Alternator Bad?

[rellis1962]

I have a whine in my radio when the engine is running. I installed a noise filter on the radio and that helped some but there is still a very noticeable whine. I think I remember reading that this indicates that something is wrong with my alternator. Is this correct? By the way, this is in a 1999 Donzi Z22, 5.7L

 

[havasuboatman]

Re: Alternator Bad?

Using resistor spark plugs will get rid of that noise. Also, don't wire your stereo on the same circut as your ign. (don't run it thru your key switch.)

 

[Bronc Rider]

Re: Alternator Bad?

Funny th noise filter didn't work, I have had good luck with them in the past. Is your battery not being charged? If its keeping a charge its not the alternator.

 

[bruceb58]

Re: Alternator Bad?

Disagree with previous post. Your alternator could still be keeping the battery charged and still have one or more defective diodes. With a bad diode in the rectifier there may be more AC getting by which would cause the noise.

 

[zbnutcase]

Re: Alternator Bad?

Agree with previous post. (bruceb58) 'nutcase

 

[Bronc Rider]

Re: Alternator Bad?

Disagree with previous post. Your alternator could still be keeping the battery charged and still have one or more defective diodes. With a bad diode in the rectifier there may be more AC getting by which would cause the noise.

This is definitely true, its just not common. I'm not a mechanic so I can only speak from experience. This is one of the issues I have heard other people having but, never to me. Its a possibility as some have mentioned.
You could take off the alternator and take it to get tested at an auto parts store.

 

[bruceb58]

Re: Alternator Bad?

This is definitely true, its just not common. I'm not a mechanic so I can only speak from experience. This is one of the issues I have heard other people having but, never to me. Its a possibility as some have mentioned.
You could take off the alternator and take it to get tested at an auto parts store.

So you don't think that if one diode was bad off of one pole of the stator that the alternator may not charge? I totally disagree! There are three poles and you could have two poles working perfectly and the other pole contributing a very large AC component.

Do you happen to know how the AC of an alternator is rectified to DC?

It may test ok at an auto parts store unless they test if there is any AC output of the alternator.

 

[bruceb58]

Re: Alternator Bad?

The whine you have in your radio may be helped by trying a different ground. As a test, run your ground all the way back to the battery. This is of course if it turns out not to be a bad diode in your alternator.

All alternators will have a certain amount of AC ripple. How high it is will vary slightly from one alternator to another. Using a handheld meter, measure the AC volts of various alternators(your cars) and compare to the output of your boat alternator.

 

[199q]

Re: Alternator Bad?

Not sure how to multi-quote but here goes:

The ground wire that was mentioned is a good place to start, along with wiring outside of the ign switch.

one thing to consider are your spark plug wires. I am relatively new to the boat world, but I have been working on high performance motors for awhile, and am about to graduate with an engineering degree, but I have seen plug wires that have so much EMI that it will keep the motor from running right.

I would check your wires and see if they are very low resistance wires. if they are, its a pretty good chance you are getting large amounts of Electromagnetic interference from them. this can come from a large amount of coil packs (not sure if they do that on boats, but this is done in high revving motors)

with the proper wiring, good shielded wires, and even a noise filter, you should be good to go. sometimes putting some shielding on your speakers head unit, and wiring can work too, but that's not really a cost effective option.

Check your charging voltage at idle and at cruising speed, and compare them to engine off key off. That will tell you how your alternator is working. A low voltage is just as much as a problem as a high voltage.

If this is a new radio that you just installed I would check your plug wires, if it is an older radio and has been used in the boat with no problems, then I would check the charging system.

 

[199q]

Re: Alternator Bad?

So you don't think that if one diode was bad off of one pole of the stator that the alternator may not charge? I totally disagree! There are three poles and you could have two poles working perfectly and the other pole contributing a very large AC component.

and that will toast your battery and electrical components in a hurry!!!

 

[Alpheus]

Re: Alternator Bad?

and that will toast your battery and electrical components in a hurry!!!

No it would not, but it will cause a whine in the radio. One diode out wont produce enough AC "to fry anything in a hurry" Some people run an alternator for a long time not knowing a pole is out...

and am about to graduate with an engineering degree, but I have seen plug wires that have so much EMI that it will keep the motor from running right.

Is "Running right" a technical term???

Unplug the alternator and give a short rev or check the charging system with a VOM and look for AC voltage. If the whine is still there I would look at your ignition system...

 

[bruceb58]

Re: Alternator Bad?

and am about to graduate with an engineering degree, but I have seen plug wires that have so much EMI that it will keep the motor from running right.

You have no idea what you are talking about.

 

[bruceb58]

Re: Alternator Bad?

and that will toast your battery and electrical components in a hurry!!!

Give me a reason! You aren't going to have huge voltage spikes, you are just going to have some ripple. If anything, you will see dips in the voltage. Most electronic devices are regulating the input voltage down before it uses it.

 

[JustJason]

Re: Alternator Bad?

Alright... Everybody is off track here EXCEPT for Bruce.

Every alternator outputs some AC right down the same cable the DC output travels down. Some alternators (depending on brand or age) are better than others. Diodes, epecially cheap ones, can leak AC through them.

A standard 12V alternator is delta wound, and uses 6 diodes in the rectifier. Diodes are what convert AC into DC by blocking half the wave. Diodes, like any other electrical component can be purchased in grades, a good, better, best if you will. Some manufactorers buy the best, some buy the cheapies when building their alternators.

(PRO TIP~ if anybody ever has a starter or alternator rebuilt at an electric motor shop, insist on high quality parts even if that effects the "flat rate" for rebuilding motors)

Now my rule of thumb is 250mv AC. Check it at the battery. (put your meter on AC MV scale) and then its positive to positive, neg to neg. If your under 250mv AC your fine. If your over 250mv AC you have failing/failed diodes in the alternator. You can always check it at the power wire going into the stereo itself, but usually the battery will suffice. Rev the engine with your meter on the battery, you should see the ac voltage to up and down with rpms.

You can check battery chargers this way as well. Cheap chargers, even sears/craftsman chargers, usually put out tons of AC on top of the DC.

On to troubleshooting your radio. If you get noise/whine through the radio AM/FM band and it goes away when on CD. It is more than likely (but not definitely) an ignition issue. Wrong plugs or wires or both. If you get noise on both radio and CD it is more likely (but not definitely) a bad alternator.

So do an AC ripple test and see what you get.

When I troubleshoot these things I have a cheap 1980's GE hand held am/fm radio that I bring with me. I'll run an engine and bring the radio with me to the engine compartment area. If that radio makes noise its usually ignition related. If that radio doesn't make noise but the boats radio does, I know its in the charging system.