Short cut to finding short circuits

There are a number of ways of tracing shorts but you need to know what to look for

Published:  27 August, 2013

One of the hardest faults to locate is a dead short. The problem is the circuit is prevented from being used by the fuse or protection device so you cannot use volt drop tests. Voltage drop testing requires the circuit to be in use and current flowing so it can't be used if the fuse keeps blowing.

Recently, a 2004 Renault Clio was recovered into our workshop. The symptoms were that it just cut out and would not start again. The recovery agent had tried unsuccessfully to establish communication with the engine management ECU. Without any clues in the form of codes or data he brought the Clio to our workshop on his flatbed. A global scan showed no communication with any of the powertrain modules. I wanted to quickly check the power supply to the ECU. I checked the fuses and the main engine relay fuse was blown. I replaced the fuse and as soon as I turned the ignition on it blew again... A dead short.

This is where using information is vital - by using a wiring diagram I was able to identify all the loads.I placed a lamp in place of the fuse and unplugged the main relay, the lamp went out. This proved the fault was after the main relay. I bridged the relay and the lamp illuminated. It was now a case of unplugging each of the loads until the lamp went out. Unplugging the EGR valve extinguished the lamp. However, I knew the short was between the harness joint and the EGR connection as I could make the lamp illuminate by wiggling the harness with the valve unplugged. By tracing the wires backwards I quickly discovered where the plastic casings had melted on the exhaust manifold causing the short circuit.

We use the term 'short circuit' practically every day but what exactly does it mean? If an abnormal connection happens between two 'nodes' or end points of an electric circuit intended to be at different voltage - in the case of the article it is the exposed wires touching the metal of the exhaust manifold. This will result in excessive heat and potential damage to the rest of the components in the circuit - and in this example, caused a sudden and unexpected breakdown.

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