GP one
New member
Last week, much to my annoyance, all four tail lights of my Grade Punto failed to illuminate and the dash board display reported "Check running lights" every time the light switch was turned on. The two front side lights were working along with the rear number plate lights, the three rear brake lights, the reversing lamp and rear fog lamp so it wasn't a general electrical problem.
I dismantled and checked the cable connections to each rear light cluster expecting to find a loose connector somewhere. They all appeared fine and the fact that the indicator lights and brake lights worked demonstrated that the majority of connections were in place, including the ground. The tail lights on the GP are fed by two circuits from the body computer with two 5w lamps connected to each. The lower 5w lamp is half of the combined tail(5w)/stop light(21W) bulb.
I finished up my first diagnosis session by re plugging the connectors to the body computer and retiring to consider the circuit diagrams.
My next step was to connect my trusty Actron ODB II reader to the ODB port, but which, unfortunately revealed nothing. Then I tried using PCMSCAN from a laptop but unfortunately it didn't recognise the Body Computer so consequently couldn't display any body computer specific information.
At this point I abandoned the diagnosis and took the car into my local FIAT dealer. They diagnosed a failed Body computer and estimated the cost to replace at approx £400. Wow! It looked as if it would cost me £400 to fix four non working tail lights. Not a great thought. Looking at the wiring diagrams it looks as if the Body Computer in central to virtually all the electrical circuits in the vehicle. Having a £400 body computer as the weak link in even the most basic of electrical circuits doesn't seem too bright.
Having a few hours free I started looking for ways to bodge a connection from the rear fog lamp to the tail lights. By sheer good fortune something I did caused the tail lights to come on for a few seconds. Perhaps there was a chance to fix the fault after all. The lights came on just after I moved the steering column adjuster. I remembered that the last time I drove the car I had discovered that the steering column also moves back and forward as well as up and down and I had adjusted the position. Removing the steering column covers revealed the movement had trapped part of the wiring loom, the looms retaining clip not being fixed correctly to its mounting. However, re-arranging and checking the cables did not fix the problem.
Just before beginning to patch the tail light wiring into the fog lamp I tried removing bulbs from the tail light circuits. My theory was that this would put less load on the circuit, perhaps allowing the body computer to supply sufficient current. When I removed the combined tail light/stop light all the other lamps came on. Hurrah I'd found the problem!
It turns out that the combined tail/stop light had an internal short between the 5w and the 21w filament. This had put a load of 21w plus (3 x 5w) on the tail light circuit, presumably causing the body computer electrics to overload.
Also, when it was quiet I noticed that the body computer made an unusual sound when I turned the side lights on. It was a small hissing noise, which I would associate with a small electric coil. It only lasted a couple of seconds before disappearing. Once I had replaced the bulb the body computer stopped making the noise.
Good job!
Now, what choice words can I use when I go back to speak with the service manager at the FIAT garage. Can they really not diagnose a faulty rear light bulb?
I dismantled and checked the cable connections to each rear light cluster expecting to find a loose connector somewhere. They all appeared fine and the fact that the indicator lights and brake lights worked demonstrated that the majority of connections were in place, including the ground. The tail lights on the GP are fed by two circuits from the body computer with two 5w lamps connected to each. The lower 5w lamp is half of the combined tail(5w)/stop light(21W) bulb.
I finished up my first diagnosis session by re plugging the connectors to the body computer and retiring to consider the circuit diagrams.
My next step was to connect my trusty Actron ODB II reader to the ODB port, but which, unfortunately revealed nothing. Then I tried using PCMSCAN from a laptop but unfortunately it didn't recognise the Body Computer so consequently couldn't display any body computer specific information.
At this point I abandoned the diagnosis and took the car into my local FIAT dealer. They diagnosed a failed Body computer and estimated the cost to replace at approx £400. Wow! It looked as if it would cost me £400 to fix four non working tail lights. Not a great thought. Looking at the wiring diagrams it looks as if the Body Computer in central to virtually all the electrical circuits in the vehicle. Having a £400 body computer as the weak link in even the most basic of electrical circuits doesn't seem too bright.
Having a few hours free I started looking for ways to bodge a connection from the rear fog lamp to the tail lights. By sheer good fortune something I did caused the tail lights to come on for a few seconds. Perhaps there was a chance to fix the fault after all. The lights came on just after I moved the steering column adjuster. I remembered that the last time I drove the car I had discovered that the steering column also moves back and forward as well as up and down and I had adjusted the position. Removing the steering column covers revealed the movement had trapped part of the wiring loom, the looms retaining clip not being fixed correctly to its mounting. However, re-arranging and checking the cables did not fix the problem.
Just before beginning to patch the tail light wiring into the fog lamp I tried removing bulbs from the tail light circuits. My theory was that this would put less load on the circuit, perhaps allowing the body computer to supply sufficient current. When I removed the combined tail light/stop light all the other lamps came on. Hurrah I'd found the problem!
It turns out that the combined tail/stop light had an internal short between the 5w and the 21w filament. This had put a load of 21w plus (3 x 5w) on the tail light circuit, presumably causing the body computer electrics to overload.
Also, when it was quiet I noticed that the body computer made an unusual sound when I turned the side lights on. It was a small hissing noise, which I would associate with a small electric coil. It only lasted a couple of seconds before disappearing. Once I had replaced the bulb the body computer stopped making the noise.
Good job!
Now, what choice words can I use when I go back to speak with the service manager at the FIAT garage. Can they really not diagnose a faulty rear light bulb?
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