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Thread: Question on New Fan Clutch

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    Question on New Fan Clutch

    Ever since my shop put on a new fan clutch (Behr I believe), it seems to be engaged right when I start the engine, and I get the "school bus" effect for the first few minutes of driving, then it goes away. This pattern is consistent and happens every time. But I've also noticed lately after the engine is good and warmed up, the electric fan will be running, but the fan clutch is not engaged (no "school bus" effect). No AC. Seems odd to me that a) the clutch is engaged when the engine is cold and b) the electric fan is kicking on but the fan clutch isn't engaged when warm. I don't seem to remember the fan clutch engaging right after start up like this. The whole thing seems backward, engaged when engine is cold but not engaging when engine is warmed up.

    Does that sound like a defective fan clutch?

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    If the fan clutch operation is normal, when first starting the car, the fan clutch should “clutch up” and an increase in noise and airflow should be obvious. After about 60-90 seconds, the fan will un-clutch and the noise and airflow will drop. The fan will continue to turn but at a much reduced speed. As the engine warms and the thermostat opens to regulate the temperature, the air thru the radiator gets hotter and the clutch will sense this, thereby increasing the speed of the fan to maintain a normal operating temperature.

    http://www.mwrench.com/Whitepapers/FanClutchtest.pdf

    The electric fan always runs when AC is on, usually in low speed and switches higher depending on temp sensor
    Last edited by shogun; 03-28-2019 at 11:14 AM.
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    Thanks much, at least I know the start up engagement is normal. I don't think my old clutch was doing that. Isn't it odd, though, that the fan clutch isn't engaged (at least I don't hear the "school bus" effect) and the AC isn't on, but the electric fan is running after the engine is all warmed up?

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    Quote Originally Posted by sienayr View Post
    Thanks much, at least I know the start up engagement is normal. I don't think my old clutch was doing that. Isn't it odd, though, that the fan clutch isn't engaged (at least I don't hear the "school bus" effect) and the AC isn't on, but the electric fan is running after the engine is all warmed up?
    I remember a couple of threads on this board where a fan clutch was replaced with a new one, and the new one was doing what yours is doing now, i.e., running high/engaged at start up. So, maybe, Yes, that is the way the new ones work. Hopefully one of those posters will chime in and let us know how it runs now, if it just needed a 'break-in' period.

    On the AC electric fan - do you have the defrost on? as that engages the compressor IIRC. Maybe try a cold startup with the HVAC system totally off from the previous shutdown.
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    That is normal: A thermal fan clutch is engaged on a cold startup because the fluid drains into the working area when the engine is shut off. The fan clutch will slow down shortly after startup as result of a pumping action produced by a difference in
    speed between the shaft and the body of a clutch. https://www.haydenauto.com/upload/Ha...gn%20Types.pdf
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    That all sounds normal. If you open the hood on a cold engine and spin the fan for a bit, it'll start to "loosen up" after the internal fluid starts to circulate inside the fan clutch housing. That's the same thing that happens when you first start the engine.
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    Quote Originally Posted by Ed CT View Post
    I remember a couple of threads on this board where a fan clutch was replaced with a new one, and the new one was doing what yours is doing now, i.e., running high/engaged at start up. So, maybe, Yes, that is the way the new ones work. Hopefully one of those posters will chime in and let us know how it runs now, if it just needed a 'break-in' period.
    The school bus at startup has been consistent and according to the helpful info posted by shogun is normal operating procedure, so I understand what's going on there now.


    Quote Originally Posted by Ed CT View Post
    On the AC electric fan - do you have the defrost on? as that engages the compressor IIRC. Maybe try a cold startup with the HVAC system totally off from the previous shutdown.
    I've been monitoring this closely since you mentioned it Ed CT. With the HVAC totally off, the electric fan has been coming on, but the fan clutch is not engaging, at least not to the point that the 'school bus' effect kicks in. So that still seems odd to me, shouldn't the main fan be operating at full-bore before the electric fan comes on, especially with the HVAC system totally off?

    The thermostat and coolant temp sensor were both replaced last June.

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    Quote Originally Posted by sienayr View Post
    I've been monitoring this closely since you mentioned it Ed CT. With the HVAC totally off, the electric fan has been coming on, but the fan clutch is not engaging, at least not to the point that the 'school bus' effect kicks in. So that still seems odd to me, shouldn't the main fan be operating at full-bore before the electric fan comes on, especially with the HVAC system totally off?.
    Hmmph. That the opposite of what I would expect. What happens after 3 minutes, and then after the engine is warm? I seem to remember reading that the electric fan will come on at first engine start as a test of the system. Does it go off after a minute or so?
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    The fan clutch dis-engages as expected a few minutes after startup. The electric fan is not coming on at startup. The electric fan is only coming on after the engine is well warmed up and have been driving for a while; my daily drive is roughly 10 minutes street>10 minutes highway>10 minutes street to home. The electric fan is running almost every day, when I'm just about home. And the weather has been pretty cool, 60s or lower.

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    Quote Originally Posted by sienayr View Post
    The fan clutch dis-engages as expected a few minutes after startup. The electric fan is not coming on at startup. The electric fan is only coming on after the engine is well warmed up and have been driving for a while; my daily drive is roughly 10 minutes street>10 minutes highway>10 minutes street to home. The electric fan is running almost every day, when I'm just about home. And the weather has been pretty cool, 60s or lower.
    That's normal behavior for the electric fan.

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    Just got home, electric was running full bore as usual. I was able to stop the main fan using the ‘newspaper’ test, so I really think the new fan clutch is defective.

    The electric fan used to only come on during very hot days with AC running and main fan in full school bus mode.

    Although it is apparently normal for the fan clutch to engage at start up, it just seems to me that it’s doing the exact opposite of what it should be- engaging when cold while not engaging when hot.

    I think I’ll see if I can’t get a replacement.

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    Behr used to be good in the 1990s. Not any more.

    Search different forums and you will see Sachs is the way to go.

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    Just one last follow-up for posterity. My indy originally put the fan clutch on because the car was already in the shop and it was expedient at the time to just have him do it. So I called him back, he says the operation I am describing is normal. I suppose my reference 'control group' data was biased since the old fan clutch was probably not operating normally, and I had just become used to how it was operating which made the new one seem odd. So case closed, I'm leaving the fan clutch on and moving on with life.

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    I recently replaced my fan clutch with a cheap ebay unit, and my electric fan still runs at full blast almost all the time. Didn't someone find an alternative fan/fan clutch to use recently that pushes more air?
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    the el cheapo ebay unit just might be your problem. have you tried the newspaper test?

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    Quote Originally Posted by mattmar1 View Post
    the el cheapo ebay unit just might be your problem. have you tried the newspaper test?
    Not yet. Been meaning to, just been lazy.
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    Quote Originally Posted by sienayr View Post
    Just got home, electric was running full bore as usual. I was able to stop the main fan using the ‘newspaper’ test, so I really think the new fan clutch is defective.

    The electric fan used to only come on during very hot days with AC running and main fan in full school bus mode.

    Although it is apparently normal for the fan clutch to engage at start up, it just seems to me that it’s doing the exact opposite of what it should be- engaging when cold while not engaging when hot.

    I think I’ll see if I can’t get a replacement.
    At 60ºF outside your electric Aux fan shouldn't need to come on at all.
    I still have my OE clutch fan that I removed when I installed a Flex-a-lite fan in place of the mechanical fan.
    If you want to try it out let me know.

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    Quote Originally Posted by Iamthebull View Post
    I recently replaced my fan clutch with a cheap ebay unit, and my electric fan still runs at full blast almost all the time. Didn't someone find an alternative fan/fan clutch to use recently that pushes more air?
    Why throw good money after bad? Seems the "cheap E-bay unit" is the issue here.

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    Ok one more follow-up. Yesterday was around 80 degrees ambient, had the AC running on the way home. The main fan clutch engaged and started school-busing after the engine was warmed up, for the first time since it was installed (besides at start-up) so I'm feeling better about it.

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