Voltmeter indicates lower than actual voltage

[WingmanSVT]

I have a weird discrepancy. I just replaced the alternator on my boat, old one had the voltage regulator fail, and now the voltmeter will show anywhere from 1 to 2 volts lower then the boats actual voltage read directly off the battery. The multimeter reads 14 volts off the battery but the voltmeter reads 13 volts, then when turn on a relatively high draw accessory like the blower the voltmeter will drop to just below 12 volts, even though the multimeter is still reading a steady 14 volts off the battery. I also connected the multimeter to the back of the gauge and the multimeter matches the indication on the voltmeter.

Absolutely stumped by this

 

[JetBote]

Re: Voltmeter indicates lower than actual voltage

Your reading at the battery is indicating the amount of voltage stored in the battery. Your reading at the gauge is showing what's available in the circuit after the blower is absorbing volts from the wiring harness.

 

[jejeosborne]

Re: Voltmeter indicates lower than actual voltage

This is ironic because I just logged in to post the exact same question. I recently installed a new depth/fish finder (off accessory switch) which has a battery alarm feature. When I am idling with the blower on, it will go into alarm. It is registering about 11.5 volts with the blower and about 12.5 without the blower. I initially thought the alternator was bad so I took it off and had it bench tested. It performed better than it the specs. I reinstalled the alternator and found 14.3 running volts at the battery as well. My battery indicator warning light is also lit. I am going to search for a loose connection this week but wonder if the wire harness is just sized too small. Any new advice out there to help me troubleshoot?

If this is a common problem and due to undersized wire, I will be running a new pair to my panel for the accessories.

Jeff

Mercruiser 260HP 350CI

 

[HT32BSX115]

Re: Voltmeter indicates lower than actual voltage

Your reading at the battery is indicating the amount of voltage stored in the battery. Your reading at the gauge is showing what's available in the circuit after the blower is absorbing volts from the wiring harness.

Howdy,


No wiring harness "absorbs volts".

If you have voltage drop in any circuit, it will be due to poor connections either in the harness itself, fuse-blocks etc or at various ground connections.

Blower motors usually don't draw enough current to produce a significant voltage drop due to the wiring from the battery to the switch and then to the blower.

Lets say you have a blower motor that draws 10A current......and it's 15' (or 3m) from the battery to the blower. (I.E. 15' of wire)





To get a full 1V drop in voltage across that wire (12.5v to 11.5v), the wire itself would need to have resistance of 0.1 ohms. (V=I*R....... 0.1x10=1 )

Lets also say that you are using #12AWG wire in the circuit.

#12AWG wire has a resistance of 5.2 OHMs per 1000m!!

Through the magic of Electrical Engineering I calculate a 15' (about 3meters) piece of #12AWG wire as having 0.02 OHMs resistance.

0.02 OHMS of resistance would only produce a 0.2v drop in voltage!! which is insignificant.

your voltage drop is from somewhere else.

In pleasure boat marine electrical systems, electrical problems are almost always poor connections at engine block ground points, wire-end connectors and the cheap switches they use.



Check all your switches, connectors, and connections.

By connections I also mean any crimped connections on power supply wiring. Crimped connections can be the source of voltage drop if they weren't adequately sealed with heat shrink tubing etc....and even if they are "sealed" they can still be corroded.....

Switches can have contacts inside be burned and/or corroded.



Cheers,


Rick

 

[Kainon]

Re: Voltmeter indicates lower than actual voltage

use the same digital meter to help find the corroded/bad connections too.. I've seen 2 new gauges out of the box have different readings.

 

[HT32BSX115]

Re: Voltmeter indicates lower than actual voltage

use the same digital meter to help find the corroded/bad connections too.. I've seen 2 new gauges out of the box have different readings.

Yeah they work great for that! On a clean tight connection, you should read pretty much ZERO volts "across" it!

 

[jejeosborne]

Re: Voltmeter indicates lower than actual voltage

It will be later this week before I will be able to investigate. I am guessing that the blower is powered directly from the dash switch rather than a relay in the stern since my voltage at the dash accessory bus is lower while the fan is running. I will be able to see then what the wire gauge is that supplies the dash and accessories. I am betting the Mercruiser harness is large enough to supply the demands unless there is a known design flaw. Does anyone have specific connections for me to focus on that are notoriously susceptible to corrosion or weak connections?
Jeff

 

[HT32BSX115]

Re: Voltmeter indicates lower than actual voltage

I cannot tell you specific connections to check.

Problem areas are always ground connections(especially the engine block grounds) and fuse blocks.

(and o course, all the crimped wire connections associated)

 

[tonyone]

Re: Voltmeter indicates lower than actual voltage

I just went through similair problem.....finally got fed up and cleaned every factory plug in,any crimped on connections were replaced and soldered with heat shrink...ya i found a lot of green corroded connections and ya it cured all my electrical issues..it is time consuming but i see it as preventitive maintenance..any connections such as solenoids were also covered with liquid electrical tape..you might be better of going this route,cuase chances are if you have one bad connection cuasing you grief then there will be more to follow...good luck

 

[Cuyose]

Re: Voltmeter indicates lower than actual voltage

I am having the same issues with my new (used) boat. Some gauges sorta work, all voltage gauges read lower than actual voltage, bouncing temp gauges, etc. When I checked the grounding busses in the engine bay they were pretty corroded with the telltale green fuzz along with connections to the neg terminals on the battery banks.

This is encouraging as almost all of these issues can be cured by redoing the connections with good marine tinned crimp connections and heat shrink, which I plan to do. I hate checking gauges and having to guess that everything is ok, regardless of if the boat is running great or not.

On a similar note. I never see it and wonder if there is a reason why. Can you put liquid electric tape or silicone on your connections after cleaning them up? I would think this would be a sure fire way to prevent moisture in the bilge to re-start the corrosion process.

 

[HT32BSX115]

Re: Voltmeter indicates lower than actual voltage

The main reason is dissimilar metal corrosion and exposed-metal corrosion.........pass an electrical current across 2 different metals in a moist or salt environment, and you'll get more corrosion.

Soldering helps, but a crimped-then-soldered connections can easily break in a high vibration situation.

Sealing them with something that keeps air out is probably the best thing you can do.

Heat shrink tubing is good as long as it seals adequately. If it doesn't, you can still get corrosion.

I have seen corrosion under the insulation in wire! ! it was bad enough that I couldn't even get solder to flow onto the wire!! If you cannot get solder to flow into the wire, it'll be hard to get a good connection using crimped connectors too.



//R

 

[JoLin]

Re: Voltmeter indicates lower than actual voltage

This is encouraging as almost all of these issues can be cured by redoing the connections with good marine tinned crimp connections and heat shrink, which I plan to do. I hate checking gauges and having to guess that everything is ok, regardless of if the boat is running great or not.

I learned this long ago when working on old cars. Favorite saying in the FIAT forum I used to frequent, was "it's the ground, stupid!"

I spent most of 2 weekends this past May pulling, cleaning, re-doing as necessary, and greasing every ground and connection I could reach on the C*C. Every switch was sprayed, 'exercised' and sprayed again with electric contact cleaner. It's tedious but really pays off.

 

[HT32BSX115]

Re: Voltmeter indicates lower than actual voltage

Every switch was sprayed, 'exercised' and sprayed again with electric contact cleaner.

Old switches are frequently a source of problems in high current circuits because there's a little spark every time the switch "makes & breaks".

The spark causes some metal to actually burn and increase the resistance of the connection with a resultant voltage drop. In extreme cases the switch will actually get hot when closed with high currents.

The best fix is to replace the offending switch.

The only switches that never have this problem are sealed, vacuum type, with mercury whetted contacts. (those would probably cost a fortune but they last "forever"!)



Cheers,


Rick