What voltage at Idle??

[turbinedoctor]

I am trying to help a friend troubleshoot his GM 120 charging system. He is only reading the battery voltage at the battery when the motor is idling but is reading 13.5 at 1500 rpms. It has been sometime since I have worked on alternators but I am thinking it should be putting out 13.5 at idle also. We have the manual for it but I did not see it mention what voltage you should get at idle.

If it is suppose to be 13.5 then what would you look at first, regulator, diodes etc...

Durwood

 

[Rocky_Road]

Re: What voltage at Idle??

I am trying to help a friend troubleshoot his GM 120 charging system. He is only reading the battery voltage at the battery when the motor is idling but is reading 13.5 at 1500 rpms. It has been sometime since I have worked on alternators but I am thinking it should be putting out 13.5 at idle also. We have the manual for it but I did not see it mention what voltage you should get at idle.

If it is suppose to be 13.5 then what would you look at first, regulator, diodes etc...

Durwood

If you have a single wire alternator, then it doesn't 'kick' in until around 1,000+.

This is called an "exciter" system, and you will not see much (if any) below that threshold.

Happy boating!

 

[Don S]

Re: What voltage at Idle??

I am trying to help a friend troubleshoot his GM 120 charging system.

Not all 120's have the same system. Maybe you could give the year, and maker of your engine/drive package. Omc, Mercruiser, etc. maybe a year?

 

[turbinedoctor]

Re: What voltage at Idle??

As far as make and model all I know right now is Mercuriser MCM 120/2.5L and I think it is around a 1987 year. The manual did say to check current output at 1500 RPMs and does mention Excition circuit testing. Does any of this help?

 

[Bondo]

Re: What voltage at Idle??

Ayuh,... At start-up, at idle, it won't show a charge...
Once exctied, it should show a charge at All rpms...

 

[turbinedoctor]

Re: What voltage at Idle??

Ayuh,... At start-up, at idle, it won't show a charge...
Once exctied, it should show a charge at All rpms...

So you are saying at idle it is normal to read just the battery voltage and then when the RPM's are raised to read some where around 13.5.

If that is the answer to my question then it leads me to another question. What excites it and why does it need higher RPM's then idle. Just trying to figure out whats happening in there.

 

[EddiePetty]

Re: What voltage at Idle??

So you are saying at idle it is normal to read just the battery voltage and then when the RPM's are raised to read some where around 13.5.
​

Wiithout knowing the type of alternator you have, the answer is two fold.

With an alternator containing an independent excitation circuit, at start-up and including ideling, the voltage will gradually elevate from battery potential to 13.25-14.25 vdc, even at idle.

A single wire alternator system must see between 1000-1500 engine rpms before the alternator will 'self-excite' thus elevating the voltage from battery potential to 13.25-1425. After 'self-exciting', the alternator will continue to charge even at idle speed.

FWIW....Ed in 'ol Virginny

(Oh, and Durwood, thanks for the response to my GPS question!!!!)

 

[Rocky_Road]

Re: What voltage at Idle??

Have you confirmed that you have a single wire alternator?

Because everyone here is trying to tell you that a single wire alternator will not show any charging until the engine goes above 1,000...that is the nature of that beast.

If you have a single wire system...and you are going above 1,000...and you DON"T show around 13.5 volts...then you have an issue. Other than that, you are fine.

Happy boating!

 

[turbinedoctor]

Re: What voltage at Idle??

How do I tell if it is a one wire system.

My friend did tell me that the voltage would increase when he raised the RPM's but I did not ask if it went back down when he went back to idle.

I am at work right now and he has the weekend off, I will try to call him and get more input.

But if it is a one wire and the voltage raises with the rpm's and then drops back down, where would you look.

Thanks for the replies so far, I know it can be hard to offer suggestions in this situation but he knows less about this engine then I do and it's his boat. I like the outboards myself.

 

[turbinedoctor]

Re: What voltage at Idle??

Bump

 

[turbinedoctor]

Re: What voltage at Idle??

Bump

 

[Bondo]

Re: What voltage at Idle??

I know it can be hard to offer suggestions in this situation but he knows less about this engine then I do and it's his boat.

Ayuh,... From what you've stated in this thread,...
Your Friend has either,...
A),... Nothin' Wrong, No Problem,....
Or,...
B),... the need to find a Mechanic......

 

[turbinedoctor]

Re: What voltage at Idle??

Thanks Bondo. He has returned to work tonight and it appears we have another problem to look into before we persue the charging any further.

He has adjusted the timing to 8 deg BTDC and now it idles fine but hesitates and back fires when he advances the throttle from idle.

Thanks for all your inputs so far!