Understanding serpentine belt drive systems
What does a drive belt do?
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 drives multiple components using both the front and back side of the belt. They also have multiple grooves. But not all multiple groove drive belts are serpentine belts.
In addition to driving accessory components, drive belts also handle vibration. Engine torque isn’t developed in a constantly smooth fashion. Quite the opposite; every time an engine cylinder fires, it delivers a torque pulse. So the drive belt system actually 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 a belt to chirp and squeal. But they can also create enough slip to affect the charging ability of the alternator and the cooling ability of the A/C compressor. Failure to examine and properly diagnose the belt system can lead to needless replacement of alternators and A/C compressors.
Why did car makers switch to a serpentine belt?
Longitudinal engine had plenty of room between the engine and radiator to install multiple “V” style drive belts. But when car makers moved to transverse engines, they had a space problem. Engineers simply ran out of room to stack multiple “V” belts on the front of the engine. At the same time, car makers were faced with 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 slip.
Belt manufacturers addressed the problem with three innovations: the “poly-groove” multi-ribbed belt that could transmit more torque, a serpentine belt that could drive components on both the ribbed and back side, and an automatic belt tensioner that maintained the proper amount of belt tension under all engine operating conditions.
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.
How to assess neoprene poly-groove and serpentine belt wear
From the early ‘80’s until the mid ‘90’s, poly-groove and serpentine belts were made with neoprene rubber compounds. As neoprene belts encounter stress from repeated torque pulses and high engine heat, the ribs crack. To assess the condition of a neoprene multi-rib belt, simply count the number of cracks in a single rib in a 3” section. Three or more cracks means 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 without to almost 100M miles.
How to judge wear on an EPDM serpentine belt
EPDM belts don’t crack, so it’s far more difficult to assess their condition. In fact, 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 since the belt has far less surface area engaged with the pulley grooves, it slips. Simply put, an EPDM belt can look fine, but if the ribs are worn, the belt will slip.
So you can’t judge EPDM belt wear with just 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) are 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 damening mechanism, the tensioner spring and arm would rebound after each power pulse and release tension.
So a serpentine belt tensioner also has a dampening mechanism to prevent rebound loosening 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 and releasing and reapplying tension with each dance move. See this article on belt tensioner wear. It the dampening mechanism wears far enough, the belt can fall off.
Enter the Serpentine belt tensioner dampening mechanism
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 starts to make 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 really 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 come back. 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 wear or belt wear affects the operation of all the driven components. A worn serpentine belt or a worn tensioner can reduce A/C cooling efficiency by as much as 20%. The same applies to the charging rate of the alternator and the efficiency of the power steering pump.
• False trouble codes
The rebound oscillations from 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 technican doesn’t check the belt system, you can easily be mislead into thinking the knock sensor is bad. If you do that and the code comes back, you’ll probably think you got a bad sensor. This will repeat until you discover that the drive belt system is worn.
You can experience the same false code situation with low power steering pressure, resulting in a P0550-P0554 steering pressure trouble code. It’s easy to see now how a slipping belt could cause a loss of steering pressure because of an slipping drive belt on the pump pulley.
And, the same worn belt/tensioner scenario can cause a B1969 A/C clutch magnetic control circuit failure or a B2119 A/C compressor failure trouble code. The PCM knows engine RPM and when the A/C compressor is engaged, the RPM sensor on the A/C compressor clutch must match the RPMs reported by the PCM. If they don’t, the PCM will set one of these two codes. Your first assumption might be that the clutch is slipping, and that’s certainly a possibility. But if you haven’t checked the drive belt and tensioner for wear first, you may be wasting a LOT of money on a working clutch or, worse yet, a working compressor.
What else causes chirp and squeal?
Any driven component that resists turning will cause the serpentine belt to
slip chirp and squeal. You’ll find at least one idler roller in every drive belt system with an automatic belt tensioner. Depending on the belt routing you may find as many as two additional idler rollers. If the bearings in those rollers wear, they’ll cause a drag on the belt that’ll cause noise.
©, 2015 Rick Muscoplat