It sounds like you may have a bad alternator, BUT, it could be a loose belt, or possibly a loose (big, like an engine/battery ground, positive battery cable to the engine starter/alternator) connection too.  
 

If I’m way elementary here, I’m sorry….. I’m impressed with the dash retrofit you’re doing but I don’t know your mechanical knowledge.  No disrespect meant here if I am breaking this down too far.  

First, let’s understand the components and function of an alternator.  1990’s saw some changes to them but essentially, they all have a regulator that co trolls charging.  

Regulator

The voltage regulator controls the amount of power distributed from the alternator to the battery in order to control the charging process. Regulators are designed with different functions and work depending on their specification.

Rectifier

The rectifier is used to convert current from alternating current (AC) to direct current (DC) during the charging process.

Rotor

The rotor is the spinning mass inside the alternator that rotates via the pulley and drive belt system. The rotor acts as a spinning electromagnet.

Stator

The stator consists of several coils of wire wound through an iron ring. The stator sits outside the rotor, when a magnetic field is created the electrical current is made.

Drive End Bearing

The bearings are designed to support the rotation of the rotor shaft.

Pulley

The pulley is connected to the rotor shaft and the drive belt system. Rotation created by the engine the drive belt system turns the pulley beginning the charging process.

(Above quoted from here) https://www.autoelectro.co.uk/alternators

Now depending on the alternator type (GM used some one wire and some with an external regulator in that era) you test them a bit differently. Almost no alternators work well at idle, so my first question is:

What happens when you raise engine RPMs and you measure voltage at the battery? What about if you measure at the large terminal stud (not the cable, actually at the stud) on the alternator (+) to the engine block (-)? You should read close to the same (within a tenth of a volt). If the alternator reads higher, you need to check your cable connections to the battery.  There is also an exciter wire that tells the regulator what battery voltage is (I don’t have that diagram, and that usually feeds the gauge as well).   
 

this has some decent reading in it too. 

 https://www.jalopyjournal.com/forum/threads/gm-one-wire-alternator-problem.199627/

what horse power is the Cadilac supercharged motor in a 2012 malibu 22 mtx

I have a 2020 axis t23 with the 409 partial closed motor. I got a warning that “coolant was hot” but i dont understand how the partial closed cooling works as i dint see coolant container anywhere?

You’re going to hear from this community that a 500hr boat could be fantastic or could be terrible, just like a 1500hr boat. Entirely depends on how it was used and maintained for those hrs. That said, 500hrs on a 2012 seems good. Less than 50hrs per summer.

On 6/14/2024 at 5:53 PM, Steve B. said:

Just a great post. What led to here? and what's on the agenda? This doesn't look like your first rodeo.

We grew up going to Banks Lake (state park in Washington) with the family but had recently mostly been doing saltwater boating. Wanted to get back into skiing fresh water and a friend introduced us to the Malibu Echelon series (he has a Presidents' edition). Grew up around boats, but this was my first full boat project (and still ongoing :)).

Not yet added to the post, we've also done stainless exhaust manifolds, LED interior lighting (got tired of the old lights rattling at idle), trailer bunks, engine cover carpet and a bunch of smaller items.

Still on the docket is interior carpet (debating carpet or something like SeaDek).