I was out on the boat (1989 454 Mag Python Vector) yesterday. She started fine, felt like power was there, and ended up anchoring it so my buddies could dive for a lost GoPro. After a few hours, we start her up to head back and because of the equipment, there was a list. While underway, I use my trim tab switch to adjust and the engine starts chugging and spitting (like the time my spark plug wire came loose). I take my finger off the switch and all is good again, and I keep trying the tab switch and it does it every time.

I get it into my slip, and check voltage on my multimeter and its only putting out 12.23 volts. The battery by itself, nothing on, was sitting at 12.4.

At this point, I realize Im low on voltage and since the battery checks out, Im thinking alternator. But before I go and drop some money on that, do you guys thing it could be anything else? I checked the tab switches, everything looks good, I checked battery connections, tight and clean. Nothing was changed or fudged with prior to going out, and it may have very well been bad on the trip out, but I never used the tabs so I never even knew.

Any help or ideas before I take off my alternator to get it tested? Anything else I could have missed?

Thanks

start it up and check alt while running.also check purple wire on back for switched 12v,,also check the red wire(little one) for 12v also.if switched and constant 12v are there,alt is done for.

The 12.23 voltage was with it running, I reread my post and it didnt make it clear.

Boat on and running = 12.23

Boat off = 12.40

Alternator should be charging around 14 volts. I had an ALT that was on its way out, it would only charge when it felt like it. Finally replaced it and once the rpms go high enough for it to start charging, I'm always at 14 volts.

What Trim Tabs are you using?

I've been on boats that the K-Plane or Drive trim pumps caused the igntion to hiccup at high rpm due to the current draw and too small of wire feeding the pumps.

Sounds like you have something similar going on, you are trying to pull to much current through too small of wire to the pump(s) and its causing your ignition to drop out.

I never had an issue running the tabs before, even when all out trying to get that last mph.

The tabs are the dual ram Bennett tabs. At idle, I can move them, but once past 1500-2000 rpm, the boat starts to miss.

My logic is the alternator is on its way out aka dead, and running the tab switch/pumps draw too much, therefore sacrificing electricity to ignition and therefore the misfire.

If it was pushing 14 volts with it on, I'd have to agree, but since its already low, something else is acting up.

Check the basics first.....

Good, clean, tight battery cables
Good, clean, tight grounds
14 to 14.6 volts at alternator

It might be something simple.

Good luck!

Dumb question, but what leads do I test on the alternator? Anything I look up comes up auto related and using the battery voltage as the output.

Pretty much any chain auto store will load test the alternator for free.

When running the large cable lug on the back should test around 14 volts.

Went down to the slip and ran the lead from the back of the alternator to a ground.

With the boat running I was getting 12.08 volts.

I ordered an alternator from iboat and did the 2 day shipping. The alternator looks original so its worth replacing anyway.