Fan clutch staying locked on.

[smdk2500]

Truck is 95 2500 6.5 TD

I'm 99% sure my fan clutch is staying locked on all the time again. I'm on my 2nd one in 2 years. If I remember right I've been using the Hayden severe duty ones. Can't find my receipts so hard to tell now. Its getting old shelling out 100+ bucks every year for a new one. What would cause them to stay locked all the time? I understand that they will roar for the first couple minutes of running from a cold start but with engine temps around 90-100 degrees it shouldn't be locked.

I've noticed a loss of power and mileage here lately and it seems like it started when I noticed the fan staying locked. With fuel prices the way they are I'm trying to do what I can to save a buck.

I cleaned my cooling stack right before winter and it is still clean. Has a few dead bugs in on the outside edge of the radiator but nothing major. Condenser is clean and fins straight oil and trans coolers have a few bugs but not much.
Is there something I am missing that would cause this or is it like everything else and manufactures are just turning out garbage parts now? If I trusted electric fans enough on a diesel I'd switch to them and be done with it but I'm not a big fan of them on trucks that didn't come with them from the factory.

Any help is appreciated

 

[Schurkey]

Is the "severe duty" clutch what is recommended for your truck?

It may be--I'm not that familiar with diesels. I know that guys toss Heavy Duty or Severe Duty clutches onto normal passenger cars even though Hayden specifically says not to. The issue is that a fan that doesn't move enough air will never properly disengage the clutch. The smaller fan spins faster than it needs to, makes more noise than it needs to, and absorbs more power than it needs to.

I'm guessing that there's two manufacturers of fan clutches: Those made by Hayden, and sold under ten different brand names that you'd recognize and respect; and the Communist Chinese knockoff sold under low-budget "house brands" or brand names you've never heard of before.

 

[smdk2500]

Is the "severe duty" clutch what is recommended for your truck?

It may be--I'm not that familiar with diesels. I know that guys toss Heavy Duty or Severe Duty clutches onto normal passenger cars even though Hayden specifically says not to. The issue is that a fan that doesn't move enough air will never properly disengage the clutch. The smaller fan spins faster than it needs to, makes more noise than it needs to, and absorbs more power than it needs to.

I'm guessing that there's two manufacturers of fan clutches: Those made by Hayden, and sold under ten different brand names that you'd recognize and respect; and the Communist Chinese knockoff sold under low-budget "house brands" or brand names you've never heard of before.

As far as I know yes the severe duty is what is recommend. Every parts store website that I buy parts from lists it as at severe duty. I tend to try and stay with brands that I have at least heard of. I'd go to a treasure yard and try one from there but the yards around here don't have many of these trucks with this engine and who knows if they are OEM anymore or not.

The clutch itself looks just about the same that is on my 95 with the 5.7. I'm sure there are differences but I am just trying to give you a idea of what it looks like. It's been a while since I've had the clutch off the 5.7 so i'm not sure on the external differences

 

[Schurkey]

Thermal

www.haydenauto.com

 

[tayto]

Fan pitch does not always dictate what duty clutch is used. I have seen HD rad, towing package and A/C with a composite fan blade and severe duty clutch in the later year gmt400s. Composite fan blade is under 2" pitch IIRC. I have one I'm swapping into my squarebody with serpentine setup I can measure it tomorrow. I went with a GM server duty clutch as it is the only one they still make.

 

[Abellmichael]

As far as I know yes the severe duty is what is recommend. Every parts store website that I buy parts from lists it as at severe duty. I tend to try and stay with brands that I have at least heard of. I'd go to a treasure yard and try one from there but the yards around here don't have many of these trucks with this engine and who knows if they are OEM anymore or not.

The clutch itself looks just about the same that is on my 95 with the 5.7. I'm sure there are differences but I am just trying to give you a idea of what it looks like. It's been a while since I've had the clutch off the 5.7 so i'm not sure on the external differences

Make sure you double check the gear ratio of the truck vs the ratio the clutch calls for. A local "green" auto parts store gave me the wrong one 3 times and my truck was doing all kinds of funny stuff. My gear ratio was a 3.42 they were give me 3.73 clutches I did 3 clutches in a week before I said screw it and bought one from my dads dealer friend. Food for thought I hope this helps.

 

[1998_K1500_Sub]

I have two positions on fan clutches:

(1) If the vehicle doesn't have AC, then as long as the engine stays at an acceptable operating temperature use whatever clutch you want. I once used the fan clutch from an 1995 S10 V6 4.3 on a 1995 K1500 V8 5.7 3.73 gears (clutch was a direct swap). The fan on the K1500 was 1" greater diameter and may have had more blades (I don't remember), but the S10 clutch sure quieted the fan down and the engine never had a heat problem (here in northern IL, where FWIW the truck was used as a grocery getter). The AC cooled well too, even when stationary, although I never checked the high-side pressures to see if they were, well, too high.

(2) If the vehicle does have AC, consider the HD clutch in order to (a) keep the high-side pressures lower and (b) provide acceptable AC performance. As noted in (1), a less "heavy" clutch may (or may not) be acceptable. Alternative: Use a "weak" clutch on the fan and put an aux electric pusher in front of the condenser to lower the AC high-side pressures / improve cooling (the "aux fan" option on the GMT400s is well known and those fans are still available on RockAuto... not the highest quality fans, but they're there).


My $0.02, from casual experiments on my vehicles. Others on the list may offer better reasoning.

 

[1998_K1500_Sub]

Thermal

www.haydenauto.com

Interesting to see the electric clutches offered in Hayden's catalog. Food for thought...

@Schurkey, Hayden's catalog indicates the 02-09 Trailblazer uses an electric clutch. Does yours and, if so, do you have any comments?

Does anyone know how they are controlled, e.g., are they "bang / bang" all-on / all-off, or are they modulated by the ECU using a PWM signal in proportion to heat load?

 

[GoToGuy]

Axle diff' friction clutches? 3.42, 3.73
Fan thermostatic fluid clutch

 

[Schurkey]

Interesting to see the electric clutches offered in Hayden's catalog. Food for thought...

@Schurkey, Hayden's catalog indicates the 02-09 Trailblazer uses an electric clutch. Does yours and, if so, do you have any comments?

Do anyone know how they are controlled, e.g., are they "bang / bang" all-on / all-off, or are they modulated by the ECU using a PWM signal in proportion to heat load?

Yes, my '03 Trailblazer has an "electric" fan clutch. When it failed, the fan was stuck "on" all the time. Noise is unbelievable. The fan is a molded-plastic deal, one-piece blades and "duct" surrounding the many blades. Looks very advanced and high-tech. And when stuck on, it really does affect fuel economy.

Photo from having removed the radiator to remove the A/C condenser; both replaced in the process of replacing a failed A/C compressor.

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Far as I know, it's on/off, not PWM. But I haven't read that service manual section, so don't take that as gospel. And PWM would explain why I never notice fan noise in normal operation. Or--maybe it's PWM to engage, so after a second or two, the thing is "on". A "soft-start" system.

I had a normal fan clutch seize in a '76 Nova. Again, heaps of noise. That lasted ONE trip home from work, and it got replaced. No way was I going to listen to that another day.