Unable to get the alternator belt tight

[Braxton]

So while performing some other unrelated maintenance on the engine I noticed that my alternator belt was extraordinarily loose.   I attempted to tightened it up as much as I could but even after moving the alternator all the way out on the adjustment bracket it was still to loose.    I replaced the belt with the handy overpriced spare that I got from Universal and surprise surprise it didn't fit any better.   The solution should be to simply move to a shorter belt but I'm worried I'm missing something.    To my knowledge the alternator is the stock alternator and all the pulleys are stock.   Why would the stock belt be to long?   Am I missing something or do I just go down to NAPA and get a shorter belt? 

(And when I sat to loose this is what I mean: well over an inch of deflection when pushed on and the belt flops around like a freshly caught fish when the engine is running.   It still seems to turn all the pulleys just fine but I'm worried about slippage when the alternator gets under load)

[Craig Illman]

Braxton - I think going down to NAPA to get a half inch shorter belt is your best plan. If that fits well, then go back again and get another for a spare. I'm not sure why your stock setup leaves things loose, but buying a belt from Universal probably isn't the best value anyway. It's not a proprietary part and you should be going for fit, form & function.
I think I've seen threads where people have recommended Gates "Green stripe" for a quality belt.

Craig

[Jon Schneider]

You'll find the proper replacement options here on the Knowledge wiki: [wiki]Replacement Filters, Belts, and Lubricants[/wiki].

[Braxton]

So how do I know if I have the new or old alternator bracket?

[Braxton]

Nevermind, I just found the bracket conversion on the project page (http://www.c34.org/projects/projects-alt-bracket.html).

I appear to have the new bracket.   Does the new bracket have a slightly smaller adjustment range then the old?   I'm thinking that this could be the source of my problem.   (The belt I bought I did so not knowing that there were alternate setups)

[Stu Jackson]

Help us to help you, like, what engine do you have?  With an '88, most likely the M25XP which CAME WITH the enhanced alternator bracket.  It was only the M25s that needed the upgrade.

For starters.  have you considered the alternator belt tensioner? http://c34.org/bbs/index.php/topic,3667.0.html (I know, it's too long, but if you get the right one this will help.)

What size belts are you using?  7390 or 7400?  Specificity does help...  These numbers are the length of the belts.  That's the key to your issue.

In Reply #21 on this thread, http://c34.org/bbs/index.php/topic,4454.15.html, John Nixon wrote:

Most alternators that die from over-heating will do so from a slipping belt. At higher rotational speeds and high loads, the slipping belt will heat the pulley and rotor shaft to many hundreds of degrees. These elevated temperatures will cause the front bearings to fail first, which then causes the whole front case and rotor of the alternator to get even hotter. I have seen an alternator rotor that got so hot in this situation that the solder melted out of the slip ring terminations of the field winding. Needless to say, this sequence quickly becomes a catastrophic failure of the alternator.

While increasing air flow to the alternator is never a bad thing, the real killer is improper belt tension. If you are running a single 3/8 inch belt and a high output alternator, you need to watch your belt tension like a hawk. The nifty belt tensioner tool in a previous article will become your alternator's best friend, and make tensioning your belt easy enough that you will be without excuse.

Braxton, I guess you know this already, but you need to let us know what change you made to the belt size before we can help you.  What else changed that would have made the belt so loose?  I know you said you just "discovered" it, and good for you on that one, but something had to have changed.  OR it's been this way for a long time.  

My regular computer is out in the shop for repair, so I can't send you an excerpt from my recent Tech Notes on my alternator replacement and belt selection.  As soon as the puppy comes back from the shop, I'll find it for you, but a shorter belt should work for you.  Buy a few different sizes and try them, return the ones that don't fit and then buy two more of the ones that do.

Only YOU can do the detective work, since we're not there to look over your shoulder.

[Ron Hill]

Braxton : Puzzling why after 20 years the belt is suddenly too long !?!
I'll guess that the too long belt has been of there awhile and the wear has now made it obvious!!  Why your replacement belt (spare) didn't fit is unknown !?!

As I recall your engine w/stock alternator needs a 390 (39 inch belt).  As previously mentioned, I get a few sizes of belts a 395, 390 and a 385.  You can take back the ones that don't fit.  Get a good brand such as Gates with the cooling notches - it'll last longer.

I also recommend a few other things.  Get a belt tensioning tool (many posts on this item/topic) and also get longer 1/2" bolts for the top (engine) and bottom (alternator) sides of the adjusting bracket arm.  It seems that the holes are threaded all the way thru, but Universal only used about 1/2" bolts? when 1" will fit.  That way you'll have more threads to hold (less strain per thread) and vibration is less likely to loosen
the bolt. 
A few thoughts.   

[Braxton]

So I'll try and answer most of the question.

1) Yes I have the M25XP.
2) I'm using the "stock" belt.   I'm guessing its a 7400 but unfortunately its on the boat and I'm not.   (Yes I'm kicking my self for not bringing one of the belts home with me)
3) The alternator belt tensioner won't do me any good, the issue isn't that I can't get enough tension within the allowed travel, its that I cannot get enough travel within the setup and therefore can't get enough tension.
4) I have circumstantial evidence that the belt has been slipping but I don't know for sure.  I haven't had any of what I would consider the classic signs of belt slippage (smell or noise) but the alternator is covered with some black gunk that looks to me could be belt sluff.    I also had one instance last season in which the alternator did not seem to charge correctly.     I don't have an amp meter to tell me for sure what's going on.   The boat is new to me as of last May and I know by now I should have discovered all the idiosyncrasies of the boat and motor but that simply isn't the case. I'm sure I'll have more surprises like this as I work my way through winter maintenance chores.

It seems to me that if I step down to a 10 mm shorter belt I'll be in good shape.   It'll be next weekend when I know for sure.   Although this could be an excuse for me to disappear from work a little early one day this week...

[Ron Hill]

Braxton : If your belt is a 7400 I can assure you that with a stock alternator it's tooo long. 
You are correct that you don't need a belt tensioning tool now with too long a belt, but you will with the correct size belt!! -- believe me!!!

[Braxton]

Thanks for the tip, I'll see if NAPA has one when I get my belts.