Our project for today is serpentine belt failure and how to prevent it. Probably since the late or mid 90s, most cars have gone to the serpentine style belt versus the conventional V belt. If you remember the V belt pretty much had 3 or 4 belts on the car, one to drive each component, such as an alter or power steering, air conditioning and other components.
What the serpentine belt has done, it has turned the project into one belt per vehicle. Sometimes there is a second belt for the air conditioning but it will have one belt that runs everything; water pump, alternator, power steering pump. What we have on the BMW is there is a second serpentine belt that runs the air conditioning compressor.
The advantage of the belt is they have extremely long belt life. So you hardly ever have to think about it and the nice thing is they typically have spring loaded or hydraulic loaded tensioners which keep the tension on the belt constantly not matter how much it stretches. Now on the old conventional belts, the V belt style it was mechanical tensioners which basically required you to as the belt stretched you had to get under the hood loosen the adjuster and tighten the alternator or whatever component was at risk.
The advantage there was the belt would start squeaking to tell you there is something going on under the hood. So that made you look at the belt, tighten the belt or replace the belt if it was worn. The downside of the serpentine belt with its automatic spring loaded tensioners that they all come with at about fifty or sixty thousand miles it still is just a rubber belt and what ends up happening is with the tensioner constantly applying pressure you never hear any kind of slippage.
So you are going to come across a belt that typically will look like new, but occasionally that belt will have a little fray coming off of it and I will show you that fray right here. There you go, that's the beginning of a failure of one type of belt. Then we have another common failure and that's just the rubber rotten of the serpentine belt and if you look at that what is happening right there is when that belt does a reverse turn what is going to happen is those V's in that belt, have a tendency to just crack and they will shred eventually.
So what's happening here with this unit is the tensioner will always keep tension on this belt, until eventually the belt just shreds and comes apart. If that belt does shred and come apart you are going to stop your water pump, your alternator, your power steering etcetera; the car literally can come to a stand still. Now another thing you want to keep an eye on are the pulleys that operate under this system. You have go the main crank pulley, it operates from anywhere from about 1000 rpm and ideal to 6000 rpm when you drive the motor.
If you look at the size of the pulley it is about six inch diameter pulley, you are looking at these idler pulleys, tensioner pulleys and what's happening here, they are about half the size. So these pulleys are traveling at double the speed of the crank shaft. So if you are driving down the highway at 3000 rpm those little pulleys are doing about 6000 rpm. At 6000 rpm you are relying on a small plastic wheel with a built-in bearing that built-in bearing is constantly running it has lubrication that was supplied from the factory one time. And what this bearing right here has done, it is a fail bearing, I will let you listen to it.
It sounds like a worn out skateboard wheel. At 3000 rpm for 2 or 3 hours that pulley will eventually fail and what will usually happen is the bearing will cease up and then the belt will shred because the alignment of the pulley goes all off. What you want to do is at about 50000 miles start looking at your belt and make sure you replace it. my recommendation is no later than 70000 miles replace the idler pulley and the tensioner pulley. These are really important and don't just do one because what will end up happening is you will replace the one that you see as bad assume that the other one will go a little further, but you don't want to start staggering the wear.
Here is a tensioner pulley right here. Now this one here doesn't have too bad of a bearing but you can still hear it. it will run like that for a while, it is absolutely dry, the bearing is still in good shape, but there is no lubrication in there. So what we want to do is we want to start off with two new bearings and a new serpentine belt to keep the car in good service over seventy or eighty thousand miles.
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