12V DC electrics question

[SpeedEight]

Sorry, didn't really know where else to ask this...

In my camper I have a switched 12V submersible water pump that no longer works when I switch it on (it used to).
Something like this: pump

It's connected to a standard block connector so I disconnected the pump end and used a multimeter to check the voltage across block:

Switched on = 12.4V
Switched off = 0V (so switch is working to block connector)

Next I put the pump directly across the leisure battery (12.4V) and it fires into life, I rewire it up and put MM across it again and get 0.3V.

What is causing the voltage drop? The pump drawing too much current for the circuit? Putting it straight across the battery is giving it the current it needs?

It worked at one point so is the pump just knackered?

Electrics isn't really a strong point of mine so any help appreciated!

[fatjon]

If you have a bad connection between the power supply (battery) and the motor you can have the meter read 12V when the motor is not connected but near zero when it is connected. This is almost certainly your problem. Check the voltage at both sides of the motor connector relative to true ground, ideally the battery negative post. If both sides show around 12v relative to the negative battery terminal when the motor is powered on you have a fault between the motor and it's grounding point. If both show almost no voltage you have a faulty in the positive supply to the motor, trace back along the positive wire until you find it. The switch is a favourite.

[rsv gone!]

So you get 0.3v across the pump, when wired in place and power on?

But the pump works when put directly across the battery?

Despite the 12v reading from the meter, there must be a poor connection in the wires supplying the pump.

[jhfozzy]

fatjon said:

If you have a bad connection between the power supply (battery) and the motor you can have the meter read 12V when the motor is not connected but near zero when it is connected. This is almost certainly your problem. Check the voltage at both sides of the motor connector relative to true ground, ideally the battery negative post. If both sides show around 12v relative to the negative battery terminal when the motor is powered on you have a fault between the motor and it's grounding point. If both show almost no voltage you have a faulty in the positive supply to the motor, trace back along the positive wire until you find it. The switch is a favourite.

This ^^^^

What we call a "ghost voltage" in the trade. Loose / corroded connections usually, see if any connectors have the tell-tale green corrosion.

[E30M3SE]

+1 for the above.

[SpeedEight]

jhfozzy said:

fatjon said:

If you have a bad connection between the power supply (battery) and the motor you can have the meter read 12V when the motor is not connected but near zero when it is connected. This is almost certainly your problem. Check the voltage at both sides of the motor connector relative to true ground, ideally the battery negative post. If both sides show around 12v relative to the negative battery terminal when the motor is powered on you have a fault between the motor and it's grounding point. If both show almost no voltage you have a faulty in the positive supply to the motor, trace back along the positive wire until you find it. The switch is a favourite.

This ^^^^

What we call a "ghost voltage" in the trade. Loose / corroded connections usually, see if any connectors have the tell-tale green corrosion.

Thanks guys, that makes sense. I'll try tracing it all back as far as it goes, it'll be a right pig though, nothing is accessible accept the final block connector without dismantling the kitchen units. The switch by the way is integrated into the tap in the sink, a likely candidate by the sounds of it.

[tvrgit]

rsv gone! said:

So you get 0.3v across the pump, when wired in place and power on?

But the pump works when put directly across the battery?

Despite the 12v reading from the meter, there must be a poor connection in the wires supplying the pump.

Or on the earthing side...