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I reckon the post found via search was mine.
I took a good hard look at that tensioner mechanism more than once when I've removed/installed alternators and I still can't work it out. The books say nothing!
Everything about it screams put the two fixed bolts in place and then do up the tensioner causing it to climb up the rack and tension against the belt, then lock it into place with its locknut and a second spanner.
Problem is that the bolts do up the wrong way and any attempt to apply any pressure results in the rack starting to disintegrate as parent poster found.
Given that time was not on my side when I did it first, the Bentley book remained silent and vehemently swearing at the unit did not cause it to fall into place, I had to improvise.
I needed a second pair of hands to do up the bolt whilst the alternator was tensioned against the belt, but I am alone when I work on the car.
The solution I came up with worked a treat.
When installing the unit back in the car, firstly push home the very difficult to locate lower bolt and then put its nut on (requires extreme wrist dexterity and a bit of tape on the other side of a ring spanner to get nut back on to end of that bolt). Only do up nut loosely so alternator can rotate about this bolt.
Insert second bolt that attaches to rack and leave loose so rack moves, leaving only the "climbing star" bolt mechanism to attach and tension the alternator against the belt.
Grab a very study blunt flat-bladed screwdriver about 14-16 inches long. Mine was nearly 1cm diameter so would not bend, a thin one is no good. Ancient tools from Grandpa's toolbox are better than modern rubbish.
Now, feed the screwdriver point first vertically down in front of the alternator, then twist it round to point towards where the gearstick is.
The end of the screwdriver will now be touching the engine block (good solid undamageable metal) near the oil filter canister.
Apply pressure to the handle end of the screwdriver to pull the alternator up towards the top of the wing of the car where you are standing, tensioning the belt.
If like me you had an ancient screwdriver, the handle will not be a hard plastic hexagon, it will be a fairly soft plastic and rounded or oval in cross-section.
When the alternator belt is tensioned, the screwdriver can be moved sideways in respect to the pressure on the belt. A couple of inches to one side will allow the handle to touch and jam up against the radiator. The combination of the soft plastic handle and the ribbed cheese-grater face of the radiator will grab the handle allowing you to release the screwdriver leaving the alternator tensioned.
(Clarification - you are pulling up on the screwdriver at this point, using the engine block as a fulcrum. One end touches the engine block, the far side of the alternator is halfway up the screwdriver and your hand is pulling the handle up and towards the top edge of the wing (fender).
Whilst holding this pressure and the belt tight, you move the screwdriver head left or right maintaining the same upwards pressure and jam the screwdriver head against the radiator. You can now let go and the screwdriver stays in place.)
How tight should the belt be?
Obviously using BMWs secret tensioner rack procedure a specific torque can be applied, on the M30 engine it's 33lb/ft. But, Mr Bentley must have stripped his rack and given up too, as he makes no mention of the thing at all, saying only "Remove upper adjustment bolt" and the utterly unhelpful "installation is the reverse of removal".
Back to the real world, you now have a 'driver tensioned belt and both your hands free.
Grab and wobble the other two belts, then wobble the temporarily incapacitated alternator belt and see if they have the same amount of give.
If too tight or too loose compared to the other two belts, grab the screwdriver firmly, (remember it's gonna pull away with the full tension you applied when you release it), and move it sideways against the pressure to release it from the radiatior, then move it either up or down a couple of radiator ridges and lock it back into place.
When the belts match tightness, do up the tensioner rack medium loose, then do up the lower bolt tight, then the upper bolt tight, then the tensioner bolt tight.
Release the hounds... err... I mean screwdriver and you are done.
If you did it right, the belts are all under the same tension.
Note 1.
As the screwdriver I had was ancient, it has a soft plastic head shaped like a squashed bulb, oval in section. This not only didn't damage the radiator, it didn't even leave a mark. I have replaced one alternator and two voltage regulators in my E34 and the radiator is still unmarked after three invasive and rude uses of this technique.
Note 2.
By matching the taughtness of the other belts in the car, mine has done some 20k miles since this procedure was first done and has not suffered from either belt slippage nor alternator damage due to overtightening. Common sense should prevail when you do yours.
Note 3.
I'd love to know how BMW do it. I bet there's a special tool which pulls the alternator up! The Bentley book notes a special tool for the M50 engines which "faciliates holding the alternator shaft with a hex key whilst loosening and tightening the pulley nut."
Note 4.
Ignore the books and do up the cables at the rear of the alternator when the unit is still free and movable in the engine bay. It is damn near impossible to attach/unattach them once the alternator is fixed in place. The Bentley book does not cover the M5 specifically and there is no room to move with the huge S38 lump and it's intake system in the way.
Kind Regards
Ivan
P.S. I would post a picture but with the fan shroud and fan in the way on my car, I can't really show you with an image how it works.
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"Never enter Karussell when on the brakes! I have gone round there on the roof, I know what I'm talking about." ~ Sabine Schmitz
Last edited by IvanDias; 13th April 2008 at 04:03.
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