Understand how a serpentine belt drive system works (2024)

A Comprehensive Guide To Automotive Serpentine Belt Drive Systems

The serpentine belt is a crucial component in modern automotive engines, responsible for powering various accessories that are essential for the vehicle’s operation and the driver’s comfort. Understanding how this system works, its components, maintenance, and common issues can help car owners ensure their vehicles remain in optimal condition. This article will delve into the intricacies of the serpentine belt drive system, providing a comprehensive guide to its function, maintenance, and troubleshooting.

Why carmakers switched from V-belts to a serpentine belt drive system

The older style longitudinal engines had plenty of

room between the engine and the radiator to install multiple “V” style drive belts. But when car makers moved to transverse engines, they encountered a space problem.

Engineers ran out of room to stack multiple “V” belts on the front of the engine. At the same time, car makers faced brutal competition from overseas and encountered pressure to cut costs and improve reliability.

The older “V” belt systems required constant re-tensioning, and customers complained about the chirping and squealing noises associated with belt slips.

Belt manufacturers addressed the problem with three innovations:

• Poly-groove multi-ribbed belt— A poly-groove belt transmits more torque than a V-style belt but drives components only on the ribbed side.
• Serpentine belt— A version of the Poly-groove multi-rigged belt that drives engine components from both the ribbed and back sides of the belt. Serpentine drive belt systems use an automatic spring-loaded tensioner to maintain the proper belt tension.
• Stretch belt— A Poly-groove multi-ribbed belt that drives components from the ribbed side and doesn’t need a tensioner. A special tool is used to stretch the belt enough to get it onto the pulleys. From that point on, the belt maintains the proper tension until it wears out.

Not all multiple groove belts are serpentine belts

But all serpentine belts have multiple grooves. Car makers often use a single poly-groove belt to drive a high-torque component. In those applications, the belt only drives the component with the rib side. In order to be classified as a serpentine belt, it must drive components with the rib and backside of the belt.

The role of a drive belt is to transmit engine torque to individual components like the alternator, power steering pump, A/C compressor, and water pump. Older drive systems used a “V” belt design, but newer vehicles use serpentine belt drive systems. A serpentine belt drives multiple components using both the front and back sides of the belt. It also has multiple grooves. However, not all multiple-groove drive belts are serpentine belts.

A Serpentine belt drive system is designed to dampen engine vibrations

In addition to driving accessory components, serpentine belts also dampen vibration. Engine torque isn’t developed in a constantly smooth fashion. Quite the opposite; whenever an engine cylinder fires, it delivers a torque pulse. So, the belt drive system transmits pulses similar to the rat-a-tat-tat hammer-like blows of an impact wrench.

In order to deliver each torque pulse without slipping, the belt must be tensioned properly, and the belt ribs must wedge into the pulley grooves. Belt wear, pulley wear, and inadequate tension can cause a belt to chirp and squeal. A worn belt can also create enough slip to affect the alternator’s charging ability and the compressor’s cooling ability. Failure to examine and properly diagnose the belt system can lead to needless alternators and replacement of A/C compressors.

How to assess neoprene poly-groove and serpentine belt wear

From the early ’80s until the mid-’90s, polygroove and serpentine belts were made with neoprene rubber compounds. The ribs crack as neoprene belts encounter stress from repeated torque pulses and high engine heat. Count the number of cracks in a single rib in a 3” section to assess the condition of a neoprene multi-rib belt. Three or more cracks mean the belt has gone beyond the 80% mark, and it’s time to replace it. Neoprene belts wear out as quickly as 30M miles.

Starting in 1995, many manufacturers switched to belts

made from ethylene propylene diene monomer (EPDM) rubber. EPDM is far more resistant to engine heat and cracking and can run up to almost 100M miles.

How to judge wear on an EPDM serpentine belt

EPDM belts don’t crack like older neoprene belts, so assessing their condition is far more difficult. The mini ribs can wear to the point where the “V” ribs no longer wedge into the tapered pulley grooves. When that happens, the pulley engages with the top ridge of the belt. The tensioner can apply more tension, but it slips since the belt has far less surface area engaged with the pulley grooves. An EPDM belt can look fine, but if the ribs are worn, the belt will slip.

So you can’t judge EPDM belt wear just by a visual examination. You must use a gauge to actually

measure how much rib material has worn away. Gates and Dayco offer belt gauges. Or, you can download a Gates app for your smartphone. To use it, take a picture of the belt and the app can measure belt wear for you.

Check for alignment wear

In addition to assessing belt wear on the ribs, check the belt for alignment issues. Any side wear or fraying is a symptom of an alignment problem (see pivot bearing and idler bearing wear sections below). Also, glazing (shiny ribs) is always an indication of a tension problem. A properly tensioned belt will NOT show signs of glazing even if it’s worn.

The Role of the Automatic Belt Tensioner

In between each torque pulse, the belt and the spring relax slightly. With no dampening mechanism, the tensioner spring and arm would rebound after each power pulse and release tension.

So, serpentine belt tensioners also have a dampening mechanism to prevent rebound and keep the belt in contact with the pulley grooves. Over time, the dampening wears out, causing the tensioning arm to “dance” with each torque pulse, releasing and reapplying tension with each dance move. See this article on belt tensioner wear. If the dampening mechanism wears far enough, the belt can fall off.

Understanding how a Serpentine belt tensioner dampening mechanism works

Belt tensioners are built in two sections. The stationary portion is bolted to the engine and holds one end of the spring. It also contains the pivot dowel and pivot arm bushing. The movable portion holds the other end of the spring, the tensioning arm, and the tensioning roller. The movable portion pivots on the pivot arm bushing.

When the bushing wears, the tensioning arm and roller move slightly, causing the belt to be out of alignment with the driven components. A serpentine drive system can only tolerate up to 1° degree of misalignment before the belt makes noise. Beyond that point, each degree of additional misalignment generates up to 30° of heat.

So a worn pivot arm bushing that causes a misalignment of 2.5° increases belt temperature by 75° and dramatically increases the likelihood of serpentine belt chirp and belt squeal. As the bushing wears beyond 2.5°, the likelihood of belt jump also increases.

What happens when the serpentine belt and belt tensioner wear out?

• Belt chirp and belt squeal— This part is pretty simple: if your drive belt chirps or squeals, it’s due to a loss of tension, a worn belt, misalignment of the belt, worn idler roller, or resistance from worn bearings in a driven component. Belt dressing and lubricant will NEVER correct any of those problems. It may make the symptoms disappear for a while, but the noise will return. See this post and this post on how to find the source of the noise

• Reduce cooling and reduced charging— Any loss of tension caused by tensioner or belt wear affects the operation of all the driven components. A worn serpentine belt or tensioner can reduce A/C cooling efficiency by as much as 20%. The same applies to the alternator’s charging rate and the power steering pump’s efficiency.

• False trouble codes— The rebound oscillations from the worn belt tensioner and the lack of dampening effect can induce the power pulses into the driven components, causing the engine’s knock sensor to pick up false signals and set a P0325-P0334 knock sensor trouble code. If you or the technician don’t check the belt system, you can easily be misled into thinking the knock sensor is bad. If you do that and the code returns, you’ll probably think you got a bad sensor. This will repeat until you discover that the drive belt system is worn.

©, 2015 Rick Muscoplat