Mercruiser alternator help!

[rrrr]

The alternator on my 7.4L Merc quit charging. Being the lazy guy I am, I just ordered up a new regulator, figured that was the problem. It wasn't.
So now I'm doing the Piglet thing (think think think :D). I seem to remember that the excitation voltage for a three wire alternator is sent from the ignition switch. If that contact in the switch is bad, no excitation and no alternator output.
Sound reasonable? I'm gonna check that tomorrow, I'm back home getting ready to go to a parrrrty.... :D

[rrrr]

Boy, how dooya like that.
A guy can get all the advice in the world on how to build a blown 540 CI stroker around here and no one knows anything about alternators.
Must be why all those flatties (mine included) just run a battery.
:D :D :D :D

[Havasu Hangin']

Uh...I hate to be a pessimist...
...but I hear more about those alternators going out than a contact switch. However, you can always test it with a voltmeter.
Heat...no ventilation...low batteries...marine alternators take more abuse than you think.
Getting it rebuilt is pretty cheap...but for $100 or so, you could have a new 100 amp marine alternator. IMHO, bigger is better in a boat alternator.

[rrrr]

I thought about just getting a replacement, but first I have to verify that the excitation voltage is there. Otherwise, a new alternator won't work either.
The regulator uses the excitation voltage to vary the field voltage. By changing the voltage, the strength of the rotor winding magnetic field reacts with the stator winding to produce the correct alternator output voltage.
The only other reason the alternator would quit is a short in the windings or a rectifier failure. That's usually a function of age and wear. The boat only has 150 hours on it so I don't think that's an issue.
I just need to get it fixed by next week because the guy I sold the boat to is gonna come get it. Then I just have to wait for the *new* boat to show up, prolly will be delivered in May sometime.
I took out the boat's interior to clean everything and make the engine work easier to perform. Yesterday when I was running the engine and laying on the floor messing with the alternator I noticed the &#Q_%#(@# seal on the engine block water pump is leaking. Guess I'll need to fix that too.
I'm really looking forward to having the 496 Mag on the new boat. Closed loop cooling is so much better for the engine. No dirt in the block and the coolant has corrosion inhibiters and lubricants. Lots easier to winterize too.

[Havasu Hangin']

Originally posted by rrrr
The only other reason the alternator would quit is a short in the windings or a rectifier failure. That's usually a function of age and wear. The boat only has 150 hours on it so I don't think that's an issue.
Or the diodes.
You already seem have all the answers...not sure what additional info you're looking for.
PS- I've seen those Merc alternators go out with less than 100 hours...but that from overheating and/or abuse.

[rrrr]

Originally posted by Havasu Hangin'
I'm a little curious why you are posting a question that you already seem have all the answers for?
Oh, I dunno. Bored. I was hoping to find someone that had found the ignition switch deal to be the cause. Right now I'm sitting at work doing some stuff instead of working on the boat.
I will prolly be able to determine what is going on in a few minutes once I get back out there. My place out at the lake is 30 miles from home so sometimes I just don't feel like driving out there.

[Just Tool'n]

You are right no 12 volts to excite the field, & no-charging.
A quick test to do is put a volt meter on it. measure output as hooked up. If no out put, do the following.
Get a remote starter switch, Snap-On part # MT3202A, Put on clamp on the battery, the other end on the back lug of the altenator, start the boat, now press the remote start switch for just a second. Disconnect the remote start switch, now take volt meter & mesure voltage from back lug of altenator. If it is now producuing voltage you have figured out your problem. No 12 volts to excite the field windings of the alt.
Also you can take it off of the boat take it to an altenator shop, have them bench test it, & they can tell you both voltage & amprage output of the unit.
Hope this helps!

[kevnmcd]

rrrr- I just went through this a couple of months ago. My alternator quit charging, so I thought I would replace the voltage regulator. Nope....that wasn't it, so I took it out of the boat and took it to a alternator shop. The guy took it apart and found that there was a wire that had broken loose on the diodes. Repaired it and is good as new. $60. :D