3 Reasons New Serpentine Belt Keeps Coming Loose

If the serpentine belt keeps sliding off in your car, it is usually caused by pulley misalignment, a defective tensioner, or, in some cases, a seized component, such as a sized A/C compressor or alternator. The serpentine belt is very important to keep your car running. It powers many important components, like the alternator and power steering pump.

Common Causes

The three most common reasons a new serpentine belt keeps coming off are that a pulley is misaligned, a component is seized (e.g., a/c compressor, P/S pump, alternator), or the belt is the wrong size.

Here is a list of common reasons why a belt often comes off:

• Pulley misalignment
• Bad belt tensioner
• The alternator is not mounted properly.
• The A/C compressor is not mounted properly
• Broken mount or bolt for one of the components
• Sized idler pulley
• Excessive resistance from the alternator, a/c compressor, or power steering pump
• Wrong belt. The serpentine belt is too short or too long.

1. Pulley Misalignment

All the pulleys on the same belt are supposed to line up and be on the same plane. If one of the pulleys is shifted or at an angle, the belt tension at the misaligned pulley will cause the belt to start skipping grooves until the belt slides off completely.

Installing a new belt will not fix the issue without first fixing the misalignment issue. If you recently replaced a component such as the A/C compressor, water pump, or alternator, that component should be checked first, as it is most likely not lining up with the other pulleys.

Lastly, check the pulleys to ensure they are not loose. The pulleys should not move too much when they are messed with.

2. Seized Component

If you replaced the drive belt on your engine, be it a car engine or a mower deck, and the strap will not stay on, the problem may not be the belt, but one of the components may be locked up.

We recently replaced the serpentine belt on a Mercedes-Benz E320, and the belt kept coming off. We noticed that the old belt was cut and shredded to pieces, and the idle pulley was damaged. After we replaced the idler, we checked the rest of the pulleys to see if they would turn quickly. Of course, the A/C compressor refused to spin even though the A/C clutch was disengaged when the engine was off.

This is a common problem if the car has sat for several months or years. The bearing for the alternator, idler pulley, A/C compressor, or tensioner starts to rust and seize up, leading to an excessive load on the belt and causing the belt to come off.

If you install the belt and the belt breaks, check the pulley, as it is most likely stuck or jammed.

3. Weak Tensioner

The tensioner pulley is a pulley in the serpentine belt path that is used as an adjustment pulley to increase or decrease tension and put on the belt as needed. When this is out of alignment, or the tensioner pulley itself is loose, the belt may fall off. We would recommend having the belt tensioner serviced by replacing it.

A serpentine belt must have tension to keep it from sliding off the pulley. These components, like the belt tensioner springs, wear out over time. Once they do, they will not be able to put enough pressure on the belt, making it slide off.

Conclusion

A loose pulley may be out of adjustment, causing the belt to come off. You will need to fix the underlying problem, or the belt will come off again. If you put the belt back, it will just keep coming off.

If you have checked the components mentioned above, you need to check the belt length next. It is not uncommon for the belt you ordered to be too short or too long. Installing a serpentine belt that is not the exact length required by the car manufacturer is another common reason a belt may come off.

Even though this article focuses on car engines, these serpentine belt problems affect anything with a motor, including Craftsman riding mowers. A deck belt usually comes off or jumps because the drive pulleys are misaligned or the belt is too big.

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