Fan always spins, not the shutterstat?

[sed6]

So my fan always spins. Read lots of threads and they seem to end with fixing the shutterstat. I was hoping that was my problem but it seems not. I disconnected the air to the shutterstat but the fan continues to turn. I can stop the fan with a 2x4 so the clutch isn't frozen. The TM says notify supervisor...Any ideas?

Thanks!

'83 M925 NHC250

 

[Rmtaunton]

Yup still frozen , it is still a clutch

 

[rhurey]

Its not supposed to spin a bit all the time? It always seemed too slow to matter much...

 

[sed6]

Yup still frozen , it is still a clutch

Ugh. Worth fixing now or wait for it to get worse?

 

[goldneagle]

It is supposed to spin all the time on the M939 and M939A1. The difference is that it spins slowly and can be stopped (with a 2x4). When it actually engages it makes a load noise that hurts your ear if you are close to it. It is a distinct sound that can be heard when you are driving and it kicks on. Mine only kick on when the engine is under load, like climbing a long incline.

 

[Rmtaunton]

939 or or 1, or 2?

 

[Rmtaunton]

A fan running is always .mmm how can I say but a great problem .

If it's an A2 it's shutterstat

An A1 or 939 ,

 

[sandcobra164]

It is supposed to spin all the time on the M939 and M939A1. The difference is that it spins slowly and can be stopped (with a 2x4). When it actually engages it makes a load noise that hurts your ear if you are close to it. It is a distinct sound that can be heard when you are driving and it kicks on. Mine only kick on when the engine is under load, like climbing a long incline.

I agree with goldneagle! My personal truck is a M939A0 but I have plenty of experience with A2 trucks and the fans operate very much the same. You will know for certain when it is truly engaged!

 

[74M35A2]

Disagree. On my 925A2, my fan does not spin at all when the engine is running, until called upon, then it sounds like a hurricane under the hood front. I think the bearings or springs are going bad in any air clutch fan that spins when not called upon. Also check for an air gap at the clutch. You could easily fit several sheets of paper or more in my fan clutch air gap. When my engine is running, you can just reach out and grab the fan blade because it is stationary. So much that I have even spun it backwards by hand with the engine running. It will eventually coast to a stop and not rotate forward, same as if the engine was off.

 

[Tinstar]

Disagree. On my 925A2, my fan does not spin at all when the engine is running, until called upon, then it sounds like a hurricane under the hood front. I think the bearings or springs are going bad in any air clutch fan that spins when not called upon. Also check for an air gap at the clutch. You could easily fit several sheets of paper or more in my fan clutch air gap. When my engine is running, you can just reach out and grab the fan blade because it is stationary. So much that I have even spun it backwards by hand with the engine running. It will eventually coast to a stop and not rotate forward, same as if the engine was off.

X2

Both mine do this

 

[Gunzy]

Is your truck getting hot? Go for a good drive and if you hear the fan engage and the engine temp stays in a normal range I don't think I would worry about it.

 

[sed6]

Thanks all. It's an A0. Seems to be some different opinions from experienced members, I like it, that means my question isn't stupid. It seems ideally it shouldn't spin at all, but since I can stop it with a 2x4 it might not be a problem, yet. What problem could it manifest into?

My truck doesn't get hot. But I'm not sure if the fan properly engages. I do get a loud sound when driving occasionally that lasts maybe 30 seconds to 2 minutes which comes and goes. This sound actually sounds like a terrible rattle, like rocks in can, that to me sounds like it's coming from the transmission. Me and the previous owner 98g couldn't ever identify the sound. Perhaps that's what a bad fan clutch sounds like?

Thinking out loud here, perhaps I should start the truck, supply shop air to the clutch see if the fan engages and then note if it sounds like a tornado or a can of rocks?

 

[M35A2-AZ]

I have a number of trucks that the fan turns a little when the truck it on, I think it is that the clutch just has a little drag in it.
When your up the speed you can tell when the fan turns on. At idle it does sound like a can of rocks sometime.

 

[goldneagle]

Disagree. On my 925A2, my fan does not spin at all when the engine is running, until called upon, then it sounds like a hurricane under the hood front. I think the bearings or springs are going bad in any air clutch fan that spins when not called upon. Also check for an air gap at the clutch. You could easily fit several sheets of paper or more in my fan clutch air gap. When my engine is running, you can just reach out and grab the fan blade because it is stationary. So much that I have even spun it backwards by hand with the engine running. It will eventually coast to a stop and not rotate forward, same as if the engine was off.

All my trucks are M939A0 and on all of them the fans spin some with the engine running. That is 4 out of 4. I have seen plenty of other trucks running and the fan blades were turning on all of them. Yes it does sound like all **** braking loose when the fan clutch activates under load.

 

[brasco]

Thanks all. It's an A0. Seems to be some different opinions from experienced members, I like it, that means my question isn't stupid. It seems ideally it shouldn't spin at all, but since I can stop it with a 2x4 it might not be a problem, yet. What problem could it manifest into?

My truck doesn't get hot. But I'm not sure if the fan properly engages. I do get a loud sound when driving occasionally that lasts maybe 30 seconds to 2 minutes which comes and goes. This sound actually sounds like a terrible rattle, like rocks in can, that to me sounds like it's coming from the transmission. Me and the previous owner 98g couldn't ever identify the sound. Perhaps that's what a bad fan clutch sounds like?

Thinking out loud here, perhaps I should start the truck, supply shop air to the clutch see if the fan engages and then note if it sounds like a tornado or a can of rocks?

You are asking the right questions. I had a similar experience on an A2 truck and thought a constantly spinning fan was normal. It is not normal on the A2s -See attached post below for my experience. I decided to buy a replacement fan clutch. I can't speak about A0 versions. Good luck.

http://www.steelsoldiers.com/showth...-Fan-Clutch-Repair-Any-Clutch-Gurus-Out-There

 

[98G]

I drove my A0 90 miles today. I just checked the fan and it's slowly turning. It kicked on and off normally on tje extended 6% grade.

Sed6's truck fan kicked on and off normally on the drive from Tucson to OKC.

I've never noticed whether or not fans on the 855 turn slowly when they aren't hot. Based on a sample size of 2, I'm I'm going to say it's ok..

I'll check 4 more trucks within the next couple of weeks and report back ..

 

[Scar59]

I drove my A0 90 miles today. I just checked the fan and it's slowly turning. It kicked on and off normally on tje extended 6% grade.

Sed6's truck fan kicked on and off normally on the drive from Tucson to OKC.

I've never noticed whether or not fans on the 855 turn slowly when they aren't hot. Based on a sample size of 2, I'm I'm going to say it's ok..

I'll check 4 more trucks within the next couple of weeks and report back ..

All my M939/NHC250 equipped trucks (4) operate the same. Fan rotates (slowly) while engine is running. The fan clutch engages at about 185* F, then the fan is engaged at really moves some air. My M813, well it's direct drive are roars all the time.

 

[ghitch75]

my 931A2 doesn't turn at all unless it gets to 185f then it spins....

 

[goldneagle]

my 931A2 doesn't turn at all unless it gets to 185f then it spins....

I guess that means it's engine specific. NHC250 VS 8.3.

 

[74M35A2]

The rattling when on is likely loose fan blade bolts. Mine made the exact sound you describe when activated. My fan bolts were rusted snug, but after breaking them loose, giving them some lube, and then retorquing them, the rattle went away. Suggest same to others. Break loose, retighten should fix it. You for sure don't want the fan getting sucked forward into the radiator and wiping it out.