Mercruiser 5.7 EFI Alternator bracket keeps cracking

[khe]

Background information: Three years ago, I noticed my volt meter at 12 volts while cruising. The issue was a broken terminal on the lead to the battery. Fortunately, there was no damage to the alternator. I noticed the mounting bracket was cracked through and the alternator was vibrating. I have no idea how long the bracket had been cracked prior to the battery lead terminal breaking.

I removed the racket, welded the broken piece back in place it, and reinstalled it. Everything was fine until yesterday. I cleared the pierhead into Lake Michigan and noticed the volt meter was at 12V. I shut everything down and inspected the alternator. The battery lead was broken off the terminal and the alternator mounting bracket was cracked again.

I repaired the terminal while on the water and the system was charging so we were able to complete our boat ride. Tomorrow, I plan to remove the bracket and weld it but can't help but wonder why it keeps cracking. After welding it, I may add some steel plates to reinforce it but there has to be something else causing the bracket to crack. I can't get the question out of my mind "Why did the original last 20 years but the repaired version barely makes it to three years?"

 

[Scott Danforth]

aluminum or steel bracket.

if aluminum, welded aluminum has a much lower strength

 

[khe]

It is a steel bracket - The power steering pump also mounts to the same bracket higher up on the engine.

 

[Scott Danforth]

look at the toe of your weld for a stress riser.

 

[khe]

I just removed the bracket. It fractured next to the weld. The lower alternator bolt (that goes into the engine block) was loose either from the vibrations when the bracket fractured or it loosened up causing vibrations that fatigued the bracket.

Now the battery is dead in my welding helmet....

 

[khe]

I made a simple weld fixture to keep the bracket surfaces parallel. It's welded up and primed, ready for phantom black paint.

 

[Scott Danforth]

phantom black paint is Ditzler DAR 9000, which is "Black". it is also the same as GM "Tuxedo black"..... also known and Rustoleum black, or any other Black

 

[Furbird]

Why not just buy a replacement bracket? Fairly certain they are the same amongst GM engines so you don't have to pay the Mercruiser mark-up if that is your deterrent. You might also want to get a pulley alignment tool and make sure that pulley (or some other pulley) is not out of alignment and causing unnecessary stress on your twice-repaired bracket.

 

[khe]

Why not just buy a replacement bracket? Fairly certain they are the same amongst GM engines so you don't have to pay the Mercruiser mark-up if that is your deterrent. You might also want to get a pulley alignment tool and make sure that pulley (or some other pulley) is not out of alignment and causing unnecessary stress on your twice-repaired bracket.

I looked at my Cadillac Fleetwood Brougham which has the same basic engine and the bracketry and placement of the alternator and power steering pump are not only in different locations, the support brackets are cast aluminum. Maybe I need to look at a Chevy truck in the junkyard and see if they are the same. The Mercruiser bracket is no longer available per their parts lookup site. New ones are available but I am attempting to determine why the original bracket failed before buying a new bracket.

I should purchase a pulley alignment gage to make sure everything is in alignment. .

The bearings in the alternator and power steering pump were fine when I checked them before installing the belt. With the engine running on hose muffs, I put a 1x1 stick against the alternator and there were some vibrations. The power steering pump has much less vibrations than the alternator. I think it is the design of the bracket that has the upper alternator mount cantilevered out. A gusset could be added to the bracket to close up the section and really increase the strength is needed.

I noticed the lower alternator mounting bracket did not have a lockwasher on the bolt to the engine block so I added one. It is possible the lower bracket bolt came loose and caused the alternator to vibrate excessively.