Voltage Drop

O

Oscarlyy

Member
Aug 25, 2015
65
Canada
Boat Info
1989 345 Sedan Bridge
Engines
1989, rebuilt Mercruiser 454, 100 hours
Hey guys!
Just bought my 1989 345 sedan bridge about 5 months ago and loving it so far! But in the beginning the boat had some low voltage issues but in the sales agreement the previous owners agreed to fix the problem and hired a electrician to fix it and he ended up doing 1500$ of work cleaning connections and tidying up some connections. The voltage is steady around a floay of 13.4 downstairs on my battery charger display. But upstairs I have a Garmin depth sounder that reads about 12.7-12.9 volts which is still fine in my opinion but than when anybody uses the vacuflush all the lights dim or than when we leave to go boating and we have our navigation on, depth sounder on and autopilot on the voltage drops to about 11.7 and than when the pump goes on on the autopilot (like it always does every 5 seconds) a low voltage alarm sounds on the autopilot meaning it is going down to 10 Volts or under says the manual... And than if you honk the horn the stereo reboots and sometimes the navigation restarts. You can see how this could be annoying. Please help! Any info at all helps! Thank-you CSR!!
 
First thing I would do would be to properly check the battery's. #2 would be t check the alt. output. Fix the vacuflush it shouldn't come on like that.
 
Nonits only when you flush than the pump goes on for about 20 seconds and the lights dim. But I will check the battery's again. What do you mean by alt. Output?
 
OK, how would I do that? Just have a load on like the autopilot than check the battery voltage?
 
Oscarlyy said:
OK, how would I do that? Just have a load on like the autopilot than check the battery voltage?
Click to expand...

Are you asking how to do a "Load Test"? Here's how:

Load test should be done on a fully charged battery.

The easiest way to heavily load the battery is to operate the starter motor.
To prevent a gas engine from starting disable it's ignition; to prevent a diesel engine from starting, shut off its fuel supply and keep the "stop" control in the stop position.

Monitor the battery voltage while cranking, but not starting, the engine for approximately 15 seconds.
This may be a 2 person job; one to monitor battery voltage and one to crank the engine.
The batteries' open-circuit voltage will drop as soon as the starter is activated and then stabilize in approximately 5 seconds. Battery Voltage should stabilize at around 10.5 volts and remain constant until the engine cranking is stopped after 15 seconds(to prevent damage to the starter).

If battery voltage goes below 10.5 volts and continues to drop after 5 seconds the battery has significant loss of capacity and should be replaced.


A hydrometer or open-circuit voltage test will not indicate the ability of a battery to provide current under load.

Hope this helps!

Cheers
 
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I assume you can just take the battery to the auto parts store to load test right?
 
Fully charge, then a draw test. the auto parts store is only there to sell you something. take it to someone you trust. Using the starter is not the correct way. Then and only then with good battery's, test the output of your Alt's. Remember it all starts with the battery's.

Electrical Doctor has a picture of a load tester. I have the same one.
 
Last edited:
Mike Blake said:
Using the starter is not the correct way.
Click to expand...

Really? That is how I learned it in the Marine Electrical Systems course/class that I took. WOW!

Can you give me or send me proof so that I can show it to my instructor so that he can stop teaching it that way and contact the publisher of the educational text so they can remove it, please?

Thanks so much!
 
Strypes said:
Really? That is how I learned it in the Marine Electrical Systems course/class that I took. WOW!

Can you give me or send me proof so that I can show it to my instructor so that he can stop teaching it that way and contact the publisher of the educational text so they can remove it, please?

Thanks so much!
Click to expand...

Well I guess if you didn't have the correct tools you would have to check it that way. My original post said to test it correctly. You can lease those tools @ the parts store. BTW 44 years in the automotive electrical business.

Have a nice day.
 
So twin motors
How many batteries?.......I'm presuming 2 start batteries, one per engine. Is there a seperate house battery??
If so has it got the crappy 1-2-Both battery switch on one motor. If so, try it on 2 which should be the house battery......see if you get the same result.
Also I would go with MB....I would just use a more modern tester, your showing your 44 years Mike...mines only 40!


Sent from my iPad using Tapatalk
 
Sounds like your boat has a lot of electrical components that require the batteries be in top condition. Do you know how old the batteries are? Has the water level been checked? If you've owned the boat for 5 months and never added water to the cells then that is most likely a problem. If the batteries are 4 years or older then I would just replace them and upgrade to larger batteries if you have the room. Buy a good brand of batteries such as Interstate or Deka.

I notice there is 20 hours on rebuilt engines. That would tell me to check all the cable connections both positive and negative made to the starter and engine blocks.
 
Last edited:
If the batteries check out fine, then it Sounds to me like you have a poor connection somewhere that is limiting the current flow resulting in a voltage drop. I'd check the battery switch and see if you have a voltage drop there when under load. To do this, put a volt meter on the two poles of your battery switch is should read zero when the switch is on. That means no voltage drop across the switch. Then put all your loads on and see if there is a voltage across the poles on the switch. If so then replace the switch. You can do this same test across relays and circuit breakers and ground busses and power busses. Eventually you will find the culprit.

Good luck,

Pete
 
my GUESS would be the battery cable / cable ends if they are the original equipment 1989 era searay stuff.
 
IanBat said:
So twin motors
How many batteries?.......I'm presuming 2 start batteries, one per engine. Is there a seperate house battery??
If so has it got the crappy 1-2-Both battery switch on one motor. If so, try it on 2 which should be the house battery......see if you get the same result.
Also I would go with MB....I would just use a more modern tester, your showing your 44 years Mike...mines only 40!


Sent from my iPad using Tapatalk
Click to expand...

OK we are tied!!!! maybe we should get the Honda jenny out !!!!!! I got the beer and popcorn. LOL
 
Mike Blake said:
OK we are tied!!!! maybe we should get the Honda jenny out !!!!!! I got the beer and popcorn. LOL
Click to expand...

Do you want lime in your beer Mike, or are you drinking it old Skool naked?


Sent from my iPad using Tapatalk
 
Just a update here, I looked around and I know my battery charger display is saying the voltage at the batteries is 13.4 and I go around the boat checking all the lights and cigarette lighters and they all read exactly 13 volts, and the sedan bridge was 12.9 volts. I'm sure my Batterys are fine as they were all replaced summer 2015 to 4, 6 volt golf cart Batterys.
 
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