The resistance is causing a voltage drop in the circuit. Like everything in a circuit, it is subject to Ohms Law (Volts = Amps x Resistance). The voltage drop across the resistance is proportional to the current flowing through it. With the control unit disconnected the only current flowing is through the multi-meter, which due to the meter’s high input resistance is practically zero. So, 0 Amps x 0.4 ohms = 0 Volts. Without current flow, there is no voltage drop. The voltage at the control unit without load will show the full 12 Volts.
In the operating circuit with the control unit drawing 10 amps, the figures become 10 Amps x 0.4 Ohms = 4 Volts. So we now have 12 Volt supply minus the 4 Volts across the green corroded wire or corroded connector, so only 8 volts are getting to the control unit. No wonder it doesn’t work.
This is one of the reasons why many manufacturers specify the use of break-out boxes when testing circuits, the circuit can be tested in the operating condition. Now breakout boxes tend to be very expensive and model specific, so what can you do as an independent? The next best thing is a set of back probes, so you can carefully slide your test lead into the back of a connector and measure voltages with everything connected.
We have established that we only have 8 volts at the control unit, so how do we pin down where the fault is? Well the answer is to use our multimeter to look for that volt drop. We are looking for a difference in voltage in parts of the circuit that should have the same voltage. If we place one probe on the positive battery terminal and the other on the positive supply terminal at the control unit they should be both at 12V (again with the circuit operating), the voltage difference between the two points should ideally be 0. If the meter shows any more than 0.3V difference then we have a problem.
In the case above, the meter would now show 4V difference between the battery positive and the supply connector at the control unit. Next, leaving one lead connected to the battery, move the other lead from the control unit and test by probing at connectors further back along the wire towards the battery. You are looking for the point where the voltage difference disappears. You should be able to find the point on the wire where you have voltage drop one side of the fault and no drop the other, this is where the circuit fault is located.
We test the ground side of the circuit in the same way as the positive side. It is important to start with the meter probe on the battery negative, as the voltage drop could be between the battery and chassis ground. To sum up, always start with a visual inspection. When we are testing for power and ground, the circuit must be in a fully operational state with everything connected. Measure voltage at the control unit by back probing the connector and don’t rely on resistance checks to prove the integrity of wiring.