Alternator reading 17 volts

[stevebonifield]

Was running under power for 2 hours engine just quit. Battery too weak to crank engine. Towed to my slip, jumped battery and engine started right up. Battery tested at 11.99 volts before i jumped it. Tested alternator while running and it tested at 17 volts. Bad voltage regulator? Amp meter also is not registering any movement. Change out the alternator I assume?? Any thoughts would be appreciated. Thank you

[Dave Neptune]

First welcome to the MMI Forum.

That's not good at all. Definitely replace the regulator 17V is way to high and could even of possibly killed the battery. Also charge the battery up and get it load tested, it may show good voltage but have very little actual amperage left in reserve.
I would also check the spark to be sure it is nice a nice and snappy blue. 17V could even damage the coil internally. It may spark OK and then open up (die) once warmed up after 15~30 minutes of running and then work again once cooled back down.

Dave Neptune

[hanleyclifford]

17 volts is excessive for many things in the system, not just the coil. I would start with a voltage regulator that has a potentiometer so you can adjust the voltage to around 14 volts at alt +. Your coil is probably bad now even though it will run for a short period. Coils need resistance to protect them, either external or internal. Check out the Transpo V1200 regulator for a start. https://www.amazon.com/Adjustable-Vo...181ee66ce61dc1

[stevebonifield]

Alternator

The alternator has a built in regulator. Is that usually able to be replaced?Thank you both for your help.

[joe_db]

What is the battery voltage when the alternator is at 17 volts? Is the connection between them still good?

[stevebonifield]

Alternator

12.05 volts which is lower than when the engine is off when it reads 12.4 volts Yes connection appears to be good

[hanleyclifford]

The alternator has a built in regulator. Is that usually able to be replaced?Thank you both for your help.

What sort of alternator do you have? The old Motorola and Prestolite have "internal" regulators which are actually external appendages. Those regulators can be removed and the field wire fed from an external regulator. If you have a newer alternator with real internal regulation you will have to replace or repair the unit - this is another reason why I switched to external regulation on every alternator I use. Currently I am running an old Prestolite with the Transpo V1200 and the whole system is happy.

[hanleyclifford]

12.05 volts which is lower than when the engine is off when it reads 12.4 volts Yes connection appears to be good

At a minimum I would take that alternator to a shop for testing. Those results sound fishy. If you post a picture of your alternator we could save some time.

[joe_db]

So with the engine running the alternator output stud is showing 17 volts and the battery is showing 12 volts
It seems like there is a bad connection between the two!

[hanleyclifford]

So with the engine running the alternator output stud is showing 17 volts and the battery is showing 12 volts
It seems like there is a bad connection between the two!

I think you are right. All batt connections should be inspected including the ground path to the engine.

[stevebonifield]

Alternator

Hopefully the photos are attached. I am first time user and the problem occurred during my first sail with this boat

[stevebonifield]

Alternator Photos 2

2nd set of photos

[GregH]

Quick question: In the Alt pics you have a green wire connected to the positive on the starter and in the battery pics you have green wires as ground?

There's not an accidental crossing going on here is there? Using same coloured wire for both positive and negative wiring?

If you can I would recommend keeping convention of yellow (or black if you must) for 12V ground wires.

[hanleyclifford]

The wiring looks neat enough though without a diagram it is impossible to know if it is correct. I see the batt isolator - those things make me nervous. I prefer a straight switch out option for each battery.

[joe_db]

This is a big mystery. Every wire needs to be traced and diagrammed. The DC-DC charger adds another layer of complication to this. This is something not normally used when batteries are the same chemistry.
First off, that is not a stock A4 alternator. The stock alternator only has two connections, + and excitation.
Here is one I found online that looks pretty much like yours and also pretty similar in layout to my Balmar alternator.

Note that this alternator has no regulator. Mine is isolated ground, not sure if yours is.
Here is yours:

What this looks like to me is you have no regulator and someone connected the field straight to the positive lead. This would drive the alternator to full output 100% of the time
EDIT
I see your other battery is a lithium battery. That explains the DC-DC charger. It may be configured to cut off charging if the input voltage passes 17 volts and also will cut off charging if the starter battery is below a certain level, probably around 13.5 to 13.8 volts.
Please do the following:
ENGINE OFF
Measure the voltage at the alternator output (red wire), starter battery, and lithium battery.
ENGINE ON BRIEFLY
Measure the voltage at the alternator output (red wire), starter battery, and lithium battery.
You do NOT want to use the boat, it seems dangerously misconfigured and a couple of those wires are really thin for their intended use besides for some wrong colors.
You will want to get the Victron app on your phone so you can connect to the DC-DC charger and see what it is doing and perhaps change some settings.