Thursday, June 25, 2015

Electrical Gremlins and Melty Cables

Driving home from work recently, I got that sinking feeling that accompanies a lack of power as I was tootling along. Initially, I thought that I may have just stalled the engine, but as the car ground to a halt and stubbornly refused to go any further, I realised I was going to have to get my hands dirty!

I must admit that I did start to panic a little as this doesn't happen very often and as ever I was in a bit of a rush to get home to take my son out, but do you ever really breakdown at a convenient moment!

After the panic died down, I started to think things through and tried to turn the engine over again. It sounded semi-promising as it seemed to want to fire but soon died as the key was released, so I thought either fuel or spark was the fault.

Luckily after the last breakdown I suffered (here), I always carry a few tool in the boot with me now, so getting out the screwdriver, I took the top off the float chamber on the carb and checked for fuel. There was a normal looking amount of petrol sat there, the float and the taper valve also looked fine, so putting it back together, I ruled out a lack of fuel.

Next up was a lack of spark, but then horror of horrors, I noticed that the cables going into the coil were smoking!! Panicking again, I quickly disconnected the battery I'd just fitted (here) and examined the cables.

Pealing back the black tape form the loom, I could see that the white/pink cable to the
positive side of the coil was damaged by the wire overheating.

Separating the cables out, I reconnected the battery and tried again and this time the engine started without any problems. Cautiously I watched the damaged cable for signs of trouble and checked it for heat for a few minutes or so as the engine idled. After I was convinced it was okay, I decided to press on and try to get home.

Gingerly I drove home and parked the car up and decided to look at the damage. Pealing back the loom further, I could see that there were other parts of the cable that had melted also, however thankfully no other cables seemed to be affected.

Checking the voltage of the damaged cable it seemed normal at 12.68v with the key in the ignition. I need to devise a way to check the voltage when cranking the starter though to see if it drops down to 7v as expected, but as of yet I haven't tested that.

... and 13.25v with the engine running

I'm fairly happy with the numbers as this means that the voltage regulator on the alternator was working properly, so at least that was okay. Tracing the white/pink cable back to it's source at the bulkhead behind the brake reservoir, there was more good news as I could see that it would be fairly easy to replace.

It's all well and good fixing it though, but what caused the problem in the first place? To be honest, I'm really not sure, but where the bonnet catch is held in place, there's a sharp edge that I suspect may have worn through the cable's insulation and caused a short circuit. It would make sense as is it was a positive cable that was at fault and the car is negatively earthed, but it could easily have been something else. Further investigations may reveal more.

One thing that I do know for sure at the moment though is that the car is off the road until I get to the bottom of the problem and get it properly repaired.

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