loose alternator belt - PeachParts Mercedes-Benz Forum

allenying · #1 02-18-2016, 06:38 PM

hi everyone,

i've been having some battery charging issues, which i think are due to the 2 alternator belts being loose. i thought it would be worth sharing the story in case others have this problem, or if anyone has any thoughts or similar experiences.

the battery is 3-4 months old. i noticed it wasn't charging that well, i had to get a couple jump starts, i thought it was because i left my light on accidentally, and it was single digit temperatures, and i didn't take it on long drives. but then even after an hr highway drive to charge it, the next morning i needed a jump.

so i charged it with a car battery charger on wednesdsay.
on thursday i used a digital voltmeter to test, it was reading 13.0v with the engine off, and 13.6v with the engine on or rev'd. as far as i understand i should be getting closer to 14.5v.

i thought it was the alternator, but just by chance my friend poked at the fan/alternator belts and they're super loose. we marked a line on them with a permanent marker to see if they were turning in sync when the engine was running, and for sure they weren't. so i'm pretty sure if they were tight, the alternator would still be working, or i'm hoping so.

now, i wasn't sure about this: i imagined loosening the adjustment bolt would make the space bigger, and tighten the belts, but it didn't work, and i'll try to tighten them next.

otherwise i'll have to bring it to the shop, when searching there seemed to be a bunch of kind of complicated issues with the bolts or brackets potentially breaking, need welding, etc.

SD Blue · #2 02-18-2016, 06:50 PM

The key to the tensioning bolt is you need to have the main mount bolt slightly loosened. Otherwise, you run the risk of bending something.

Also, the belts for the alternator need to be from matched stock numbers or one can be slightly different than the other in length. I ran into this issue several years ago when my car slung belts.

allenying · #3 02-18-2016, 06:59 PM

aha, i was very careful about turning it before since i was worried something could break,

would that be the nut at the bottom end of the bolt?

and would tightening the bolt tighten the belts? or loosen them?

thanks!

SD Blue · #4 02-18-2016, 07:26 PM

To help clear it up, the main alternator mounting bolts need to be loosened. The bracket is a tensioning bracket not a mounting bracket. In fact, the tensioning bracket and the alternator come out as a "unit". Where folks get into trouble, with bending and breaking, is not keeping this idea foremost.

After the alternator main bolts are loose, then adjust the tensioning bracket.

allenying · #5 02-19-2016, 01:47 AM

thanks,
i was just reading more on the forums, it sounds like someone 10 yrs ago also had loose alternator belts.

i was also reading that 13.6v was about normal for when the engine is running, at idle or higher rpms. i was also reading that 12.5v was normal for when the engine is off, i was getting 13v, so the small difference made me think the alternator wasn't doing enough, but maybe the battery alone was just reading 13v because i had charged it with a charger the day before and then had been driving it an hr or 2...? i'll take readings again in the morning. even if i'm getting the right voltage reading, i don't think the belts are supposed to be loose.

there's so much discussion about alternators, 55 amp, 65 amp, or even higher alternator upgrades. i'm guessing it's simplest to stick with 65 amp bosch at most, in the case that it is the alternator?

thanks!

allenying · #6 02-19-2016, 04:48 PM

ok i brought it to the shop and they tightened it, i really didn't want to break anything. without mentioning it's what i was getting, they confirmed that 13.6v was about normal for the older models, when at 2000rpm, since the alternators are not the strongest.