BMW 840 CI fitting new fan belt

It is fairly straightforward on the M113 V8s. I can't remember if that automatic tensioner has the 15mm socket spot on it just under the tensioner pulley or not. The tensioner is located on the driver-side lower but next to the power steering pump pulley. Move it from YOUR right to left, when facing the engine and remove the belt. Put the new belt on starting at the crank pulley. The rib side routes all the way around the idler pulley-from the crank shaft, Power steering pulley, top guide/idler pulley and then alternator on the lower right(passenger-side). The back or smooth side goes around the tensioner and water pump.fan pulley. I hope that helps.

One of the routine maintenance items that you should perform on your BMW is the checking and replacement of your accessory drive belts. The belts are driven off of the crankshaft and power accessories such as the water pump, power steering pump, alternator and air conditioning compressor. There are typically two belts on the car - one that powers the air conditioning compressor and another that powers everything else. Both should be checked periodically (every 3,000 miles, or when you change your oil), and particular attention should be paid to the main belt. The car can run fine without the air conditioning belt installed. Some of the early E30 cars use a standard v-belt design, and some of the later cars use what is known as a poly-ribbed belt (having many channels or ribs on the underside of the belt). The poly-ribbed belt setup utilizes a spring-loaded belt tensioner pulley that provides the proper tension for the belt at all times, making adjustment unnecessary. The traditional style v-belts need to be tensioned using standard types of clamps and tensioners. When inspecting your belts, the one thing that you want to look for is cracks (yellow arrow - Figure 1). If you see any cracks at all, you should replace your belts. The cracks will usually occur on the inside of the belt (the surface that typically rides on the surface of the pulley). With the poly-ribbed belts, this is the grooved surface. With the v-belts, this is the surfaces on the legs of the 'V'. With the poly-ribbed belts, replacement is a snap. The tensioners that hold the belt tight can be easily released using a socket and or 8 mm hex tool. Different tensioners turn different directions, so you may have to rotate the tensioner clockwise or counter-clockwise depending upon your particular car. The description of this process is one of those things that is difficult to describe, but very easy to do. First, pry off the small plastic cap that covers the tensioner (green arrow Figure 1). Then, place your tool into the tensioner and try rotating clockwise or counter-clockwise - will become immediately apparent how the tensioner releases the belt (Figure 2 and Figure 3). Removal of the two belts is easy - you do not have to remove the fan. Simply release the tension on the belt from the tensioner, and then the belt should simply slide off. Release the tension, and then you should be able to unwind the belt from the engine. The belt should be able to be maneuvered around and through the fan - you do not need to remove the fan to swap out any of the belts (Figure 4). It is important to note that if your BMW has air conditioning, you will need to remove this belt first, as it typically blocks the other belt. Another tip - if the belt is worn, simply snip it with some large tin cutters and pull it out of the car, after you have released the tension on it. Installation of the new belt is easy. Simply slide on most of the new belt onto the pulleys, release the tension on the tensioner, and slide the belt onto the tensioner. Check to make sure that the belt is securely seated in all of the pulleys. Verify that the ribbed portion of the belt is set against the crankshaft pulley. The proper orientation of the belt is shown in Figure 5. Replace any plastic caps that you may have removed from the front of the tensioner pulleys. Now, start the car and peek in at the belts. Verify that they are turning smoothly on all of the pulleys. For engines with the older-style v-belts, the procedure is nearly identical, except for the tensioning. The alternator is mounted on a bracket that rotates and is used to keep tension on the belt. In addition, there is a small rack-and-pinion device on this bracket that allows you to crank up the tension on the belt (Figure 6). The first step in setting or releasing tension is to release the nut on the back of the bracket that keeps the whole assembly secure. Do not attempt to turn the geared bolt without first releasing this nut on the rear - you will most likely damage the bracket. With the nut released, you can now turn the geared bolt counter-clockwise, releasing tension on the belt. Belt replacement is nearly identical to the poly-ribbed belts. Well, there you have it - it's really not too difficult at all

There should be a serpentine belt routing diagram under the hood somewhere. If not, all I can tell you is that serpentine belts wrap around 3/4 of the diameter of each pulley; and the ribbed pulleys contact the inside ribbed part of the belt and smooth pulleys wrap around the outside non-ribbed part of the belt. Also it cannot be installed wrong because it's length will be too long or too short if not wrapped correctly. Hope this helps!

Hope this helps.

Verbal description of the complete serpentine belt loop on a 99 Olds 3.5 V6:
This is from Benny 1345 (Answers.com) - off the top of my head it sounds right:
Top of CAM to top of ALTERNATOR - bottom of ALTERNATOR to top of nearby (front) PULLEY - bottom of PULLEY to top of A/C COMPRESSOR - bottom of COMPRESSOR to bottom of CRANK - top of CRANK to bottom of middle PULLEY - top of middle PULLEY to bottom of rear TENSIONER - top of TENSIONER to back to top of CAM.
Remember the serpentine belt has ribs on only one side, the ribs face AWAY from the two guide pulleys and TOWARD the pulleys on the Alternator, Compressor, Crank, Cam and Tensioner. You should tell by the pulley being smooth or ribbed as needed.

Locate the tensioner pulley; as you face the engine it is located two in from the right and midway down. It can be identified as a the smooth wheel with the inside ribs of the V (fan) belt facing outwards. In the middle there is splined (torx) nut and a black collar at its base against the wheel. An 8mm socket should fit on this splined shaft pretty well if you don't have the correct size of torx socket. Set the socket wrench at the 3 O'clock position and lift it, as if to undo the torx shaft. With a bit of effort the tension pulley can be lifted against its spring and in the process the belt will become slackened. Make a note of the pulleys and the direction of the belt traced around them. Lift the tensioner pulley and slip the belt forwards. Release the pulley and remove the old belt. Fit the new belt all the way around the pulleys, using your previous sketched layout of the belt th paas a guide, but leave the tensioner pulley until last. Re-engage your socket wrench on the splined shaft and as before lift the pulley against its tension spring. Slip the back of the belt over the tensioner pulley and release the pulley to put the belt back under tension. Note the order of pulleys on which you fit the belt is not critical. On some engine models the belt is first fitted to the tensioner and then fed around until a small pulley, such as for the alternator on the right hand side, is the only one not fitted. By creating a bit of slack by lifting the tensioner pulley it is possible to slip the v belt over this last pulley. Just as note sometimes the clatter from the diesel unit is not just the engine. The tensioner pulley has a damper unit fitted to it to stop it jumping around when the engine is running. The bushes for this damper can wear and the metal mounting collars can rattle loudly result. This can be checked whilst the engine is running by locating the damper and pressing a long handled screw-driver to the end of the damper. If applying pressure to the damper creates a change in engine noise then it is likely that the bushes are worn. If that is the case it can be replaced, I have tried fixing the bushes but nothing has worked for me. I mention all this as this is a good time to address this when replacing the the v belt. The plastic collar at the base of the splined shaft on the tensioner pulley can be prised off and beneath it there is a 15mm tensioner pulley retaining nut; obviously the belt needs to be off first. Removing the tensioner pulley allows access to the 13mm damper mounting bolt.

Follow diagram on the radiator housing. Put belt on harmonic balancer first then work you way around each pulley according to the diagram. It is a tight fit but it can be done. Get the belt on all the pulleys except the alternator. Use a 3/8" breaker bar or socket wrench to pull up on the tensioner and slip the belt over the alternator pulley. Install with ribbed side of belt to ribbed pulley, smooth side of belt to a smooth pulley. Each type of truck will have a different routing pattern.

There should be a diagram under the hood on the fan shroud or radiator support. If not, let me know and I see if I can find one. For the most part, if the pulleys it rides on are grooved, the belt ribs ride on it, it the pulley is smooth, the back side rides on it. From a picture of the alternator I have, from the passenger side coming up, over the alternator, then down and under the belt tensioner (smooth pulley), then up and over A/C compressor, then down and around P/S pump, up and around water pump, down around crankshaft pulley.

There is a sprint loaded tensioner (has smooth pulley) with an opening on the arm to insert a breaker bar or rachet you push to releieve tension to remove the belt....I think it is 1/2" drive.

Use the belt routing diagram in the engine compartment to install the belt....smooth side goes over smooth pulleys and ribbed side goes over ribbed pulleys.

I always save my old belt in case the belt ever breaks on a trip...