1981 300D Tightening Alternator Belt - PeachParts Mercedes-Benz Forum

zippyclown · #1 02-14-2011, 08:58 PM

I must be dim because I can't for the life of me wrap my head around how to simply tighten my alternator belt on my 1981 300D. Can someone outline this for me?

zc

vstech · #2 02-14-2011, 09:04 PM

SIMPLE.

get 17mm box end wrench, and loosen the three bolts/nut on the back of the alternator. (two bolts pass through the alternator, one attaches the adjuster bracket to the block bracket... )

spray penetrating oil of choice (I prefer Kroil) onto the 13mm adjuster nut on the long bolt off the top alternator adjuster bolt.

gently work back and forth the adjuster bolt until it's turning freely, then tighten the nut until the belt is tight.
then tighten up the three 17mm bolts on the back of the alternator.

vstech · #3 02-14-2011, 09:06 PM

also keep in mind that the belt is supposed to be 12mm thick at the top... if yours is 10mm or really thin, there is no way to tighten it, you will have to replace it.

Yak · #4 02-14-2011, 10:09 PM

If the '81 has the dual alternator belt pulley, buy two belts at once from the same source. It increases the chances of getting both belts tight.

Make sure you loosen ALL three bolts before you try to torque down on the adjuster. There are a few threads on different style adjuster bolts - the 90-degree ell, or the ratchet style.

If you have the 90-degree, it's possible that it's bent/scored/grooved from a PO or mechanic. If the 17 mm on the back of the alternator isn't loose, the adjuster won't do a darn thing - except get stripped or break.

vstech · #5 02-14-2011, 10:17 PM

agreed!

however, the 81 if stock, should be a single belt, 12mm... 1036mm length.

if you have a gear driven alt adjuster... somebody swapped for a pre79 bracket, should be the same belt though.

if you have DUAL belts, it could be 1000mm or 1036mm depends on what the pulleys were taken from. but the belts will both be 10mm NOT 9.5mm and they need to be a matched set from the same box!

zippyclown · #6 02-19-2011, 02:49 PM

Quote:

Originally Posted by zippyclown

I must be dim because I can't for the life of me wrap my head around how to simply tighten my alternator belt on my 1981 300D. Can someone outline this for me?

zc

Thanks! My car is stock, so these directions worked like a charm. Love how straightforward the Germans built this thing.

It was a b-tch to reach the bolt in the front... More awkward than anything... And the penetrating oil was a good tip.

Did a diesel purge while I was at it. I'd say it was a good morning.

Have a great weekend.

zc

GreaseNugget · #7 12-05-2011, 08:33 AM

I had a heck of a time trying to loosen the 17 mm bolts, particularly the one towards the center of the engine.

For those with turbos, start by removing the plastic U-shaped air tunnel from the front of the turbo. It'll open a lot more room up.

Brian Carlton · #8 12-05-2011, 09:07 AM

Quote:

Originally Posted by vstech

SIMPLE.

get 17mm box end wrench, and loosen the three bolts/nut on the back of the alternator. (two bolts pass through the alternator, one attaches the adjuster bracket to the block bracket... )

One sad commentary to this:

If you attempt to loosen the nut that secures the adjuster, from the rear, it is entirely possible that the nut tries to turn the "bolt" that is part of the adjuster. If this occurs, the very thin threaded portion of the adjuster (the 90 degree section) must take the counter-torque. Naturally, it cannot do this because it's just cheap soft steel. You can guess the result.

So, when this nut is loosened, another wrench is needed on the forward side for counter-torque. I'm not sure how this is easily done by one person.

vstech · #9 12-05-2011, 09:13 AM

Quote:

Originally Posted by GreaseNugget

I had a heck of a time trying to loosen the 17 mm bolts, particularly the one towards the center of the engine.

For those with turbos, start by removing the plastic U-shaped air tunnel from the front of the turbo. It'll open a lot more room up.

agreed. also, as stated at the top, the turbo's have two belts, and they should be 10mm, not 9.5mm american belts. continental belts fit correctly, and you should get a matched set from the same box/order lot.

vstech · #10 12-05-2011, 09:16 AM

Quote:

Originally Posted by Brian Carlton

One sad commentary to this:

If you attempt to loosen the nut that secures the adjuster, from the rear, it is entirely possible that the nut tries to turn the "bolt" that is part of the adjuster. If this occurs, the very thin threaded portion of the adjuster (the 90 degree section) must take the counter-torque. Naturally, it cannot do this because it's just cheap soft steel. You can guess the result.

So, when this nut is loosened, another wrench is needed on the forward side for counter-torque. I'm not sure how this is easily done by one person.

... hmm, I've never had that happen... I can totally see it happening though.
for me, the nut always pops loose without turning the bolt.
the bracket would be in the way to get a wrench on it I'd think...

Brian Carlton · #11 12-05-2011, 09:59 AM

Quote:

Originally Posted by vstech

... hmm, I've never had that happen... I can totally see it happening though.
for me, the nut always pops loose without turning the bolt.
the bracket would be in the way to get a wrench on it I'd think...

Yep, never had an issue for nearly 10 years.........until I went to tighten the belts after replacement..............

It's not an easy task............but the alternative is even more unpleasant.

Air&Road · #12 12-05-2011, 10:04 AM

Quote:

Originally Posted by Yak

If the '81 has the dual alternator belt pulley, buy two belts at once from the same source. It increases the chances of getting both belts tight.

Make sure you loosen ALL three bolts before you try to torque down on the adjuster. There are a few threads on different style adjuster bolts - the 90-degree ell, or the ratchet style.

If you have the 90-degree, it's possible that it's bent/scored/grooved from a PO or mechanic. If the 17 mm on the back of the alternator isn't loose, the adjuster won't do a darn thing - except get stripped or break.

This is a suggestion in the right direction, but not far enough. For two belt systems you need to buy a MATCHED PAIR! This might be foreign to younger parts people because these systems haven't been used in a long time. An old time parts guy will know how to match codes on belts to come up with a matched pair if they are not packaged as such.

It would be best to ORDER a matched pair. A properly matched pair will give better and much longer service.

Hope this helps.

Brian Carlton · #13 12-05-2011, 10:33 AM

Quote:

Originally Posted by LarryBible

It would be best to ORDER a matched pair. A properly matched pair will give better and much longer service.

Hope this helps.

Larry, Continental doesn't make any matched pairs anymore. You get what you get...........and the shorter of the two belts just stretches to accommodate............

Air&Road · #14 12-05-2011, 10:52 AM

Quote:

Originally Posted by Brian Carlton

Larry, Continental doesn't make any matched pairs anymore. You get what you get...........and the shorter of the two belts just stretches to accommodate............

Sorry to hear that.

In the old days (as everyone knows, I'm old enough to know plenty about them) there were codes on the belts that were used to match up belts for pairs. You could check the ones in stock and see if these codes matched to put together your own matched pair from available stock.

I don't know if these production codes are still on belts, but maybe this info will help someone.