Join Date: Aug 2008
Location: East Tennessee
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Re: 12.4V DC open but drops to near 0 when load applied
Ohms law, E = IR, where E is the voltage drop, I is current and R is resistance.
You have a high resistance connection some where in the circuit.
Disconnect the battery, both the positive and negative leads.
Measure the resistance from the positive wire that WAS connected to the battery to the positive side of the pump. It should be a few ohms because of the fuse and maybe the switch. Any more than a few ohms start measuring back towards the battery connection. Measure at the fuse, switch, ect.
If the above measurement is around a few ohms, measure from the negative wire that WAS connected to the battery to the negative side of the pump. Again it should be a few ohms, any more than that measure back towards the battery until you find the bad connection or corroded wire.
More than likely it is a corroded connection, a bad switch or corroded wire(s).
The other way of finding the problem is to leave the battery connected, turn on the pump, attach the negative lead of the volt meter to the negative battery terminal then start measuring the voltage along the circuit. Measure positive battery terminal, should be the same as when the pump is off. If the voltage across the battery is good, move on down the circuit (wires) until you get to zero. Such as you measure the battery side of the fuse and it is 12.4 volts, the pump side is zero, all the voltage is dropping across the fuse so its either bad wire connections to the fuse, or the fuse it self is bad but not blown.
Note I suggest the negative lead of the meter to be connected to the battery, not the ground of the engine or pump.
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Last edited by Skipper Jer; 21 Hours Ago at 12:40 PM.