[theladybug]

Over about a month, I heard a faint knocking sound coming from the engine. It sounded like it was coming from the serpentine belt. I bought my car with a full dealership history, so the serpentine belt it had when I bought it is months ago was still just 30k miles new. When the belt finally snapped a few weeks ago, I investigated.

I don't know much about engines, but I noticed the serpentine belt wraps around four wheels. Three of them are ridged, one of them isn't. The upper ridged wheel (the uppermost one, right next to the oil dipstick) spins easily to the touch, as does the non-ridged wheel immediately below it. The other two ridged wheels I could barely spin at all; I wrapped my hand around the back to move them but they were all locked up.

I think they're the problem, maybe they seized up and caused the belt to snap. But I don't know what makes them spin, or if they're supposed to spin as easily as the uppermost one does.

 

[Insightful2]

The top ridged wheel you deacribe is the water pump. It is supposed to spin freely. The non ridged wheel directly below that which is slightly smaller is the serpentine belt tensioner. It should also spin freely. The ridged wheel below that is the A/C compressor. That should spin freely when the A/C compressor has been disengaged, but it should be very difficult to move when it is engaged. The largest ridged wheel behind the A/C compressor wheel is the wheel connecting to the crankshaft. It should also be hard to move due to the compression in the engine.
It seems unlikely to me that any of these items caused your belt to fail. My first assumption is that the belt was overtightened from the earlier replacement, and that caused it to fail early.
It's possible I'm completely wrong, and maybe the A/C compressor being locked up when the car is off is what caused the belt to break early.
I'm sure someone else with some more expertise will chime in soon.

Best of luck,
Tim

 

[theladybug]

Hello Tim,

The ridged wheel below that is the A/C compressor. That should spin freely when the A/C compressor has been disengaged, but it should be very difficult to move when it is engaged.

Now that you mention it, I did notice in the minutes before the belt snapped that the AC wasn't getting very cold. It seemed to be blowing regular temperature air. Usually I only have it one degree below neutral since I'm sensitive to air conditioning, but I remember turning the climate control way down by about eight degrees, and it didn't get any chillier.

I thought at the time it was symptom, but could it be the problem?

 

[Insightful2]

I'm not an A/C expert by any means, but it sounds to me like your A/C compressor locked up causing the belt to be driven over the wheel at a high rate heating the belt quickly and causing it to fatigue. This could possibly cause it to snap from the heating and strain.
I would check to see if your A/C compressor has any problems. When the climate control system is completely off in the cabin, the compressor should be disengaged and spin freely. If that is not the case, I would suspect that you have a locked up compressor.
This may be a wrong assessment, but it sounds likely from what you have described.

-Tim

 

[theladybug]

Would turning the key once and hitting the climate control off button be enough to disengage the AC compressor? Or would the engine have to start for it to disengage?

 

[Insightful2]

I'm not sure on that one. My guess is that the engine would have to be on to disengage it, but I am not sure on that one. It's possible that the "on" position would be enough to disnegage it, but like I said, I'm not positive. I would not run the engine for more than a few seconds without your water pump being driven. If done, this could result in over heating and damage to the engine. For a few seconds wouldn't hurt it, but any longer period of time would not be best.

Someone will probably reply soon to say whether or not the engine has to be on to disnegage the A/C compressor.

-Tim

 

[larryd5]

I'm not sure on that one. My guess is that the engine would have to be on to disengage it, but I am not sure on that one. It's possible that the "on" position would be enough to disnegage it, but like I said, I'm not positive. I would not run the engine for more than a few seconds without your water pump being driven. If done, this could result in over heating and damage to the engine. For a few seconds wouldn't hurt it, but any longer period of time would not be best.

Someone will probably reply soon to say whether or not the engine has to be on to disnegage the A/C compressor.

-Tim

The AC compressor has an electric clutch that engages and disengages as the AC runs. The clutch is just a 12 Volt activated unit. With the engine off and no belt on the car you should be able to easily turn the AC compressor wheel. If not then you have found the problem. A locked up clutch on the AC compressor. Will need to replace the compressor to fix it.