Alternator wiring issues

[Kdixon1029]

I have a 2019 Mercruiser 5.7 350 MAG MPI. My alternator isn't charging the batteries. Here's what I've done in the last few months. Replaced 50amp circuit breaker, replaced 90amp fuse on starter and bought 2 new batteries. With the motor off, I get a reading of 12.2 at the alternator and roughly 12.5 at the batteries. This should be telling me that the circuit is good from the battery to the alternator. When the motor is running I get a reading of roughly 15.5 at the alternator and only 11.8 at the battery telling me the alt isn't charging the battery. I've replaced the exciter wire from the alternator to the 50 amp circuit. I "believe" there may be an issue at the starter where the orange wire from ALT is connected. I had to drop the starter to replace the fuse and am thinking there may be a short or a loose connection as this issue has been happening intermittently. I disconnected the orange wire from the alternator and ran a jumper wire straight to battery #1 and ran on the river for 3 hours no issues it charged perfectly. This tells me, I think that the regulator is fine. My question is can/should I run the jumper wire directly to the battery switch so it can charge either or both batteries? Also, should I add a fuseble link to the connection? Is there any danger in doing this? I do plan to drop the starter and check/reconnect wiring next season. I'm having surgery next week and won't be able to climb down in the engine compartment until next spring. We live in the south and can definitely get several more trips before winterizing. Thanks for any advice

 

[Eric 3.7LX Alpha 1]

Check your grounds. I had a similar issue on my work truck, was the main battery ground to the block. The end was corroded and broke off when I moved it. Try a temporary ground from the (-) on the battery to the alternator body.

 

[Mrwonkey]

I have a 2019 Mercruiser 5.7 350 MAG MPI. My alternator isn't charging the batteries. Here's what I've done in the last few months. Replaced 50amp circuit breaker, replaced 90amp fuse on starter and bought 2 new batteries. With the motor off, I get a reading of 12.2 at the alternator and roughly 12.5 at the batteries. This should be telling me that the circuit is good from the battery to the alternator. When the motor is running I get a reading of roughly 15.5 at the alternator and only 11.8 at the battery telling me the alt isn't charging the battery. I've replaced the exciter wire from the alternator to the 50 amp circuit. I "believe" there may be an issue at the starter where the orange wire from ALT is connected. I had to drop the starter to replace the fuse and am thinking there may be a short or a loose connection as this issue has been happening intermittently. I disconnected the orange wire from the alternator and ran a jumper wire straight to battery #1 and ran on the river for 3 hours no issues it charged perfectly. This tells me, I think that the regulator is fine. My question is can/should I run the jumper wire directly to the battery switch so it can charge either or both batteries? Also, should I add a fuseble link to the connection? Is there any danger in doing this? I do plan to drop the starter and check/reconnect wiring next season. I'm having surgery next week and won't be able to climb down in the engine compartment until next spring. We live in the south and can definitely get several more trips before winterizing. Thanks for any advice

Hi the numbers described don't sound to good to me 12.2 and 12.5 would be questionable as there is no load at all so therefor there would be no voltage drop, if there was say 20 amp load then depending on the length of cable then 0.3 I could live with.
The 15.5 volts would say that its an open circuit voltage ( broken loose wire/ connection ) and that's too high for a lead acid battery any way. ( that will come down more than likely once you rectify the voltage drop ) Personally i prefer a soldered fusible link rather than a fuse particularly in a marine environment fuses can be problem waiting to happen.
So fit the fusible link from the starter post/ solenoid connector to the main harness with every thing going through the fusible link from and to the battery not the main start cable of course. And I would run a new cable as you described to replace the existing one, being conscious that there is no other connected wires to it the original along the way and then disconnect it each end.

 

[ROY WILLIAMS]

so you ran the engine the alternator is 15.5 VDC ..then the battery is 11.8 VDC ...
did you inspect and clean and tighten the connectors or the battery switch ..USE A VOLTMETER .....WIRE BRUSH IT ON THE CONNECTORS . USE THE COPPER SCREWS/BOLTS NUTS ............

 

[alldodge]

I disconnected the orange wire from the alternator and ran a jumper wire straight to battery #1 and ran on the river for 3 hours no issues it charged perfectly.

Your ALT is fine, just need to fix the bad connection. Might be your Bat switch

Edit: Might just run a new orange wire to the 90 amp fuse

 

[Kdixon1029]

so you ran the engine the alternator is 15.5 VDC ..then the battery is 11.8 VDC ...
did you inspect and clean and tighten the connectors or the battery switch ..USE A VOLTMETER .....WIRE BRUSH IT ON THE CONNECTORS . USE THE COPPER SCREWS/BOLTS NUTS ............

Yes everything is cleaned, brushed and tight. I'll have to recheck those numbers again to be exact I didn't write them down as I was testing so they're best estimate

 

[Kdixon1029]

Check your grounds. I had a similar issue on my work truck, was the main battery ground to the block. The end was corroded and broke off when I moved it. Try a temporary ground from the (-) on the battery to the alternator body.

Thanks for the reply. Yes both batteries are grounded to the bock with solid connections.

 

[Kdixon1029]

Your ALT is fine, just need to fix the bad connection. Might be your Bat switch

Edit: Might just run a new orange wire to the 90 amp fuse

That's the thing, the 90amp fuse is on the starter which is impossible to see much less reach! I just wanted a safe temporary fix. I'll drop the starter next season.

 

[alldodge]

Thats the thing, the 90amp fuse is on the starter which is impossible to see much less reach!

Ok, then attach another fuse to the Bat post and keep the direct wire

 

[Scott Danforth]

The alternator works fine trying to push 15.5 volts to the batteries as the excite/sense wire is reading low

Something between the alternator and battery is in need of attention

Wiring/connections/switch/isolator/etc

 

[nola mike]

I ran into this issue in the spring, pita to figure out. Turns out the fuse was barely connecting internally. Enough to sorta sometimes charge and give me weird readings. Lemme see if I can find my post

Edit: here. Guess it was spring before last

Help with charging system?

Had some fluctuating voltages last year after installing the new engine. Didn't stay very long, but would overcharge for maybe 30 sec, then undercharge. Took the boat out for the first time this season, and on the way out my voltage spiked to 18v for a minute before settling to 14.2. after a...

forums.iboats.com

 

[Kdixon1029]

I ran into this issue in the spring, pita to figure out. Turns out the fuse was barely connecting internally. Enough to sorta sometimes charge and give me weird readings. Lemme see if I can find my post

Edit: here. Guess it was spring before last

Help with charging system?

Had some fluctuating voltages last year after installing the new engine. Didn't stay very long, but would overcharge for maybe 30 sec, then undercharge. Took the boat out for the first time this season, and on the way out my voltage spiked to 18v for a minute before settling to 14.2. after a...

forums.iboats.com

Was just reading through your old post. Did you ever install the 90A fuse at the alternator rather than starter?

 

[dubs283]

install the 90A fuse at the alternator rather than starter?

Fuse needs to be mounted on the starter main post with alternator and main 12 volt harness supply on the output side to protect not just the reg/rectifier on the alternator but also the main harness and associated circuitry

Older models did not have the fuse so its possible to abate the fuse and have normal operation but the fuse is there for added protection, mainly from connecting a battery backwards to protect associated curcuits

 

[Bondo]

Was just reading through your old post. Did you ever install the 90A fuse at the alternator rather than starter?

Ayuh,...... That would protect Only the alternator,......

On the starter, it'll also protect the electronics on yer Mpi, computer controlled motor,......
Smokin' the ECM will make an alternator look pretty cheap,......

 

[nola mike]

Ayuh,...... That would protect Only the alternator,......

On the starter, it'll also protect the electronics on yer Mpi, computer controlled motor,......
Smokin' the ECM will make an alternator look pretty cheap,......

On both the mpi and the carbed, the 50(?)a circuit breaker is protecting the electronics, no?

 

[dubs283]

50 amp breaker is ubiquitous on mercs.

90 amp fuse was added 20+ years ago as a failsafe towards alternator/main harness circuitry

Not exactly necessary but according to nmma/marketing/liability it's a part that makes sense

 

[nola mike]

50 amp breaker is ubiquitous on mercs.

90 amp fuse was added 20+ years ago as a failsafe towards alternator/main harness circuitry

Not exactly necessary but according to nmma/marketing/liability it's a part that makes sense

Yes, but what is it protecting aside from the alternator if you have the circuit breaker in line with the only other powered lead? And then why would it matter which end of the wire it was on?

 

[04fxdwgi25]

Should be on the power feed end, to protect the wiring, in the event of chaffed insulation developing a short to ground. Much like house circuit breakers protect the wiring after it leaves the breaker box.

 

[nola mike]

Should be on the power feed end, to protect the wiring, in the event of chaffed insulation developing a short to ground. Much like house circuit breakers protect the wiring after it leaves the breaker box.

Yes, the orange wire itself would be protected in that case. But that's the only benefit I can see. The wiring from the battery to the starter is similarly unprotected.