What you need to know:
The correct belt tension is a balance between being tight enough not to slip into the pulley channels, but not so tight that it puts undue stress on the pulley bearings, eventually causing them to seize or snap the tensioner mountings
Modern fan belts are remarkably tough and reliable. They seem to last indefinitely without stretching, perishing or cracking. Almost a fit-and-forget item so adjustment is correspondingly rare.
It was not ever this way. Fan belt failures used to be so frequent that most motorists constantly carried a spare, and theories abounded on what was the best substitute if a spare was not available. Women’s nylon stockings, which fashion later replaced with tights, was a leading choice. Sourcing this remedy became the topic of much bawdy folklore.
Clever chemistry then gave us universal stretchy belts such as very heavy-duty elastic bands as a quick temporary fix, but much improved materials on modern belts have largely put that trend out of business. And bawdy quips with any gender association are no longer admissible.
The correct belt tension is a balance between being tight enough not to slip into the pulley channels, but not so tight that it puts undue stress on the pulley bearings, eventually causing them to seize or snap the tensioner mountings. There is no handy machine or gauge to measure that, but there are wide enough margins between “perfect” and “still okay” for a test-by-hand to suffice. The traditional guide is to press down with your thumb on the longest stretch of the belt between pulleys. It should deflect about one centimetre at the centre point.
If it does not deflect at all, loosen it. If it deflects as much as two centimetres, tighten it.
Belts are multi-taskers
Belts on modern cars do a lot more work than their predecessors. On older cars, they only worked the water pump to circulate coolant, the generator/alternator to charge the battery, and of course the fixed radiator fan, which is how they got their name. Early cars travelled relatively slowly, so the fan, which sucks air in through the grille, was constantly essential to increase air flow through the radiator.
Nowadays, they also have to run the power-steering pump and often an air-conditioner, which is the biggest load of all, so many cars now have at least two belts and sometimes, three or more. The fact that most fans now have electric motors and most of the time are switched off by a thermostat, is scant off-set for the extra loads. Some steering pumps are electric powered.
Most of the pulleys involved in running all those elements are in a fixed position, and only one, (usually the alternator), is designed to move by loosening its locking bolts and sliding it closer to the other pulleys. It is then easier to fit a new belt into the pulley grooves before levering the alternator back away from the other pulleys until the belts are taut, and then retightening the locking bolts.
Getting enough tension requires a lot of force, sometimes needing the help of a tyre lever between the alternator and the engine block. This task is easier and more exact on cars that also have intermediate “tensioners”. These should always be well loosened before moving the alternator, and then used to fine-tune the tension after the alternator has been relocked.
The tell-tale sign that belts might be too loose and slipping in the pulley grooves is a screeching sound when the engine first starts or when the engine is suddenly accelerated. The warning that belts might be too tight is less immediate and less obvious. Most likely, it will be a new “rattle” noise in the pulley or tensioner bearings…a sign that they are beginning to fail.
Servicing pulley bearings
The hard-working bearings of pulleys and tensioners are usually described as “sealed for life”, but many can in fact be serviced, which helps them work more smoothly and last longer with a much less chance of failure.
Service can sometimes be done while they are still fitted to the car by removing the bearing caps and using a syringe and needle to ‘inject’ a thin lubricating oil through the edge of the rubber inner-seal into the bearing-race. If the bearing itself is removed, the rubber seal can be carefully removed with something akin to a metal toothpick and the bearing race can be packed with fresh grease before replacing the rubber seal and bearing cap and wiping away the excess.
Most sealed-for-life bearings are filled with a lightweight lithium grease during manufacture. Heavier duty grease will benefit vehicles used in harsh conditions.
Clearly, the efficacy of this extra attention is greatest for cars that operate in especially dusty conditions or often forge through deep water.
Modern V-belts are designed to perform well up to 100,000 km. Depending on the weather conditions you live in, some belts may even last longer. If you drive your car in a dry and hot climate, remember to park your car in a shaded area. Keep your vehicle away from direct sunlight and park it in a garage or under a tree. Whenever you are conducting maintenance procedures, make sure that you avoid spilling any coolant or oil on the belts to avoid damage and slipping.
It is also advisable to avoid overheating your engine to keep your belts in good shape. Your best bet is to inspect your belts at least once a month and watch out for signs that indicate wearing or damages. Keep in mind that the belts are essential in keeping your engine running. So, it is essential that you keep them in good condition. If there is damage, consult an expert technician and ask them if a belt replacement is necessary.