have the engine "roar" or "wrrrr" sound when the temps are hot and at highway speeds.

I know some will chime in and say that it's "fan noise" when the truck gets hot, I say that it is not the cooling fan making the noise, but the tranny making the engine rev.

Thoughts?

Well if it is...my handshaker does the same thing!

MaxRock

if the tranny made your engine rev, then your rpm's would change right?? that would be noticable.
I think you are in fact hearing the fan clutch engage which sounds like a loud jet engine. normal.

happens when you first start up in the mornings and while towing/highway etc

let us know what you find out.

Trip notes.

After reading all the posts on overheating, I thought yesterday would be a good day to test things out.

Temps were to reach 115 degrees in the palm Springs area, and the trucks temp guage was vacillating between 114, and 123 degrees during the start of the trip.

Going East out of Indio Ca. there is a large hill ( I don’t know the grade) , but yeterday I saw dozens on cars and big rigs to the side of the road with overheating problems, so suffice it to say it was a miserable climb in one of the most brutal days of the year. My driver’s side window felt like oven glass.

Half way up the engine started making a wrrrr sound or high rev sound while climbing, no power loss as I was testing the bone stock rig, with 1000 miles on it, I floored it and reached the max speed of 95 mph, so power was not an issue, and all gauges were in the normal range, no temps climbing ( tranny a couplke of pegs higher than center). So loud it was hard to listen to the radio, or talk.

After I got to the top the wrrr noise kept coming on on it’s own and going off on it’s own. Now at this point one would think OK, cooling fan. But, it just didn’t feel or sound like any cooling fan I know of, so I kept at trying to find the cause, and make the noise stop.

Now if anyone knows the route from Palm Springs to Phoenix then you know after you get to the top of the hill you get plenty of arrow straight and level highway to play with. I set the cruise at 75, while the noise was on and sat back, then as I dipped in a small hill the noise stopped – hmmmm.

That got me curious to see if I could make the wrrrr go away on my own. The next time it came on ( now mind you the cruise is on, so no hills to strain the engine, straight flat road, ambient temps 114, gauges all normal except for tranny still a couple of pegs high of center) so I wondered why is the wrrrr coming on nothing has changed to make it happen?

I pushed in to hold the button to enable Overdrive Disable feature of the tranny, and floored it, the engine reved, and then the noise stopped and stayed stopped until it decided to kick in,
( I could NEVER make the noise kick in, it had it’s own mind on that) I could only make it stop.

Ahha – I now could make the noise stop! And when the wrrrr stopped you absolutely could not hear the engine and could now whisper and be heard.

If I can make the noise stop, now I know it’s not the fan, if it was the fan, and the temps were making it come on then with all systems the same ( ambient, speed, temps etc. ) then the fan should stay on no matter what I did, and that’s not the case.

Now it’s time to test my theory – It would come on and I would no nothing for 5 minutes, then do my button test, the noise would stop, next time 10 minutes then hit the button noise stops.
Again and again, it would only sometimes stop on it’s own, but ALWAYS stop when I did my button push. At one point I got tired of fighting with it and it just stayed on for nearly an hour.
By the way the turning off process only lasted between 5 and 10 minutes each time. It would always stay on longer than I could make it go off.

One time, while climbing another hill as I was continuing on to Flagstaff on I40, the noise came on, this time I floored it without pushing the button and the tranny seemed like it hit the top gear, and the noise went away, this was near 95MPH. Under 70MPH the noise won’t come on between 70-80 the noise won’t go away, unless I do the button thing.

It seems the software is delaying the 'shift busyness' so much that it parks itself in a gear shy of the top gear, and with my fiddling, I was confusing the software into shifting to top gear thereby lowering the wrrrr and calming the engine revs down.

Here’s what I found on Allisons site to day, which now confirms all my findings AND problems.

Shift stabilization is a software feature in the Transmission Control Module (TCM) that is intended to minimize 'shift busyness', or 'hunting' between ranges. Based on several inputs and a 'map' of engine torque at various RPM and throttle position, the TCM determines before making an upshift whether the engine will be able to maintain vehicle speed in the next higher range. If it senses that it does not, it will prevent the upshift from occurring.

Now when I reached Flagstaff the ambient temps went from high teens in Palm Springs to the Low 80’s. Same level, style of road, same cruise at 75, and now the noise is gone, and stays gone for the remainder of the trip – did I win the battle with the beast, who knows, I know I gave up fighting it. I did notice the tranny temp was a couple of pegs below middle in Flagstaff. And while I was having all the wrrr the temps were a couple of pegs high of middle.

interisting I just had a SuncoastIV put in, and mine still does this wrrrr noise it does not happen as much as you have had it though, mostly at park or if it is hot out and I run hard with it(today its 90 plus degrees and it did it today) only after parking it and leaving it run, and after some serious hot rodding (or trans learning as I say) it does not stay on as yours does it usually goes of in 30 seconds or so

For what's it worth, our Durango has a viscous (sp?) fan and it behaves/sounds just like one on the 2500HD. Another "illusion" is the loud noise is taking away power...I think its just the louder noise making it seem less powerful.

I called Allison, but they are all off for two weeks, at the main plant. I know people will swear up and down it's the fan, but yesterday convinced me that it has to do with the tranny, not going or "searching" for that next higher gear.

Each time it hit that higher gear it was like a different truck smooth no roar ( like a jet engine).
My wife was in the truck and could see I was obseced with the noise, but she also agreed the truck was much more pleasant with the noise gone. And again I was making the noise go away by either powering through with enough RPM's to catch the next higher gear, or by manipulating the overdrive button.
I mean you could feel the shift into the higher gear, and instantly the noise would cease ONLY after the next higher gear.
Water temps never budged the whole time I was at this task. Only the tranny temps rose and lowered.
One time while going 80 MPH I took my foot off the throttle, and coasted, and put the tranny in (N) and the engine revs stayes at 2k on an idle engine, then noised stopped with this manuever also.
Was this nuts, you betcha, but either this noise stopped or GM was buying it back. No way could i put up with this amount of engine roar.

I have only had my truck for a short time and yesterday I returning home from picking up my new plow about 50 miles away. I brought my truck down to the shop tuesday afternoon and the temps outside were in the high 80's and I never hear the wrrr sound, yesterday when returning, the temp needle was slightly higher then normal cruising along at about the same speed 65mph and the wrrr sound came and went, when it did the temp needle dropped to where it normally sits and then it would turn off, however the rpms never changed when it was making the noise thus my conclusion is that is the fan.

Trust me, it's the fan. I thought it was the trans too. Apparently it's sensitive to air temperture, not engine temperature. I might go electric. You're right, it's very irritating.

Its most likely the fan. I have had my fan kick in when the mirror thermostat only says 80 and the edge shows engine temps at 195.

Sounds normal to me, what you are experiencing.

So during all this button pushing and high speed runs to shift into the next gear, what happened with the RPMs each time the noise appeared or went away?

Agree it's the fan clutch making the noise.

In talking to the Allison guys it's sorta like the chicken and egg thing. The fan clutch kicks in, which makes the roar, and the higher engine RPMs by pushing the button, makes the tranny shift and makes the roar go away. He said their are some after market things that will change shift points, but asked me if it was worth losing the warranty over - Duh!

For me in the extreme temps the water temp never moved off of center when it was nearly 120, ans got to the low 80's temp never budged. But I have to say on the way home it never happened again, with temps from the low 80's to low 100's.
It obviously is working as designed, just irritating as heck when it's roaring, and when it's gone I don't think their is a quieter truck.

I know they say these transmission "learn" well I quess mine learned I was pretty
Pi$$ed off, and decided to fly right...or I'd sick the "button" on it

Also the Allison guy said between 20-30 degrees cooler with transyn, and I agree that's what I put in my 04.5, and it made a huge difference.

It is the fan. Remember reading in a GMC brochure that the fan was designed with "submarine technology" to make it run quiet. WWI technology maybe.

aka108 - ROTFLOL

One time, while climbing another hill as I was continuing on to Flagstaff on I40, the noise came on, this time I floored it without pushing the button and the tranny seemed like it hit the top gear, and the noise went away, this was near 95MPH. Under 70MPH the noise won’t come on between 70-80 the noise won’t go away, unless I do the button thing.

By 75 mph you're in the top gear. It's the fan. A fan that moves a lot of air will make noise. Relax, it won't be 100+ degrees for long.

My 1990 R3500 dually with 454 has a fan that does exactly the same thing, only louder. The 454 stays cool at highway speeds but overheats at idle (even with the factory electric fan.) So far, my 04 LLY has not overheated.

Jim

It is the fan. Remember reading in a GMC brochure that the fan was designed with "submarine technology" to make it run quiet. WWI technology maybe.

that is funny sh#t , but true

By 75 mph you're in the top gear. It's the fan. A fan that moves a lot of air will make noise. Relax, it won't be 100+ degrees for long.

This is my point, I wasn't in top gear at 75, it would hit top gear whenever it felt like it OR I manipulated it. Once it didn't hit top gear till I hit 85, other times it didn't hit top gear at all, just rode in 4th for miles and miles at 75plus. You knew it once it hit top gear, RPM's would drop, and noise would quit, and it ran smoooooth.

If you think that's irritating, try to tow in the same conditions. The water temp will budge.

An inherent design problem of a thermo-viscous clutch, is the short cycling nature of it. It is a self-contained device with the viscous clutch in the middle. No electronics input at all. The problem is that the fan is what cools the turn-on/off clutch mechanism. It speeds up air over the stack, the temp of that air lowers across the clutch, disengaging it, less air flow, higher temp air, reengages. A short-cycling noisy nightmare (when empty) if you ask me. If you were towing anything it would have been on the entire time.

If there was an electric fan with decent output, that fit in place of this one, I would pull this one out today.

I have a bit of a theory I will test out soon. I have a feeling that at higher speeds, the pressure under the truck increases, keeping cooling air from dumping out of the engine bay. If flow is severely reduced through the rad etc, then this short cycling can occur with more frequency.

Normally I would expect flow to increase with speed.

I agree, but I had no load and the fan clutch was on constantly, taht's what was driving me nuts. Granted it was doing what it is supposed to do, and granted I was driving it in 116 degree temps on the highway.
I'm thinking between synthetics for engine oil, and tranny fluid should drop the temps enough to perhaps prevent this

I agree, but I had no load and the fan clutch was on constantly, taht's what was driving me nuts. Granted it was doing what it is supposed to do, and granted I was driving it in 116 degree temps on the highway.
I'm thinking between synthetics for engine oil, and tranny fluid should drop the temps enough to perhaps prevent this

I believe it was performing correctly, just that there was very little flow without the fan (obviously), that is the problem. I run synthetics, it may help a little, just because a little less heat is produced, but nothing like airflow will

It bothers me that you were at 75, unloaded, but not in 5th? Doesn't make sense. Tranny issue? 40 mph headwind?

K - I don't own a Duramax, but oh how I want one...

I've read this post, and I'm wondering a couple things.

1.) Could this be related to the throttle position sensor? I've got a Dodge, and let me tell ya, the truck will NOT keep the TC locked in O/D. The problem with the Dodges is that... well, it's that the TPS is a cheap piece of crap. As the position of the pedal changes, the voltage ouput from the TPS *should* smoothly change, following the pedal position. What the bad TPS does is send out very erratic voltage changes. This causes the tranny to disengage during spikes as the ECM believes a shift will be needed due to the false interpretation of the pedal position by the TPS.

I've replaced the TPS 2 times, and both times it solved the problem for about 1 month. Then the problem returned. The results sound very similar to what you are experiencing. Engine RPM would increase ~300, engine noise would significantly increase, finally the TC would lock up and the noise would drop again, RPM would drop. When this occurs, it very much feels like the truck is shifting.

2.) If the truck really is downshifting to 4th / i.e. when forced to with the push button, does the sound immediately stop. If it does, I'm really leaning towards the TPS. However, if it hangs on for 10 - 15 seconds, could it be related to the higher circulation volume of coolant (possibly engine and tranny)? At higher RPM, the cooling capacity increases (to a point), and in theory, this higher volume of fluid could theoretically pull more heat from the engine / tranny radiator, thus reducing the need for the fan.

I'm definitely not a mechanic, nor do I own a Duramax. I'm just throwing out a strange similarity. If you want to read ALL about the TPS issues with Dodges, search for "tranny surge" at pavementsucks.com. As I do not own a Duramax I cannot tell if these symptoms are the exact same - perhaps a few people experiencing this issue can verify for other members of the forum. I think a throttle position sensor is only about $30 or so. It might be worth the money to give it a whirl.

This is my point, I wasn't in top gear at 75, it would hit top gear whenever it felt like it OR I manipulated it. Once it didn't hit top gear till I hit 85, other times it didn't hit top gear at all, just rode in 4th for miles and miles at 75plus. You knew it once it hit top gear, RPM's would drop, and noise would quit, and it ran smoooooth.

What RPM were you at when going 85 mph in 4th gear?

wouldn't that be like 500 over red line?

I have a bit of a theory I will test out soon. I have a feeling that at higher speeds, the pressure under the truck increases, keeping cooling air from dumping out of the engine bay. If flow is severely reduced through the rad etc, then this short cycling can occur with more frequency.

Normally I would expect flow to increase with speed.

Tested that already in New Mexico at 110F ambient temp. Driving 75 cycled the fan every 3 min. - driving the same street, same temp only 65 increased that cycle time to 5 min or more. That was without load altitude 5k.

"This is my point, I wasn't in top gear at 75, it would hit top gear whenever it felt like it OR I manipulated it. Once it didn't hit top gear till I hit 85, other times it didn't hit top gear at all, just rode in 4th for miles and miles at 75plus. You knew it once it hit top gear, RPM's would drop, and noise would quit, and it ran smoooooth."


Faulty TPS could cause this exact scenario. The T/C unlocked could feel like 4th gear given the higher RPMs and increased engine sound. Also explains the manual shift fixing the problem. The tranny knows it is now in 4th, therefore, it would not unlock the T/C even with the voltage swings - there is nowhere for the tranny to go even if the driver is asking for more power / acceleration.

Tested that already in New Mexico at 110F ambient temp. Driving 75 cycled the fan every 3 min. - driving the same street, same temp only 65 increased that cycle time to 5 min or more. That was without load altitude 5k.

so that I understand, are you saying that at 75, that you had more or less fan "on" time. Looks like you are saying you had better cooling at 75.

so that I understand, are you saying that at 75, that you had more or less fan "on" time. Looks like you are saying you had better cooling at 75.

I am saying that with 75 I the engine needed the fan to come on more frequently than with 65. The cooling should be the same - temp was always shown the same (205 or little less when cycling).
I thought that was your theorie regarding pressure build up under the truck - higher speed more pressure up to a point were the pressure is worst than the added cooling because of more airflow
75 - more incoming airflow but more pressure under the truck
65 - less airflow but less pressure under the truck also

Is that what your idea was?

I am saying that with 75 I the engine needed the fan to come on more frequently than with 65. The cooling should be the same - temp was always shown the same (205 or little less when cycling).
I thought that was your theorie regarding pressure build up under the truck - higher speed more pressure up to a point were the pressure is worst than the added cooling because of more airflow
75 - more incoming airflow but more pressure under the truck
65 - less airflow but less pressure under the truck also

Is that what your idea was?

Yes.

Tough to say in your case, since 75 will produce more heat to overcome additional drag (a little bit more). But your observation is noted. And that 10 mph is a lot of added frontal pressure.

We do know that static P on the front gets higher with speed, normally that is. So something like an air dam, whick lowers undercarriage P, discussed on the OH thread, would have exponentially significant improvement, the higher the speed.

Funny, I did a search and couldn't come up with a single aftermarket air dam for the GMC. The Chevy stock config is a bit different, appears to have a lower air dam, but who knows. I also don't know which model has worse heatup tendencies. A poll should be conducted on that alone.

Fingers seems to have improved his flow with a makeshift dam. We'll see where this goes. The plastic guard underneath appears to improve the situation (when removed), and at a glance, it looks like it would "plow" air underneath, and perhaps increase undercarriage P.

I guess the trick is coming up with a configuration where the undercarriage P doesn't increase at the same rate as the front P, with speed changes.

Trust me, it's the fan. I thought it was the trans too. Apparently it's sensitive to air temperture, not engine temperature. I might go electric. You're right, it's very irritating.

I agree with Mcrat, the fan does kick in at those temps and makes a loud growling sound. No worries...

Mahalkita,

Looks like today we have some test results that prove there is no speed induced flow through the stack, so your observation is validated. The airflow through the stack at 25 mph is exactly the same as airflow at 70 mph. So, yes, you will have more fan actuation the faster you go, even empty.

The OH thread is coming right along. A simple air dam addition, or hood vent mod, and you won't have to listen to that thing any more. Fingers crude air dam double flow at 70 mph from its stock value.

Kendall69, it IS the cooling fan engaging. It can sound different at times and it does come on and off in a very unpredictable manner.

You can make it go away, but it will come on again, unexpected or not.

I promise you that it is the fan. It's just people's perceptions differ and you might swear that it's not the fan, because you know what a fan sounds like, and it sounds different. But the fan can sound weird, and well, it's just a matter of time until you will know that it is in fact the fan.

Later.

Before I go on and on about this subject, how many peopel have the engine "roar" or "wrrrr" sound when the temps are hot and at highway speeds.

I know some will chime in and say that it's "fan noise" when the truck gets hot, I say that it is not the cooling fan making the noise, but the tranny making the engine rev.

Thoughts?

I had the same problem. a wrrrr sound. Turns out it was the power steering pump. Dealer replaced and sound went away for a while but it's back again...

Update,

did the same run again Friday, this time I had a 4k camper on - out side temps 116 - 122 pe rthe mirroe temp guage ( I was going to take a picture of the 122 temp in the mirror for fun) but going 75 plus would have been a problem.

Anyway I just got the Helms manuals when I got home and sure enough, there "throttle angel" for what's it's worth.

The added concern unloaded VS loaded is the water temps shot through the roof, with the weight of the camper. I did not experience and water temps increase during the last run. Tranny temps still on the higher side.
Again no problems under 70, always on over 70. I can never make it come on, as it does it on it's own, but I can always make it go off, by various methods. One this time was just to floor it through the sound till either the last shift came of the noise stopped.
Again sure I think it's the fan coming on - but if it is the fan coming on why does it not do it's job nad "cool" the engine down.

Granted I am putting this thing through some of the most brutal conditions on the planet ( up steep long grades with 4k on it's back in 120 degrees plus) , BUT, if it's going to break I want it to break on GM's dime not mine.

After all the dealer never said don't drive at highway speeds during the hot part of the day with a load, when they took my 50K.

I will have to admit, it did beat me, I just gave up fighting it on the way home and left the wrrr stay on and on and on and on.

I'm goint t otry and cool the whole rig down with synthertics in the tranny ( transynd) and Delavac 1 - lets see what happens.

You need to read the last 10 pages of the overheat thread. You are in good company. Seems many of these trucks have airflow problems, some even appear to recycle hot engine bay air. That is likely the reason for the erroneous oat reading. The fan is cycling hot air that finds its way to the front of the stack, we have all but proven this. Kinda tough to cool a truck that eat's its own heat.

Find out if your oat is reading hotter at specific speeds. That will help narrow down if spped is the issue. I am guessing that it reads hotter as you speed up. Indicates that Hi pressure (hot) air from the engine is somehow getting back to the front of the grill area.

killerbee, I have been reading those threads, and for the first time my temp ran up to VERY high, because I had a load on. Prior to the load I just had the noise and the water temp didn't budge so I didn't think I was in that over heating camp ( now I am).
I', thinking of taking a video camera to document the temp situation, so when the stealer tells me " can not duplicate problem" I will show him the video, and re shoot the video every time I get it back from them not fixing a problem they can't fix.
Interesting how power was never an issue, I had all the power I needed - even blew away an empty Dodge, going up hill who after trying to pass me ducked in behind me, and only passed me when I backed off, and was at the top of the mountain, on level ground - so I wouldn't melt the block.

I just picked up my new Duramax last week. I live in Hemet Ca. (hot) I get the fan noise that you are all talking about. I was not sure it was the fan until I did a little test. Before I started the truck I popped the hood and turned the the fan by hand it spun very easily. I went for a long drive until I heard the fan noise, I stopped, opened the hood and tried to spin the fan by hand, It hardley spun and would not spin freely like it did when it was cold. It also sounds like my old suburban when the fan would kick in when it got hot.

Paleale

So far, in the short time I've had my truck, the clutch fan has only came on when I was pulling my 5er on a 200 mile jont. It would come on at slower speeds and off again at cruise... logical. But it is noisy!

Didn't you lose power as it heated up? What's "very high" we are talking 240+ ECT?

If so, your iat's were off the chart, per the tests we have done, and HP should have dropped precipitously.

I believe I read somewhere that the '05 has a computer controlled fan clutch rather than mechanical thermo control. BTW, there is no electric fan available that will deliver anywhere near enough air to cool this motor.

I believe I read somewhere that the '05 has a computer controlled fan clutch rather than mechanical thermo control. BTW, there is no electric fan available that will deliver anywhere near enough air to cool this motor.\

nope. It was slated, but scratched. The EV, electo-viscous, design appears promising, but not here yet.

You are correct about flow, only because the vehicle itself provides no speed induced flow, the fan provides it all. If that weren't the case, go electric.

I believe I read somewhere that the '05 has a computer controlled fan clutch rather than mechanical thermo control...
Hmm... I don't think so. I just went outside and checked and my (05) fan and clutch appear to be the same as my father's truck (04.5). Unfortunatly it looks as if GM is still using (stoopid) thermal clutches for these trucks. Come on guys you're using hydraulically actuated belt driven fans on the HMMWV why can't you adapt that system to be computer controlled on our vehicles. The computer we're using is more powerful than what put a man on the moon I think it can handle the additional data computation of a fan!

ohhh oww stung by the killerbee... doh oh well darn near duped your post there ):h

Someone needs to go get dirty looking at an 06. The shroud and intake have changed for sure, but the fan...could be a different design? Is the blade pitch/design different? Diameter?

how would you get an electrical signal to a rotating viscous fan?

Someone needs to go get dirty looking at an 06. The shroud and intake have changed for sure, but the fan...could be a different design? Is the blade pitch/design different? Diameter?

how would you get an electrical signal to a rotating viscous fan?

Connect 2 wires to the fan motor and 2 wires to the fan clutch....):h

why does my fan come on when it is 30 and snowing out? Or what my wife calls my jet.