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Back to Basics – David Wagstaff AAE MIMI Master Technician

By autotech-nath on July 22, 2017
We all know the customer thinks we have a magic machine that we plug in that tells us what’s wrong with their car, even some garage owners believe this, but I’ve never come across a fault code that says, ‘mouse chewed through wiring under cover on top of engine’. The reality is, you’ll get some sort of generic code, a non-functioning sensor or non-communicating control unit. Finding the real fault is down to basic electrical testing, a skill often overlooked.
Screen Shot 2017-07-20 at 19.17.13I was presented with a car with an underlying mouse-induced fault, with a fault code for the coolant temp sensor. The fan was running and in live data the coolant temp showed as -40⁰C. A visual inspection near the sensor showed the chewed wiring, but what would have happened if the wiring had been somewhere more difficult to see? Traditional testing techniques would have easily found this, a resistance check on the sensor would have shown it to be OK and a continuity check on the wiring between the ECU and the sensor would have shown an open circuit. The difficulty comes when we have a fault caused by corrosion in the loom and this is where traditional techniques fall down.
I have seen far too many control units wrongly condemned after a technician has carried out all the checks they believe were needed. They have disconnected the control unit and with their multimeter tested for power and ground at the multi plug. Happy they have 12 volts across the supply wires they carry on and check continuity on the wiring and possibly shorts to ground and positive. When it all checks out OK they fit a control unit, to find the fault remains. The error here is in the testing technique. Corrosion in the loom creates resistance. “Ah but I did a continuity test and it was OK,” I hear you say. A resistance as low as 0.4 Ohms in the supply wire to the control
unit could be enough to cause it to not work, well below the 100 ohms or so at which point most multi-meters will sound their continuity buzzers.
If we actually measure the supply voltage to the control unit with everything connected and working, we find we have a voltage of only 8 Volts. So, what is happening here?
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.


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Autotechnician is a magazine published nine times a year, delivering essential information to independent garage owners and technicians in the UK. Delivered both digitally and in print, autotechnician provides readers with technical, training, business advice, product and news, allowing our readers to keep up to date with information they need to run and work within a modern workshop.
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