Alternator putting out 17.25 volts

[mark_53]

Hi all, I just installed an ACR with a start isolation feature and connected the alternator directly to the house battery but the voltage at the house battery is now 17.25 volts. Before the switch over, voltage was at around 13.7. Seems like a dramatic change. First of all, is this voltage to high for my two group 27 house batteries?
The other problem with the higher voltage is the ACR goes into lockout mode at 16.0volts and prevents the start battery from being charged. Is there a way to reduce the voltage the alternator puts out?

[Stu Jackson]

Mark,

1.  Did you disconnect the wire between the alternator output and the starter?

2.  Do you have an external regulator?  If so, where is the battery sense wire connected:  back of alternator or at the house bank?

3.  Have you measured the voltage at the back of the alternator, too?

4.  What alternator?

[mark_53]

Mark,

1.  Did you disconnect the wire between the alternator output and the starter?

Yes.

2.  Do you have an external regulator?  If so, where is the battery sense wire connected:  back of alternator or at the house bank?

No external regulator.

3.  Have you measured the voltage at the back of the alternator, too?

Yes, it slowly increases from 16.9 to 17.25

4.  What alternator?

Motorola 55amp

Much better now at 13.85 volts at the battery with the start isolation disconnected. Can't imagine what would make alternator voltage increase simply by connecting the start isolation to the start button.  This is a Blue Sea ACR.

Thanks!

[J_Sail]

That sure sounds like an alternator that is not sensing it's putting out any voltage, when in fact it is. Stu was right to ask if it is configured with an external regulator. Is it possible that it has an internal regulator but that its sense wire, instead of being connected at the alternator to its output, has been brought out to a separate post that's still connected to your starting battery circuit? That's a long-shot but in that case with the ACR open due to the Start Isolation the alternator sense wire would see just 12.6v even though the alternator is attempting to crank up to 13.x volts. In that case the alternator would continue to increase its output voltage until it maxed out. In the meantime the ACR, which normally would have reconnected the house battery to the start battery, would see the excessive voltage and lock itself open, allowing the situation to continue.  Without Start Isolation connected, the ACR would have a decent likelihood of closing and tying the two battery systems together. That would restore the connection between the alternator output (connected to the house battery) and the alternator regulator's sense wire (connected to the Start battery circuit).

Paradoxically if the ACR didn't invoke its over voltage override then everything might have returned to normal when it saw a charging voltage and tied the two batteries together.

I don't know how or why your regulator would have a separate sense wire (and how it's still connected to your start battery circuit), but it sure sounds like that's what's going on. I suggest you look at it extremely carefully and identify/trace every wire leading from it.

[mark_53]

That sure sounds like an alternator that is not sensing it's putting out any voltage, when in fact it is...

J-Sail, great analysis.  That sure sounds like a good probability.  When I connected the alt. output to the house battery, I left the other wires as is.  If one of those was the voltage sense wire, it is connected to the start battery which has a sitting voltage of about 12.4v.   Sounds like I should connect the voltage sense wire to the house battery or Alt. out?  Only problem is my schematic shows no voltage sense wire.  Alternator wires are shown as orange=output goes to house battery, purple=engine panel goes to starter positive post, grey=tach.
When I physically look at my alternator I can see the orange output wire, a blue wire attached to the starter post, a purple wire that looks goes towards the engine panel and a second orange wire I need to trace yet.  One other thing I noticed was if I isolate the start battery by switching it off while the engine is running, voltage at the house bank drops from 17.25v to a normal 13.4v.

[J_Sail]

I don't know how that alternator is constructed, but if indeed it has a separate sense wire, then it would be great to run it back to your house battery so that it sees the actual voltage being delivered to the battery after any voltage drop thru your wiring from the alternator output to the battery. That's how hi-end external regulators are wired for best performance. What's critical, though, is that the sense wire never end up isolated from the alternator output, so make sure an open switch or ACR can't result in the sense wire failing to sense the alternator output.

I have never heard of a Motorola 55amp alternator with an exposed separate sense wire, though, so that part still has me puzzled unless someone has previously done some modifications. Hopefully someone such as MaineSail who knows the various different marine alternators inside and out will comment.

[Stu Jackson]

Unlike an external regulator (Alternator Regulator Wiring Diagrams - all three http://c34.org/bbs/index.php/topic,4548.0.html), the OEM alternators, which you have, do NOT have an external sense wire, it is part of the internal regulator.  The link's first sketch shows the OEM alternator wiring and colors.

How Alternators & Regulators Work PLUS External vs. Internal Regulators (by Maine Sail):  http://forums.sbo.sailboatowners.com/showthread.php?t=125392

[mainesail]

This applies to any alternator that is externally sensed... Some internal regulators can be externally sensed if they don't create a parasitic load by doing so...

Regulators & Voltage Sensing - Why It's Important (LINK)

[mark_53]

Turns out my OEM Motorola alternator does have a voltage sense wire contrary to the owners manual wiring diagram. I connected it to the house battery and voila, start isolation works!  Alternator is putting out 15.0v at the house battery then combines after about 30 seconds with the start battery.
Thanks all for the valuable insight. 

[Noah]

Congrats! I hope you didn't blow out your engine panel volt meter. PIA to uncaulk/recaulk panel to replace.

[mark_53]

Congrats! I hope you didn't blow out your engine panel volt meter. PIA to uncaulk/recaulk panel to replace.

Unfortunately, it looks like I did.  Was thinking of replacing with something like this...
http://www.amazon.com/gp/product/B006G52A7O?ref_=cm_cd_al_qh_dp_i

[J_Sail]

I'm glad to hear the sense wire theory worked out. If you have photos of the alternator and the specific way in which it ended up with the sense wire misconnected that might be useful to others. 

Also I would triple check before concluding the meter is blown. A 12v meter should take 17v without damage.  That's not to say it's impossible it got blown, but it seems a little unlikely. MaineSail may have some experience to share here.

[tgsail1]

Most of the newer Leece-Neville alternators, the successor to Mototrola, have a remote sense wire on the output terminal. On my new alternator it is a small gauge red wire with an eye terminal.  I actually still have my C34 OEM alternator in my garage and it too has the sense wire on the output terminal. Will send a photo later.

[Stu Jackson]

In Reply #6, I wrote this:

Unlike an external regulator (Alternator Regulator Wiring Diagrams - all three http://c34.org/bbs/index.php/topic,4548.0.html), the OEM alternators, which you have, do NOT have an external sense wire, it is part of the internal regulator.  The link's first sketch shows the OEM alternator wiring and colors.


I just went back and read it and looked at the first diagram.  There WAS a red sense wire on the back of the Motorola.  Take a look.  Sorry about the wrong text to the right sketch.

[mark_53]

I'm glad to hear the sense wire theory worked out. If you have photos of the alternator and the specific way in which it ended up with the sense wire misconnected that might be useful to others. 

J_Sail, you were spot on with the diagnosis.  Much appreciated! 
Complete photo of the alternator back side is impossible to get without removal.  Suffice it to say, there are four wires coming out of the back of the OEM alternator, sounds like colors vary quite a bit.  Two go the the engine panel and two go to the starter + post.  When moving the alternator output to the house battery, it is very important also move the sense wire!

Also I would triple check before concluding the meter is blown. A 12v meter should take 17v without damage.  That's not to say it's impossible it got blown, but it seems a little unlikely. MaineSail may have some experience to share here.

I didn't think 17v would harm the volt meter either but I'm getting 12v to the post but the needle doesn't move.  It is 27 years old.  I'll remove it eventually and check it on the bench before ordering a new one.

Most of the newer Leece-Neville alternators, the successor to Motorola, have a remote sense wire on the output terminal. On my new alternator it is a small gauge red wire with an eye terminal.  I actually still have my C34 OEM alternator in my garage and it too has the sense wire on the output terminal. Will send a photo later.

A photo of the backside of the OEM alternator may be helpful to those adding a start battery in the future. I don't know much about alternators so a description of what each terminals function is would be helpful too.  On mine, the two terminals on the back right side go to the engine panel, the two on the left that went to the starter post need to be routed to the house bank.  I had my alternator out a few years ago but couldn't find a model number anywhere on it so tough to look up specs.

In Reply #6, I wrote this:

Unlike an external regulator (Alternator Regulator Wiring Diagrams - all three http://c34.org/bbs/index.php/topic,4548.0.html), the OEM alternators, which you have, do NOT have an external sense wire, it is part of the internal regulator.  The link's first sketch shows the OEM alternator wiring and colors.


I just went back and read it and looked at the first diagram.  There WAS a red sense wire on the back of the Motorola.  Take a look.  Sorry about the wrong text to the right sketch.

Stu:  I couldn't find the sense wire on the diagram.  Suggest that diagram be updated with a photo that tgsail1 can provide, also model number if anyone knows it, and labels on the terminals.  I'd be happy to help with that.