New Alternator

[crjflyer32]

So I have a 1984 Mercruiser 140 with a 12v 55amp alternator that has gone bad and either needs replacing or rebuilding. If all the holes match up and everything fits right, do you see a downfall to increasing it to a 110amp alternator instead? I will be installing a stereo system and fishfinder this winter and want to make sure that the electrical system can handle everything. As I understand it now, unless I am running about 3000 rpm's and under ideal conditions, I will not get the full 55 amps from the one that I have (provided it actually worked.)

What are your thoughts?
Thanks!

 

[Volphin]

Re: New Alternator

As long as it is a marine unit and uses the same wire hookups, you should be OK I think.

V

 

[larsonrider]

Re: New Alternator

I don't think that will work. I had to replace the alternator on my boat a few months ago. Once I got the new installed and everything I realized that I did not have any power to the igniton switch when I turned the key. After looking at an electrical diagram I realized I had a 50amp circuit breaker in the charging system. I am pretty sure that when my alternator shot it put a surge through the system and popped the breaker. And even though the breaker was the size of my thumb it still cost me $50.00 to replace. Might want to double check that before hand. Just my thoughts. Best of luck.

 

[Volphin]

Re: New Alternator

You could be right... I forgot about the CB.

V

 

[stonyloam]

Re: New Alternator

IMHO you are better off investing in battery capacity rather than the alternator. Add a second deep cycle marine battery and a switch, run your stereo, fish finder and other accessories off of that. Gives you emergency start capacity, and lets face it most of the time the stereo will be used with the motor not running anyway. Keep them both fully charged with a battery maintainer (all year if you can) and never worry about the batteries.

 

[sschefer]

Re: New Alternator

There is nothing wrong with upgrading your alternator except the additional HP it will consume if the load ever gets that high. The previous poster that replaced his Alternator and then found the bad circuit breaker probably replaced a perfectly good Alternator. The current rating of the Alternator simply means that if the demand is there it can produce the current to accomodate it. It doesn't mean that it produces it all the time. Go ahead and upgrade if you want. You can even put in a distribution panel and split off some circuits if you want.

 

[fishrdan]

Re: New Alternator

I have a 100 amp alternator on my 140 Mercruiser and it works well, $60+- from DB Electric. They don't have an idle output spec for this alt, though I guess it can put out around 50-60 amps at idle when needed,,, I charge my trolling motor batteries off it. If upgrading to a 100 amp alt, you should also upgrade the wire (orange?) between the alt's + post and the starter. The stock wiring is not designed to handle 100 amps and could overheat and melt the insulation, cause a fire hazard and potential to a bad wiring melt down. I used 6ga wire as I had some extra laying around. If there is a fuse link between the starter and alt wire, you should upgrade that too as the stock one (50-60 amp?) could pop if the alt is charging heavily. The new 100 amp alt had a smaller pulley than my stock alt, to spin it faster, so you will probably need a smaller belt than stock.

As the other poster said, you should also get another battery to run your stereo off of, so you are not draining your start battery playing the stereo. That and add in a battery switch or battery switch/ACR combo kit.

If you don't want to go through all the hassle of upgrading the alt wire and fuse link, just replace the stock 55 amp alt and run a second battery. I would add the second battery irregardless.

 

[FunInDuhSun]

Re: New Alternator

Since the OP's alternator is bad, now's a good time to upgrade it.
When replacing them on my twin-engine cruiser, I discovered the replacement high output alternators were a little less expensive than the originals because the HO's are more popular.

 

[bruceb58]

Re: New Alternator

You really don't need a circuit breaker in there. For example, Volvo I/Os don't have one. More than likely your car doesn't have one. If you have a circuit breaker and it pops for some reason, you just wiped out the regulator in your alternator. Just make sure you have the proper gauge wire to handle the current.

As far as how much current an alternator puts out at idle, most alternators can put out half their total power output at around 1500-1700 alternator RPM which is in the ballpark of equivalent engine idle RPM since the pulley ratios are probably more than 2 to 1.

 

[crjflyer32]

Re: New Alternator

Thanks for all the good info! I will definately be adding that second battery for the electronics. I had completely forgotten about changing the wiring from the alt to the starter to handle the increase in load. I'll let you know how it turns out. It's gotta be water ready end of next week.

 

[bruceb58]

Re: New Alternator

As I understand it now, unless I am running about 3000 rpm's and under ideal conditions, I will not get the full 55 amps from the one that I have (provided it actually worked.)

The 3000 RPM number you heard is probably the Alternator RPM. The engine RPM will probably be around 1500 RPM to give you full output.

 

[StevNimrod]

Re: New Alternator

It is my understanding that for a given case size, a loaded higher amp alternator will run hotter. Also, alternator output follows a curve specific to the alternator build; you really need to compare curves to get meaningful information about what's happening at a certain RPM to determine if the alternator suits your driving.

Depending on the stereo system; it sounds like you should be more interested in capacity. Especially if you're going to have a big system and fishfinder running while the engine is off - the alternator won't know, nor will it help, if it's not running. Point being, in some cases a "bigger" alternator isn't solving the actual problem of marginal capacity.

 

[bruceb58]

Re: New Alternator

It is my understanding that for a given case size, a loaded higher amp alternator will run hotter. Also, alternator output follows a curve specific to the alternator build; you really need to compare curves to get meaningful information about what's happening at a certain RPM to determine if the alternator suits your driving.

Very true that charge curve for every alternator is different and the same size case will be hotter if the alternator output is higher. In most cases where the function of the alternator is to recharge the battery and not to keep up with huge loads the actual time at peak output is pretty low. The current will taper pretty fast. That is why for most people unless they have DC refrigerators with huge house banks, the 55A alternator is sufficient.