Low voltage at idle.......voltage regulator????

[Nucknoilerfan]

1975 175 mercruiser with a 302. At idle the voltage drops to below 11 volts but gets back to 14 volts at higher rpm. Would this be indicative of my voltage regulator failing? How can I tell if the voltage regulator is internal on the alternator or external. I don't even know what one looks like. At first I thought my gauge might be faulty but my fish finder also alarmed in on under volts.

 

[81 Checkmate]

Re: Low voltage at idle.......voltage regulator????

Just a guess but is your alt belt tight, it could be slipping at low idle.

 

[Nucknoilerfan]

Re: Low voltage at idle.......voltage regulator????

Not many things that I'm sure of but that is one that I checked first. Seems nice and tight to me.

 

[elkhunter338]

Re: Low voltage at idle.......voltage regulator????

how fast do you have to rev it to get it to charge, then when you back the rpms down how low will it idle until it stops working.
In 1975 do you have a generator or an alternator?
Sometimes mine stops charging at 650rpms, then rev to about 1100 then I can back it off to troll at 800 rpms and it will stay charging.

 

[Nucknoilerfan]

Re: Low voltage at idle.......voltage regulator????

Without taking it out on the lake mine seems similar to your situation. At idle (650rpm) it shows low voltage and probably returns at over 1200rpm. Problem is when I want to troll at slow idle speeds I'm worried about the engine stalling and not being able to start. How can I tell if I have an alternator or a generator?

 

[bruceb58]

Re: Low voltage at idle.......voltage regulator????

Check your voltage with a multimeter at the battery to see what you are actually getting.

 

[lcmains1]

Re: Low voltage at idle.......voltage regulator????

. Would this be indicative of my voltage regulator failing? How can I tell if the voltage regulator is internal on the alternator or external. I don't even know what one looks like.

If the regulator is external it will be on the back of the alternator, if it is internal you will not be able to see it. I just had this same issue, had a belt slipping and the alternator housing was cracked. I raplaced the alternator ($98 for a rebuilt one) and all is well again. It really is amazing how much better the engine runs with the new one!!

 

[tswiczko]

Re: Low voltage at idle.......voltage regulator????

Sounds normal to me. Most alternators don't charge well until you are running about 900-1000 rpm. If you are having a low voltage problem it could be a weak battery.

 

[bruceb58]

Re: Low voltage at idle.......voltage regulator????

It really is amazing how much better the engine runs with the new one!!

Hard to imagine an engine runs better with a new alternator. One would think you would actually have more power with a bad alternator since it puts less load on the engine!

 

[Nucknoilerfan]

Re: Low voltage at idle.......voltage regulator????

Had a look under the cover today, it's been wet for the last couple of days, and it looks like the voltage regulator is external. I threw the charger on the battery to give it a good long charge at 10A. Hope to get it out on Friday for a rip around the lake. Someone else pointed out that it may be a battery issue so I'll see how soon the voltage drops at idle on the next trip. I figure a good battery should hold the voltage at least for a while without dropping below 12 volts. The only accessory running is the fish finder which shouldn't draw that much power.