Your BMW has anywhere from one to four V-belts running directly or indirectly off the crankshaft, located between the radiator and the engine. The number of belts depends on the model of BMW (e.g. a 1967 BMW 1600 has one belt, a 1975 BMW Bavaria four), and whether the car has power steering and/or air conditioning.
Sooner or later, all belts will wear, tear, fray, and, if not replaced, break. This article should help you with preventive maintenance - changing belts before they fail -- and with the emergency replacement of a broken belt.
Experienced BMW drivers always keep a set of spare belts in the trunk (check your owner's manual for sizes, then double check with the existing belts -- the manual could be wrong). For even if you find a mechanic who can replace a belt on a rainy Saturday night, chances are he won't have the correct metric-sized belt to fit your car. Metric belts come in sizes such as 12.5 x 850, which means the belt is 1 2.5-mm wide at its widest point, and 850-mm in circumference.
Even with the right circumference, a belt that's too narrow or too wide will fail prematurely. Too wide a belt will protrude from the pulley, while too narrow a belt will wear at its outside edges because the lateral force of the pulley against its sides will be excessive. Some American belts can come close in size, but won't be a precise f it.
Tools needed for belt changing include:
- Ratchet set and metric sockets
- Metric wrenches (your tool kit)
- Jack and jack stands if you will be working under the car
- Couple of screwdrivers
- and a piece of wood.
Wear heavy gloves when working near the radiator to avoid nasty cuts on the knuckles from the radiator fins. Also, it's much easier to change belts on a cold car, although you may not have a choice in an emergency.
First, inspect your belts for wear, and get in the habit of doing this regularly. Physical signs of belt rigor mortis are cracks, frayed threads, a hard, brittle condition, or even high mileage. Generally, the smaller the belt (in circumference) the less time it will last, as it's making more revolutions per minute compared to a longer belt. Pulley size also affects the number of rpm's and consequently the wear.
Inspect the belts with the engine off, then with it running. Sometimes belts simply need adjustment rather than replacement; as a general rule, there should be about 1 3-mm or 1/2 inch of play or "give" when you depress a belt midway between two pulleys.
If replacement is necessary, remove the old belt, only by hand, using screwdrivers to hold the portion you have removed off the pulley. Many shade tree mechanics have successfully "pried" a belt off; however, this may warp a pulley. Never cut a belt off if the replacement doesn't fit, you'll have a disabled vehicle.
Each belt has a point at which it can be loosened or tightened, and it will be necessary to loosen the tension before you can remove the belt. The belt from the crankshaft to the water pump and alternator is adjustable at the alternator; the belt driving power steering at the power steering pump; and the air conditioner belt either at the compressor unit or at the idler pulley (a free-wheeling pulley that doesn't drive anything). There will be one or two bolts which you can loosen to release tension on the belt, following which it can be slipped off. Later, in the installation, you will put tension on the belt by tightening these same bolts and moving the apparatus.
Disconnect the battery ground strap when working near the alternator. It's easy to go across the positive lead with a metal tool and cause a short circuit or fire. You may have to remove one or two good belts to get at the bad belt; in this case, be sure to remove the good ones extra carefully to avoid damage.
On late model BMWs, getting belts over the fan to remove them from the car can be a problem. The plastic fan blades can be bent slightly to do this watch your knuckles on the radiator fins. With the smaller belts it may be necessary to remove the plastic fan N it's held on with five or six bolts.
This is the hardest part of the job because new belts are stiff and hard to work with. So some mechanics' tricks of the trade are called for.
Some silicone spray on the belt will make it slide better. Get the belt as far as you can over the pulleys, but don't force it or it may break or tear. Bear in mind that BMW engines rotate counterclockwise, as viewed from the driver's seat. With the transmission in neutral, you may be able to turn the engine by hand to get the belt over the pulley. Or, you can put the car in fourth gear (if manual transmission) and push it slightly to move the engine. If your driveway slopes, let the car coast a foot or two in reverse gear to move the engine. Do not use the starter motor to move the engine. This has blinded more than one mechanic, and also results in broken fingers.
Some belts, such as air conditioner compressor belts, are easier to replace from below the car. Never crawl under a car supported only by a jack; use jackstands or something solid under the wheels; chock the back wheels and apply hand brake securely. A lift greatly simplifies this job.
This is the final step. A block of wood, a screwdriver or anything can be used to hold the apparatus containing the adjustment pulley in place while you tighten the bolts. Again, check tension 1 3-mm or 1/2 inch. Too tight a belt will wear out bearings on water pump, alternator, etc., while too loose a belt will squeal and can cause overheating, lack of electrical power, or lack or air conditioning.
If, after tensioning the belt, there is still excessive play, check the bushings (rubber mounts or sleeves that go around bolts) on the apparatus itself (alternator, etc.) or pivot arm. If the bushings are worn out, it will be impossible to achieve correct tension until they are replaced. Often worn bushings will cause fan belt squeal .
With new belts, check the tension again after a few hundred miles of driving to make sure it hasn't loosened. You'll find that the ability to change your own belts can be very handy for preventive maintenance or emergency service.
Authors: Dick Neville and Michel Potheau