12v power drops off.

[CharlieO.]

Electrical diagnosis is not my strong suit.
13.5 volts at batteries and at battery switch. Start the engine battery voltage at 13.4 and gets back to 13.5 but reading on the battery switch drops off to 4ish volts measuring the switch where it feeds the 12v panel.
Suggestion to resolve? What should I be looking for? I've gone through and tightened up every connection.

 

[Comodave]

A connection can be bad internally with corrosion. I would use the meter at every connection and see if the voltage drops at any specific connection.

 

[CharlieO.]

Dave, I did find one cable at the battery that feeds 12v side that had some green corrosion that I plan on ordering and replacing.

 

[twistedtree]

It could also be the contacts internal to the switch itself.

 

[Comodave]

Dave, I did find one cable at the battery that feeds 12v side that had some green corrosion that I plan on ordering and replacing.

It can be corrosion inside the connector too. I always use a copper paste when I am doing large electrical connections. If you put the volt meter on each connector tou may see the voltage drop. Then redo that connection.

 

[CharlieO.]

Well I replaced a couple cables, still no resolution. But I did find that I don't need to start the boat for the voltage drop, just turning the ignition key to on gets the drop.

 

[SteveK]

My thought is you are measuring at the switch but power is diverted before the switch, even when the switch is off. Thus a reading shows drop of volts. This however does not make sense as the ALT should increase the volts with engine running. Corrosion alone is not causing this. Maybe a charger hooked direct to Battery keeping it at 13.5v but also keeping the ALT from charging.

13.5 volts at batteries and at battery switch. Start the engine battery voltage at 13.4 and gets back to 13.5 but reading on the battery switch drops off to 4ish volts measuring the switch where it feeds the 12v panel.

 

[twistedtree]

On your key switch there is a wire that brings power to the switch from the battery and is energized all time. And there is another ignition wire that is only energized when you turn the key on. Using a meter. Which wire terminal shows a voltage drop when you turn the key on? Only the ignition wire? Or both the ignition wire and the battery wire?

 

[TowLou]

If the voltage is higher before the switch and drops after the switch you have resistance in the switch causing a lover output voltage. It may even feel a little warmer then normal after some time on due to the resistance.

 

[CharlieO.]

I get good voltage on both sides of the switch. But lose voltage at places like my vhf, depth sounder and GPS.

 

[twistedtree]

Does your engine have a pre heater that comes in when you turn the key on?

 

[CharlieO.]

No. Old Ford Lehman.

 

[twistedtree]

So when you turn the key on, voltage drops at the Mac equipment, but not at the key switch?

 

[CharlieO.]

Correct

 

[Delta Riverat]

Sounds like you have a bad wire between the key and the electronics.

Kinda strange that the electronics would be connected to the key. On my boat all the circuits are on the main panel. Everything can be run w/o the ignition on. You even have to turn on the helm breakers before the ignition will work.

 

[CharlieO.]

Yes, I can run everything when the key is off, but when I turn the ignition on or start the engine that’s when the stuff drops off due to low voltage,
I’ve already run a new buss bar to bypass whatever the problem is. I’ve only run the GPS/fishfinder so far to the new buss bar and it works well now. I’d like to figure out what the problem is instead of bypassing it.
May need to plan for a rewire next winter but would really like to find out what the issue is.

 

[Delta Riverat]

OK, that is good info. Also tells me that there is a new excessive draw happening when the ignition is turned on. You are looking for a short and not an open or bad connection.

 

[CharlieO.]

I’ve been kinda wondering if the problem could be with a sending unit or sensor.

It drop out occurs even when everything in my 12v panel is switched off.

 

[SteveK]

If a field wire to the ALT comes from the run position on ignition, that could be the draw.

 

[TowLou]

Check your negatives to them. Check from the harness and then go straight to the battery negative.

 

[Barking Sands]

Do you have your original wiring harness on the engine? See if you have any of those old trailer plug connectors. They are terrible. I had two between the engine and batt and panel. My voltage would be all over the place on the panel. Shake the plug and I could get voltage back. Shake it again and it would drop to 5..lol. Anyways..see if you have any of those plugs and wiggle them while watching voltage.

 

[CharlieO.]

No such plugs that I have found. The only ground from the battery bank goes to the starter housing.
We currently only have one battery bank that serves as house and start. There used to be a generator installed but it has been removed by a PO. All the wiring is still the same as when the generator was there. I’m wondering if the problem started when the generator battery was removed.

 

[Delta Riverat]

How old is your one battery? You may need another.

Have a look see. I had 2 start batts that would not start the engines. Voltage 13.2. Had a look and both were dry. As in needed a gallon and a half of water. After fill and charge, worked fine.

Plenty of voltage, but no current.

 

[CharlieO.]

There are 2 batteries together, 3 years old, no problems starting the boat.

 

[Delta Riverat]

We can take the batts off the list of suspects then, plenty of current.