Boat Voltage Question

[The_Only_Gert]

Hey guys have more of a generic questions so don't really think that my engine matters but I have a 92 Thompson carerra with a 4.3 merc...long story short I have battery voltage to my key, verified with a multi-meter, when I turned the key on I get a significant drop in voltage at the multi-meter. battery voltage was around 12.5 and when I turned the key on the voltage dropped to 11.5 or lower (can't remember the exact number) anyway on my volt meter in the gauge cluster the voltage was the same. I checked the battery and was still at 12.5 so I know I have a drop somewhere. I disconnected the output wire from the key and tested voltage and had the same 12.5 so I know the key is good. Now when i power the rest of the harness up is when I get drop. so I know its past the key, I've been cleaning connections and still haven't found the reason for the drop.
Here is why I started looking for the voltage drop. When running my battery's are at 14.2 give or take so I know they are charging. The volt meter however is only showing 12.8 ish and throughout the day the voltage on the volt meter drops lower (the battery's are still at 14.2 with a multi meter) my worrying is that I don't want to under-voltage my fuel pump and burn it up, since I know they run on the same wiring basically. Why am I getting this drop througout the day, alternator tested good, factory volt meter jumps to 12.5 when a direct connection is made between it and the battery. Thanks for any help guys.

 

[tpenfield]

Don't assume that the electrical connection problem is 'down stream' of the switch. When you re-connected the switch to the rest of the wiring harness, current would now be flowing to the down stream components that are active (namely the ignition system) what ever amount of current that load is drawing is causing a 1.0 volt drop from the battery to the output of the switch.

Clean the connections 'up stream' and see what difference it makes. Also check for any undersized wires along that path.

 

[Rick Stephens]

Exactly right ⇧

Start at the battery, clean connections with a wire brush till shiny bright. Pay particular attention to the ground connection at the back of the motor.

 

[The_Only_Gert]

just wondering this, I have a audio amplifier in my boat, so I have 8 gauge power wire from the battery to the amp which is next to the key, If my factory wiring to the switch is to small for the load when the key is turned, could I attach my 8 gauge wire to the key and see if that changes anything? Just don't want to hurt any gauges or such. Thanks, I will also be checking my grounds. When I turn my key on the gauge illumination comes on, would all 6 lights be pulling enough power to cause the drop? I really wouldn't think so but I'm new to finding these voltage drops.

 

[GA_Boater]

I disconnected the output wire from the key and tested voltage and had the same 12.5 so I know the key is good.

All that tells you is the key switch supplies 12 volt without a load. The switch contacts could be damaged from arcing, dirt, etc. that only shows the voltage drop with the load.

With the key on, the gauge lights and the gauges themselves are powered, so it's more load than you're thinking. To eliminate the key switch, jumper across the switch. If the voltage still drops, it's downstream, if not, it's the switch.

Your factory wiring is OK, so don't use the amp feed to the switch. When I say OK, I mean the factory wires are sized properly, but some connection(s) might need cleaned.

Also the fuel pump is not powered through the key switch, so don't worry about low voltage burning up the pump. Not yet, anyway. :smile:

 

[gm280]

Very first thing you need to do is CHECK THE GROUNDS everywhere they go. I don't care if they "LOOK" perfect and shiny metal. It is what is underneath the contacts that matters. And every connection, without fail, needs to be clean bright like new shiny, metal include the GROUNDS!

Second issue. Don't confuse voltage and current! With no load, even a corroded connection will test perfect voltage. It is when you apply a load that the voltage drops because there is corrosion making resistance somewhere.

For a good example about voltage and current, If you stretched out a long extension cord made with very small wires, it will read the proper voltage until you try and plug something into it that requires more current then the extension cord can handle. Then the voltage drops like a rock and the cord will probably even get warm or hot depending how long you try to make it pass the current.

A friend on mine years ago did that exact situation. I had a small compressor and it run like a top at the wall receptacle. But he plugged it into a 50 foot extensions cord and it would not even start up. It merely hummed. He kept telling the voltage at the end of the extension cord read 115 volts but the compressor would not run, and he stated the compressor was bad. I tried to explain it to him and had to prove the compressor was good by plugging it into the wall receptacle again. It run like a top. He learn something that day...

 

[muc]

Google "voltage drop testing" and you can learn how to find the problem.

 

[tpenfield]

What you know right now is that you are getting a 1.0 volt drop at the ignition switch when there is a load. This is showing that there is significant resistance in the wiring/connections leading up to the ignition switch.

By example:

If the load were drawing 1/2 amp (500 ma) with a 1 volt drop from the battery, then you have 2 ohms of resistance ( R = V/I ) from the battery to the ignition switch, which is not ideal.

 

[QBhoy]

I really think you might be worrying about nothing. You will always get a little drop on voltage when you turn ignition on. For lots of reasons and one or two main reasons.
as for the slight voltage difference between the multi meter actual reading and the dash gauge...id bet that half of the boats in the world have dash voltmeters that are either affected by tributary loads or just not accurate.

 

[Lou C]

I had a similar problem on my '88 FW with the OMC engine wiring harness and the Medallion gauges harness. What I had was low readings at the dash gauge, like only 12 V --12.5 V with the engine running, yet the alternator output was 14.2V as was charging at either battery. So I thought it was just a bad gauge. Then got curious and found the gauge was reading exactly right on when checked with the same digital voltmeter. So now to start checking, I had battery volts at the B terminal on the ignition switch but it dropped a 1.0 to 1.5V lower when I turned on the ignition (measured at the I terminal on the switch).

Then went on a fishing expedition checking every single connection on the engine and all the dash terminal connectors. All looked good. I knew there was a splice somewhere in the engine harness but could not find it without really taking it apart (didn't want to damage any wires) so tried an experiment. Checking the ground side, I added a temporary supplemental return negative wire (like a big one 8 gauge) to the neg terminal on the fuse box to the engine ground. No change. Then tried the same thing on the positive side, immediately got full voltage with ignition on. So, as a temporary fix, I wired this supplemental feed in, with a fuse from the battery common terminal. I have left it that way for 2 years lol and its just fine. everything works as it should. Maybe the original harnesses had wiring that was just too small, who knows. If I get daring I might try to find that splice and take it apart.....

above are a couple of pix, both with engine running, top is before bottom is after...

 

[achris]

Google "voltage drop testing" and you can learn how to find the problem.

Or just look at the one I did years ago (it's in the stickies)....

https://forums.iboats.com/forum/eng...ives/10259654-how-to-perform-a-volt-drop-test

Chris........

 

[The_Only_Gert]

Thanks for all the help guys! great info ill be working on it this weekend.