AFT_B912

BY Neil Currie D o you ever find some jobs are like buses? You don’t see any for a long period of time then all of sudden two or three come at once. Well, in this article I am going to go over one of a number of short circuit faults I had in the workshop within a few days of each other and how I approached them. The vehicle in question was a 2006 Subaru Impreza which had been recommended to be brought to me after another local garage had taken a quick look and decided it was not for them. Even though this vehicle may present a challenge due to lack of technical information – it’s a Subaru, not a common model on the road nowadays - we agreed with the customer to spend an initial hour on the vehicle and see where that led us. The customer’s complaint was that fuse 17 in the interior fuse box was blowing which was labelled A/C. The central locking wasn’t operating correctly and would randomly go off. Also, at times the indicators worked by themselves. Interviewing the customer revealed that they had purchased the car a just few weeks earlier so could offer little information other than the fact that all had been well until the electrics starting playing up. Dead short I decided the best place to start would to be to investigate the blowing fuse complaint then look into the locking and indicator issues afterwards. My first port of call was to check my information sources for power distribution information to find what components fuse 17 fed. However, none of my sources listed wiring at all for this vehicle, checking the manufacturer web portal found they only listed the newer models so I would have to look elsewhere to find what I needed. Thankfully with the power of networking and forums I managed to acquire the whole workshop manual with everything I could ever possibly need. Next I found the relevant information and found that the fuse in question powered the automatic air con control module/panel, the air con pressure switch, blower motor relay and air con relay for the compressor. I then confirmed if the fuse problem was a permanent or intermittent issue as it sat in my bay. The customer stated the fuse would sometimes blow each time a new fuse was fitted and then other times would last days and the A/C system would function as intended. Luckily for me the short was a ‘dead short’. This means that the short to ground is permanent and easier to find, in theory, as it is constantly there. How I tested for this was by placing a 501 bulb across the pins of the fuse and the bulb lit indicating a short circuit (fig1). The reason for this is we have a 12v supply coming into the fuse and should then have the same 12v supply then leaving the other side of the fuse and off to power whatever consumer is on that wiring. However, because we have a ground on the other side we have a voltage differential of 12v which lights a bulb brightly. With no resistance in the circuit, the amperage has no limit so the fuse is in place to blow and protect the circuit. There are tools on the market to aid in finding short circuits, and many technicians also use a current clamp but I am a fan of the bulb method alongside a multimeter. It has always worked for me. Path to ground With the bulb lit brightly, I then proceeded to visually 28 AFTERMARKET DECEMBER/JANUARY 2020 TECHNICAL www.aftermarketonline.net SHORT CIRCUIT 2019 Top Technician winner Neil Currie is back, and this time he’s got his work cut out with an electrical mystery Whoever had carried out the repair decided to fabricate their own support bracket ” Right: Fig. 2 Right: Fig. 1

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