The following article gives detailed instructions on replacing the air conditioning “stretch” style drive belt with a tensioner pulley on the N62 8 cylinder engine. Even though we have used a 2005 BMW 645ci to perform this installation, this article can be applied to any vehicle with the N62 8 cylinder engine and a stretch style AC drive belt with minor modifications.
A BMW N62 AC belt tensioner retrofit is a simple solution for those who wish to delete the “stretch” belt system from their 5,6 or 7 series vehicles.
We make it a point here at the Repair Guide to never question the logic behind BMW engineering. We firmly believe that the intelligent men and women who make the decisions on how BMW vehicles are manufactured have forgotten more about automobile engineering than we will ever know.
That being said, we will not express our opinion on BMWs move to a “stretch” style AC drive belt from a belt tensioner on many of their vehicles. We can only speculate that it was done for the same reason that many other car manufacturers have made the switch; to eliminate mechanical tensioners that need to be serviced.
The BMW AC stretch belt is a simple design that defeats the need for a tensioner pulley. It is comprised of a drive belt with an elastic core that allows it to be “stretched” around the AC and crankshaft (harmonic balancer) pulleys without the need of a mechanical tensioner. Since it is recommended that AC stretch belts are only installed once, removing one is easy; you just cut it off with a pair of shears. Installing a new AC stretch belt requires a special tool that is attached to the AC belt pulley. The tool will slowly push the new belt onto the pulley as the crankshaft is turned with a socket wrench.
The stretch belt sounds like a simple solution to avoiding an extra mechanical part that can wear out…but it has its drawbacks. Using the the AC stretch belt installation tool is not always easy; many home mechanics complain of damaged or snapped belts while trying to get them on the pulley. Another disadvantage is the fact that stretch belts cannot be reused. If for any reason you need to remove it to service the vehicle, you are stuck ordering a new one. And finally there is just the “old school” mentality of why change something that has worked so well and for so long.
Stretch belt enthusiasts argue that there is seldom a need to remove the AC drive belt on your BMW, and one less mechanical part that can fail is always better. At the end of the day, switching to a mechanical tensioner on your AC belt is purely personal preference…there is no right or wrong answer.
Whatever your reason is for performing a BMW N62 AC belt tensioner retrofit, the good news is it’s inexpensive and easy to do. BMW left the threaded mounting boss for the belt tensioner on the front of the engine, so it is as simple as cutting off the stretch belt and bolting on the new system. We give you a parts list below with everything you need to complete the repair. You should be able to make the switch for less than $50.
1. AC tensioner pulley (required) – These are the same AC belt tensioners used on other BMW models with the N62 engine, including the E65 745i (pre 10/2003) and the E53 X5. Both choices are high quality replacement parts at a reasonable price.
2. AC drive belt (required) – You must change the AC drive belt when retrofitting a mechanical tensioner to your vehicle. The stretch belt you are replacing is not the right size to work with the new tensioner.
3. Tensioner pulley mounting bolt (required) – You must purchase the tensioner pulley mounting bolt to complete the retrofit (it does not come with the above pulley). It is recommended that you only use Genuine BMW factory bolts on your vehicle.
4. Accessory (serpentine) drive belt (optional) – Since you will need to remove the accessory drive belt to complete this repair, now is a great time to replace it.
The main accessory belt must be removed first to access the AC drive belt which runs behind it. Now is a great time to replace your main accessory if haven’t done so recently.
- You will need to jack and support the front of your vehicle to install the AC belt tensioner (there isn’t enough clearance to get under the front of the car on your back). If you are not sure how to do this, please see our article BMW Jacking and Supporting for Repairs and Maintenance for more information.
- Starting from the top of the N62 engine compartment, identify the main accessory drive belt on the front of the motor.
- Use a small flat blade screwdriver or metal pick to remove the dust cap off of the tensioner pulley.
- Using a T50 torx bit and a long handle ratchet (or breaker bar), turn the tensioner pulley clockwise to release the pressure on the main accessory belt.
- With the tension released, remove the belt from the water pump pulley. Remove the belt from the engine compartment.
- Moving under the front of the vehicle, locate the AC stretch belt.
- Using a pair of shears, cut the belt off of the pulleys and remove from the vehicle.
Installing the new belt tensioner is simple…BMW left the original threaded mounting boss on the front of the engine when they switched to a stretch belt.
- Locate the threaded mounting boss on the lower part of the engine as shown below.
- The new BMW N62 AC belt tensioner comes “pre-loaded” in the open position. In other words, the manufacturer most likely installed a pin so you can simply loop the new belt over the pulley without releasing the pressure on the tensioner (see image below).
- Using a new M10-50 bolt, install the new belt tensioner. Hand tighten the bolt. Note – the BMW Repair Guide strongly recommends only using Genuine BMW bolts on this repair.
- Use a torque wrench and a 16mm socket to torque the mounting bolt to 47 Nm (34 ft-lb).
- Install the new drive belt on the AC and crankshaft pulleys.
- Using a T60 torx bit, turn the belt tensioner counter-clockwise just enough to retract the pulley and release the pressure on the locking pin. Pull the pin from the new tensioner (watch your fingers).
- Release the tensioner allowing the pulley to rest in its operational position. Check the belt to make sure it is seated correctly on both pulleys.
- Reinstall the accessory belt, making sure it is routed correctly. Pay careful attention to where it loops around the power steering and deflection pulleys…this can be confusing.
- Using a T50 torx bit, turn the accessory belt tensioner clockwise retracting the pulley. Loop the accessory belt over the tensioner pulley then release it into its operational position.
- Check the belt to make sure it is seated correctly on all of the pulleys.