Alternator and fan belt slowly keep turning after the truck is off

Anyone experienced the alternator and fan belt slowly keep turning after the truck is off and the keys removed? My 04 3.4 just did this, kept creeping turning around for about a minute.

 

Welcome to TacomaWorld
Saw it happen on my 1954 Chevy stepside when the pulley broke off.
Kept turning till I disconnected the battery.

 

check your harmonic balancer bolt.

 

i can`t wait to see how this plays out!!

 

So your 54 is belt driven, the pulley broke off and you disconnected the battery somehow prevent the belt from rotating?

 

Was the truck shaking?

 

Since the water pump is driven by the Timing belt .

If some How the coolant pressure went really high from a Blocked radiator so the pressure could not vent to the over flow tank .

It could be a really long shot and I think a freeze out plug would fail before the engine turned over .

I would hope the OP knew if the truck was knocking or the starter was stuck engaged.

 

What I would like to know is, when the fan/belt is moving, is the engine still running without the ignition being on?

Maybe op see the fan&fan clutch spinning and think everthing else is still spinning with it?

 

Generator WAS belt driven till the pulley broke off, then it became a 12 volt motor driven by battery.

 

Truck was not shaking, totally off, keys out, engine off, engine only warm, the fan belt was turning really slow creaking with enough power to turn the alternator. I’m suspecting a compression issue, it doesn’t do it every time. Maybe a cylinder is under pressure and that is causing it to slowly turning the crank shaft until the pressure is released?

 

That's something I've never heard of, I would love to see a video of that.
Only time I've seen anything like that it was a manual that was parked on an incline in gear with the parking brake off.

 

Is the engine turning counter clockwise when you notice a slow rotation?

 

Yes counter clockwise from my view from the front bumper. And it is a manual it was on a slight incline in gear with parking brake on. The truck was not moving.

 

This is a good one.....
there's only 3 sources of power in that loop...

piston compression....alternator.....and lastly steering pump....

of these the alternator is only one that can put out any more than a quick poof...

Surely the water pump wouldn't have enough to go reversed...unless maybe super heated water vapor or whatever...

Does it make it difference if vehicle is running a while?

 

OP you been sippin’ on grandpa’s ole cough syrup huh?
All jokes aside, I’ve never seen this at all in my career. I’d definitely love to see a video.
Scenarios that I can think of that it could be possible.
-1 harmonic balancer/crank pulley failure of some sort. Rubber completely missing between balancer and crank pulley, magically not making any noise. Woodruff key missing from crankshaft, and the crank pulley bolt loose enough to allow the momentum of the fan and everything to continue turning after the engine has stopped. If this were the case, I think highly unlikely, but all the accessories and belts would continue turning, not just the alternator and fan. P/S and A/C as well.
P/S and Alt belt both drive the fan.
-2 belts so completely loose that it will not grab the crank pulley at all. It would take just as long for the alternator and fan to start turning which would probably make some kind of noise or something. Also the alternator probably wouldn’t be charging very well, if at all, and no power steering.
-3 most likely scenario for me is that the fan clutch is completely dry and just the fan continues turning after the engine has stopped.
You say that it does this with the truck in gear, stopped, truck not moving with the clutch engaged. It’s physically impossible for the crankshaft to turn, assuming the clutch is good, and the truck actually will move under it’s own power when running.
I’m very confused.

On the other hand, I have found a problem on a brand new F-150 that another dealership was unable to find. Concern was that there was a light “click click” noise and only happened 15-20 seconds after the engine was shut down. It ended up being the internal fan of the alternator not connected to the alternator shaft, and when it finally came to a stop it made the click noise. I replaced the alternator under warranty. This was before sprag alternator pulleys were popular.

 

"OP you been sippin’ on grandpa’s ole cough syrup huh? "
I seriously don't know what to make of this scenario and by all accounts a video of this bizarre act is in high order.

 

I’ve been a tech for a loooong time and never seen or heard of a symptom like this, only thing I can think of is that on shutdown the alternator is becoming a 12 volt motor and driving the belt around. I like the cough medicine theory as well…

 

https://youtu.be/i_lIYthFeLY

 

Alternators can not become motors!!

Generators can become motors.

 

https://www.dropbox.com/s/k2a43cobfhpffrq/IMG_0042.MOV?dl=0

Here is a link to the video. I’m pretty sure it was just the incline and not having the brake set tight enough. Thank you for all the insight. Keep the ghosts for Halloween now it’s time to get into that cough medicine for real.