Fan belts on my 300d where a problem. Short lifespan and too many adjustments for tension. I used new continental belts. I also noticed I had to go about halfway on my adjustment to obtain normal tension with new belts.

This did not allow much more adjustment for normal seating in and stretching during the belt break in period. I noticed the outer pulley edge was getting thinner or less blunt than I expected .

To put it simply the pulley was worn. I decided to check the next size american standard belt for width. It appears to have been perhaps 1/16 inch thicker. I had to assume the v would be at about the same angle as the metric belts.. It appeared it was or close enough. The belts when installed even though a slight amount higher and wider do not contact each other edge to edge. If they did contact the alternator pulley might have to be considered totally worn out.

Decided on a 39 inch length uncogged belt. About 5.00 apiece at anyplace selling utility belts. Tried a 40 inch length but was going to consume too much of the adjustment tensioning it. The 39 inch belts require little in the way to install other than the back belt must be put on the crank pully and the alternator pulley with the belt behind the fan pulley. Otherwise you may damage the cords in the belt by forcing it. The front belt goes on easily.

The belt rides the groove with only about 1/32 inch above the pulley edge. After break in it should drop that 1/32 inch. I think the right size belts for this model in width were finding the bottom of the alternator puleys v and slipping. Requiring more than normal amount of tensioning adjustments. Also it was defeating the side traction component of the belt somewhat as well.

I suspect I am not the first to have this issue. I also preffer unclogged belts on the chance there are cords then running through the v area and just not along the top. Much less prone to stretching if stronger I imagine.

Not a rocket science issue either. Most of us have become accustomed to serpentine self adjusting belt setups. This 300d v belt drive is older and just thought I might comment on it for the younger members if you have or eventually have issues with your belts. Some of the old diesels have so many miles on them there must be a lot of semi worn pulleys that might benifit from wider belts.

As almost always there were clues involved. A pile of worn belts where in the trunk. On reflection I suspect one of the previous owners perhaps grabbed a used high milage alternator or a pulley was transfered from alternator to alternator without anyone noticing it was worn. This pulley eventually landed up on this car somehow. Or since the alternator turns much faster with it's smaller pulley than the fan or crank pulley. This was increasing the wear factor on it.

Excellent analysis.

Thanks.