How many times have you returned a new serpentine belt to a parts supplier because it squeals as badly as the old belt? We’ve all been there: we replace a squealing serpentine belt, which fixes the squeal, but the customer returns a few days later complaining that the new belt is as bad as the old belt had been, or worse, that we had done something wrong when we replaced the old belt.
Most of us blame the belt in such situations simply because "they don't make them like they used to, anymore", and so we remove the new belt, and claim a replacement from the supplier only to find that the second belt also begins to squeal after a few days. So where do we go wrong so often? If the truth were told, many of us do actually “get something wrong” during serpentine belt replacements, although we could argue that many aftermarket belts squeal no matter what we do. However, this is not a valid defence from a customer’s perspective, so in this article, we will look at why some serpentine belts squeal, and what we can do to either fix the problem definitively, or prevent serpentine belts from squealing in the first place, but let us start by saying that-
We tend to think that accessory drive belts, aka serpentine belts just drive things like alternators, water pumps, A/C compressors and others, but we often ignore or discount the magnitude of this task. Things like extreme under-bonnet temperatures, small bend radii, shifting load patterns as a result of changing loads and most importantly, the effects of billions of tensioner oscillations (over a belt’s lifetime) place extreme loads and strains on even the best drive belts that are available today.
Thus, if we say that drive belts are not what they used to be, it is because modern drive belts are several orders of magnitude stronger, more flexible, and more resistant to chemical attack by oil and fuel vapours than they used to be. In fact, when drive belts were made mainly from neoprene, few drive belts lasted for much longer than about 50 000km, even in the best of operating conditions. Moreover, the amount of wear on neoprene belts was always clearly visible, and it was easy to assess the condition of these belts simply by looking at the amount of wear on them.
Now, however, drive belts are made from a highly advanced compound known as EPDM, or ethylene propylene diene monomer-class rubber, to give it its full name. This material holds many benefits, including the facts that drive belts are now much stronger and have improved friction coefficients, which make for quieter operation, and render modern drive belts practically immune to chemical attack and degradation caused by dry rot.
However, there are downsides as well, the biggest of which is the fact that although modern drive belts made from EPDM typically have service lives of 100 000km or more, EPDM belts typically do not exhibit signs of wear (in the way that neoprene belts did)- even towards the end of their service lives. Moreover, while neoprene belts often started showing cracks about halfway through their useful service lives, EPDM belts, on the other hand rarely crack in the way neoprene belts did.
As a practical matter, the cracks in neoprene belts happened because the neoprene in the rubber compound degraded rather quickly, which dried out the compound in much the same way that excessive exposure to UV light dries out the rubber compounds in tyres. So, since the number of cracks in the ribs of neoprene belts served as a useful indicator of the belt’s overall condition, we often replaced cracked drive belts before they could fail catastrophically.
However, this valuable diagnostic is not available on EPDM belts, since the rubber compound does not dry out and crack. In fact, unless an EPDM belt is visibly frayed along its edges or scuffed on its smooth side, it is impossible to assess the condition of an EPDM simply by looking at it. This no doubt explains why we see so many snapped drive belts because the old-style wear gauges that worked so well on neoprene belts are all but useless on modern EPDM drive belts, which begs this question-
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As stated elsewhere, it is often impossible to assess the condition of these belts just by looking at them, unless an EPDM drive belt is frayed along its edges, which is always an indication that two or more pulleys in the system are misaligned or that one or more pulleys have some kind of damage on their outer edges.
The image above is a representation of the shape that the ribs on a new drive belt should have but given that this shape typically does not change much (if at all) over the life of an EPDM belt, it is tempting to think that the lack of visible wear on An EPDM drive belt means that the belt is still fit for service.
While this is true in some cases, it is not always true, so this writer would like to suggest that a better way of assessing the condition of a modern drive belt involves seeing the belt as a critical component of a system, as opposed to the belt being just a part or component that drives various accessories. As a practical matter, this system is common to all vehicles that use internal combustion engines, and while the number of components may vary, all such systems comprise at least the following parts-
So, if we consider all of the above to represent a single system, it becomes easier to-
The last point is particularly important because we often fail to consider the condition and operation of the drive belt or another component in the system as possible (and likely) causes of issues like persistent undercharging of the battery or above-normal engine temperatures. Consider the image below-
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The above images show two common conditions that can (and often do) affect the operation of a drive belt, even though the belt may be tensioned correctly. Note the shiny appearance of the ribs on the belt on the left. This is often caused by faulty tensioners that do not maintain the belt's tension, or equally commonly, by fitting a new belt onto worn pulleys. None of us would do this on purpose, but then again, if the belt is not squealing, and given that we often cannot see the working side of a belt clearly, few of us routinely remove a drive belt just to see if one or more pulleys might be somewhat worn.
The image on the right shows foreign matter embedded in the grooves, which can, and often does, lift some of the ribs of the belt out of the grooves in the pulleys as the foreign matter passes between the belt and the pulleys- even if the belt is tensioned correctly. As might be expected, this could (and does) cause the belt to slip on some pulleys, which not only affects the accessory on which the belt is slipping but also places extreme cyclical loads on belts that run on fixed tensioners.
It is particularly important to note that while a glazed EPDM drive belt (as shown on the left) may produce belt squeal under some conditions, embedded foreign matter (as shown on the right) in an EPDM belt will rarely (if ever) produce audible belt-squeal under any condition(s). Nonetheless, both of the above conditions have the potential to affect the operation of both the alternator and the water pump as the belt slips over the pulleys that drive these accessories. Again, as might be expected, this could produce symptoms like chronically undercharged batteries and/or persistent above-normal engine temperatures.
Of course, it is not always easy to distinguish between the causes and possible effects of something like a glazed drive belt, especially when the belt does not squeal, but this is where the benefits of seeing EPDM drive belts as critical components of larger systems come into the picture.
For example, if we are trying to diagnose persistently above-normal engine temperatures, and everything else seems to be in good order, it might help to remove the drive belt to inspect the pulley on all accessories. The object of doing this is to see if the ribs on the belt are “bottoming out” in the grooves in the pulleys; if both the belt and the pulleys are in good condition, the contact area(s) between both the ribs on the belt and the pulleys will not extend below about two-thirds of the depth of the grooves on either the belt or the pulleys.
If the ribs on the belt are bottoming out in the pulleys, both the drive belt and the affected pulley(s) are worn beyond an acceptable limit, and both must be replaced as matter of course. If, on the other hand, one or both edges of the belt shows signs of fraying or scuffing, one or more pulleys are misaligned, which is rare on modern vehicles except in cases* where bearings in alternators, belt tensioners, idler pulleys, or water pumps are worn or damaged.
* Or in cases where some accessories have been replaced with aftermarket parts of uncertain provenance.
This might sound self-explanatory, and even obvious, but if you are new to the car repair industry, you should know that-
So given that modern EPDFM drive belts typically do not exhibit signs of wear caused by misaligned pulleys unless the belt’s edges are frayed or scuffed, it would be a mistake to think that no visible wear on the belt is evidence that all the pulleys on the engine are aligned perfectly. Also, given that most modern engine compartments are so cramped that it is sometimes very difficult to see all the pulleys in the system, much less see marginally misaligned pulleys, how do we know that all the pulleys in the system are aligned or otherwise, as the case may be? Well, there is a way, so let us look at-
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The importance of ensuring that drive belts on modern engines run true on all the pulleys it passes over cannot be overstated, and the image above shows an example of a laser-based tool that makes it easy to verify whether all the drive and driven pulleys on the engine are aligned correctly, or not, as the case may be.
If you have never used such a tool, here is the short version of how they work-
The laser beam is indexed to the centerline of the tool's base, so if you index the base of the tool to the centreline of a pulley, the laser beam will be referenced to the centreline of the pulley as well. Therefore, if the tool is moved with a pulley as you rotate the pulley, the laser beam will (or should) fall on the centreline of the next pulley in the series of pulleys. It goes without saying that you need to remove the drive belt for this test.
However, the trick to using this kind of tool effectively is to check the alignment of as many pulleys as possible (or practical) from the pulley that you originally indexed to the tool to. For example, if you indexed the tool to, say the water pump pulley, you should try to gauge the alignment of the A/C compressor pulley, alternator pulley, harmonic balancer, and any other pulleys that are in the laser beam’s path from your original position on the water pump pulley. Doing this ensures that the alignment of all the pulleys is referenced to a single point, which in this example, is the centerline of the water pump pulley.
Of course, it is not always possible to reach all the pulleys in the system from a single point, but in such cases, the tool should be moved/ indexed to a pulley that shares a centerline with at least one other pulley, but preferably, two other pulleys. In some cases, it may be necessary to move the alignment tool several times, but in all cases, the alignment of all the pulleys in the system should be referenced to as few points as possible to increase the oval accuracy of the test, which brings us to-
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Ideally, a belt tensioner should be replaced along with drive belts, but in many cases, customers decline to pay for tensioners that are not visibly damaged, worn, or defective in some way. It is not for this writer to question what many customers are unwilling to pay for but on other hand, this writer has never passed up an opportunity to inspect drive belt tensioners thoroughly and to make customers aware of damage to their tensioners- damage that may not always be visible during a casual inspection.
Consider the image of the fixed drive belt tensioner shown above: in this case, the plastic wear surface had worn away to the point where the pulley is on the point of splitting into pieces, which will inevitably destroy the drive belt, and with it, possibly the engine as well as a result of overheating. Note the two red arrows in this image; these indicate wear indicators that become visible as the plastic wear surface wears down. In this particular example, the drive belt had been replaced three or four times but the same tensioner was re-used every time. In this case, the defective tensioner pulley was only discovered because the tensioner bearing had become noisy.
So, here is the problem with re-using fixed tensioners, and especially tensioners with plastic pulleys: you don’t always know what condition the actual pulley is in unless you remove the tensioner-
These checks are vitally important because, unlike automatic tensioners that can largely compensate for some of the above issues, fixed tensioners can suffer severe damage as a result of-
It is worth pointing out that while steel idler pulleys generally do not suffer damage as a result of the above points, it is mostly impossible to detect some or all of the above issues on plastic pulleys unless one removes the tensioner from the vehicle. So, as a practical matter, not performing this basic inspection could easily translate into severe damage to the drive belt and some accessories if the pulley should fail, or worse, severe or even fatal engine damage if the failed pulley causes the engine in a customer's vehicle to overheat.
Of course, automatic tensioners must be inspected as well, but the amount of belt flutter one sees when the engine speed changes suddenly is a good indicator of an automatic tensioner’s overall condition. The less belt flutter one sees the better, but even if no belt flutter occurs, there is still a need to inspect and verify the condition of the tensioner’s-
It is also worth pointing out that with only a few exceptions, drive belt tensioners cannot be repaired, serviced, refurbished, or rebuilt in ways that will magically restore a tensioner to the condition it was in when it was new. The only reliable remedy for a suspect or visibly damaged tensioner is to replace the old unit with at least an OEM-equivalent replacement. This is particularly important in the case of hydraulic tensioners, which usually require a kind of bleeding priming procedure to remove air from active hydraulic chambers in the damping mechanism.
Failing to perform such priming or bleeding procedures invariably result in the tensioner being unable to a) establish and maintain the proper belt tension and b) offer effective damping to eliminate excessive belt flutter. Both conditions usually lead to belt squeal and/or belt slippage, mechanical noise from the tensioner, issues such as persistently high engine temperatures or chronically undercharged batteries, and premature belt failure, which leaves us with this-
We hope this article has given you some new and actionable insights into modern drive belts and how to ensure that modern drive belts perform optimally throughout their very long service lives. On today’s vehicles, there is no reason why a drive belt should not last beyond 100 000km, but whether it does (or not) depends on how carefully we inspect and maintain the system of pulleys that drive critical engine components.
Put differently, we could say that an absence of evidence should never be taken as evidence of the absence of faults or defects in one or more components in the drive belt system. The only way to be sure that there are no hidden defects or faults in the system is to remove the drive belt from the vehicle, and to perform a thorough visual inspection on every pulley, bearing, and tensioning device in the system because it is no longer good enough to say that everything "looked fine". Chances are that something was not "fine" the last time we checked, but we do not want to discover this unfortunate fact only when a customer brings his vehicle back because his new drive belt is squealing- again.
