Only bad things happen when a serpentine belt, now known as a multi-rib V-belt system, breaks under the hood of a customer’s car. Paying close attention to the belt and all of the components that it touches will save your customers aggravation and increase your bottom line.
The system has a number of names: Gates calls it the Accessory Belt Drive System® (ABDS) and uses the name Micro-V® for its belts, Dayco refers to it as Automotive Belt Drive System and uses the name Poly Rib®, Continental/ContiTech calls its Poly-V®.
As a tech I have always referred to it as a serpentine belt. But these systems are all doing the same thing: moving power from the engine to the belt-driven accessories in today’s vehicles.
The drive belt system is something with which we are all familiar. For years almost all engine driven accessories depended on the simple single V-belt design, but that started to change 37 years ago. The V-belt system worked adequately up until then, but its weaknesses and limitations were starting to become apparent and they had to be addressed. Noises, vibrations and insufficient power transmission were just a few of the concerns of the V-belt system. The belt-driven power steering pump, alternator, cooling fan, A/C compressor, water pump and in some cases air pump and vacuum pump were all being stuffed into an engine compartment that was becoming tighter and more compact as a result of aerodynamic designs and smaller vehicle platforms.
Vehicle manufacturers needed a solution that could address the V-belt system weaknesses, but it had to be of a compact design, diminish noises and vibration, increase durability and be capable of transmitting the ever increasing power demands of the devices that it was intended to operate. Jim Vance created the serpentine belt drive or multi V-belt (as I will call it from now on) system while working for the Gates Rubber Co. It was the solution the automotive engineers were looking for, it eliminated many of the problems inherent in the existing V-belt system but it created new challenges that engineers and techs are still dealing with today. The 1979 Ford Mustang was the first vehicle that incorporated this new multi V-belt system, but this system started a revolution that continues to evolve.
Power transmission, durability and design
The multi V-belt has only one task to perform, according to Gate’s Robert Bassett: to transfer torque from the engine to the accessories that need to be driven. This torque is transferred from the crankshaft pulley via the multi V-belt to the pulleys that are on the driven accessories. As the belt is driven or pulled over the pulleys, the ribs of the belt are wedged into the pulley’s grooves, creating friction; it’s this friction that transfers the engine’s torque.
The wedging force is provided by the drive belt tensioner, either automatic, manual or by the belt design itself in a stretch-to-fit system. The multi V-belt allows engine and accessory designers to use smaller diameter pulleys on alternators and other accessories to increase the mechanical advantage by lowering the pulley ratio and increasing the belts arc of contact (the amount of degrees the belt stays in contact with the driven pulley). These features create less drive belt slippage, better fuel economy and more available engine power because of reduced engine loads.
The multi V-belt has to be manufactured from a material that will allow it to remain flexible under an extreme temperature range, deal with engine oil, coolant, road grime and a host of other things and still be able to bend over 100 times a second. While the first designed multi V-belts were made from a neoprene-based rubber compound, it worked well but it had an issue with cracking or chunking after about five years or 50,000 miles of use and needed replacement. This wasn’t the durability that the manufacturers wanted from their drive belt system, so the switch was made to an EPDM (Ethylene Propylene Diene Monomer) based compound to increase the life expectancy of the system to 10 years and 100,000 miles. EPDM isn’t the only material used in the belt’s construction. Aramid fibers, polyester and polyamide tensile cordares are just a few of the very proprietary bits and pieces that go into making the multi V-belt.
The stretch belt takes the drive belt system to the next step; it eliminates the need for a tensioner, lowers NVH (noise, vibration and harshness) caused by longer drive belts, allows better engine and accessory packaging and takes some of the load off the main drive belt system. Although they look very similar to a multi V-rib belt, the technology inside them is quite different. Stretch-to-fit belt technology is very proprietary. The material that they are composed of allows them to stretch over the pulley during installation. Some stretch belts use the heat from friction and the engine to activate (internally) the belt’s interior component structure. This locks the belt[‘s length and sets a permanent tension on the belt.
NVH
NVH is a major concern to the manufacturers and they realized that any belt-driven device is going to have some inherent NVH: it’s a reciprocating mass that is not perfectly balanced. One multi V-belt can now have as many as 10 contact points and eliminates the maze of brackets, required hardware and machining of the old V-belt systems. Eliminating these parts lightens the engine’s weight (lighter means better fuel economy) and lowers its NVH properties. But a multi V-belt’s construction also aids in lowering its NVH properties.
Jay Swope, regional commercialization center manager of belt technology for Dayco, states the following:
“Because of specific NVH (noise, vibration, and harshness) concerns and/or drives where the tensioner take-up is minimal, automotive OE manufacturers have decided to specify more and more accessory belts that incorporate aramid cord into its construction. This much higher tensile modulus cord helps to eliminate belt elongations, which will aid in dampening the NVH concerns of belt spans, and will ensure the belt length will remain stable so that the tensioner’s capability of providing the proper tension throughout the life of the belt is sufficient.”
But that’s not the only way that NVH is dealt with in the multi V-belt system. The use of the overrunning alternator decoupler (OAD), overrunning alternator pulley (OAP) and torsional vibration damper (TVD) are now widely used. These devices diminish vibrations, noises and extend the service life of the drive belt and its rotating components. A rotating alternator has inertia, and wants to continue to spin at the same speed, but during a sudden deceleration, or engine shut off, this inertial force is transferred back into the drive belt system.
The OAP uses a one-way clutch to allow the alternator to freewheel or overrun when the drive belt speed suddenly drops, diminishing the stresses induced into the drive belt and the alternator (think older Dodge Cummins diesel belt squeal on engine shut off — they lacked an OAP).
The OAD goes one step farther. It combines a one-way clutch to allow freewheeling and a torsional spring to absorb the vibrations that are induced by the crankshaft. Each time a cylinder in the engine fires, the crankshaft is sped up for a very short time. These cylinder firing pulses are transferred to the alternator rotor by the drive belt system. These pulses contribute to drive belt tensioner motion, belt wear and increase NVH in the system.
To counter these pulses and the issues they create, the manufacturers first turned to increased tensioner spring pressure and the use of a wider drive belt. Because the OAD pulley absorbs these base engine vibrations, the drive belt tensioner’s spring pressure can be reduced and the belt can be made narrower. This reduction in drive belt tension means that bearing wear in the rotating accessories is reduced and the entire system becomes more robust and dependable. The TVD absorbs the crankshaft vibrations and stabilizes the drive belt speed by the use of a special bearing and elastomer to lessen NVH, belt slip and extend the drive belt and its components’ service life.
Diagnostics, tips and issues
Just a 5% loss of material on the rib’s face of a multi V-belt or 10% slippage can result in a loss of tension, noise and increased heat from friction in the drive belt system.
The EPDM compound provides outstanding flexibility and good frictional grip over its lifetime, but it doesn’t show the cracks indicating wear as the neoprene belts did. These belts wear very similar to a tire, wherein the material is slowly worn off the surface of the ribs. This wear creates a number of issues that can often go unnoticed until the belt has failed or there has been a component failure.
Worn multi V-belt ribs go further into the pulley groove before they seat, reducing the wedge effect that creates the friction to transfer the power. This lack of friction creates heat, noise and loss of tension.
The rib wear also decreases the amount of clearance between the pulley and the belts ribs and can trap debris and water that could result in the drive belt slipping due to hydroplaning. If enough material is worn off the rib surface, the tips of the pulley’s grooves can actually cut or wear into the backing of the drive belt assembly causing a catastrophic failure.
Because the EPDM belts don’t show wear like the older neoprene belts did, most of the major companies have special tools and gauges to help identify the wear that may not be visible. Automatic tensioner wear is something that can’t be overlooked either. The life span for today’s EPDM belts varies, but with the average vehicle lasting over 10 years now, many still have the original equipment belt and tensioner installed.
The industry recommends the multi V-belt, the tensioner and other parts of the drive belt system be carefully examined after 60,000 miles and often replaced as a complete assembly (multi V-belt/tensioner/OAD) at about 90,000 miles. Remember that the pieces were installed new as a system and wear out together. Not replacing a tensioner when installing a new multi V-belt is just asking for problems.
Gates claims that many new or remanufactured alternators are returned to vendors for not charging (up to 70%) have nothing wrong with them. They won’t charge properly because the multi V-belt was slipping, worn out or not tensioned properly.
Stretch belt service has its own unique challenges. Stretch belts should never be removed and reinstalled; they are designed to be cut to be removed and special tools are intended to provide damage-free installation. Many stretch belts use a special curing process to maintain their tension and removal and reuse will compromise this feature, no matter what the vehicle manufacturer states.
Drive belt noises are a common complaint and pulley misalignment is the number one cause. Brian Wheeler, corporate marketing and communications manager for Dayco, says, “Noise is the No. 1 warranty/claim issue in both original equipment and aftermarket drive systems. Of course there are other failures that can and do occur, but solving a noise issue requires proper understanding and training. Installing a new belt will usually solve a misalignment noise chirp, at least for a short time. But once the fibers wear off from the belt’s ribs, the belt will no longer ‘slide’ down the misaligned pulley, but will begin to chatter, causing a rib tip vibration and ultimately chirp noise. It does take some investigative work to determine the cause of the belt noise, and then understand the necessary steps to correct the root cause, whether that is a tension issue, a worn bearing, a pulley misalignment, etc. This is the only way to ensure that the customer’s noise issue is resolved for the long term.”
Press fit power steering pump pulleys are a common cause of belt misalignment that causes belt chirp, but worn harmonic balancers, bad bearings in idlers and tensioners are also culprits. Misalignment noises are typically diagnosed by applying a small amount of water on the back of the belt. If the noise changes, you have to get to the root of the misalignment, as a new belt isn’t going to be the answer. Belt squeal is often caused by a lack of tension (failed/failing tensioner) but extreme belt wear, seized or seizing components or belt surface contamination (oil, coolant, other fluid leaks) can also be the cause.
Other noises can be created by a failed OAD, OAP bearing or broken TVD, but these are not the only issues. False misfire codes are now becoming common on a number of vehicle makes due to deteriorating or failed drive belt components.
The software monitoring the speed of the crankshaft can be affected by a bad automatic tensioner not providing enough tension on the multi V-belt allowing slippage or a torsional spring going bad in the OAP on the alternator.
These failures can induce false crankshaft variations that are detected by the PCM as engine misfires. The same goes for the vibration damper on the crankshaft. Most of these components now have a prescribed lifespan and are very often overlooked as causes of an engine misfire.
The multi V-belt system is the system of choice now, and for good reason. The design is compact, it reduces vibration, and is capable of transmitting the needed torque and power required to run the high output devices that today’s vehicles require.
But as techs we have to remember that this is a complete system.
The multi V-belt system was designed that way, and will age and wear out that way. Tensioners, special pulleys, and idlers may not show any outward appearance of wear and tear, but similar to today’s EPDM belts they are slowly wearing out as a complete assembly and need to be treated that way when it comes to proper accessory drive service. ■
Jeff Taylor boasts a 31-year career in the automotive industry with Eccles Auto Service in Dundas, Ontario, as a fully licensed professional lead technician. While continuing to be “on the bench” every day, Taylor is also heavily involved in government focus groups, serves as an accomplished technical writer and has competed in international diagnostic competitions as well as providing his expertise as an automotive technical instructor for a major aftermarket parts retailer.
To read more articles by Jeff Taylor, see:
Leak Detection: Smoke, Dye and Electrocin Methods
Brake System NVH Woes: Addressing and Avoiding Noise, Vibration and Harshness Issues
HFO 1234yf Refrigerant and Coolant Mapping
Some Diagnostics Hurt Your Brain: Fixing Difficult and Perplexing ProblemsMazda
