I owned a 2009 Audi A3 with a rear lightbulb error that eventually led to a lightbulb failure. For months there was a nagging error icon on the instrument panel that indicated one of the rear brake lightbulbs was out. When I finally checked, I found that all the brake lights on the rear of the car were working, even though the indicator displayed a problem. On a subsequent check, I found that two of the bulbs had burned out.
With the annual safety inspection looming, and half the brake lights not working, I finally had the incentive to fix the problem.
When I removed the rear light assemblies, I discovered that the non-functional lightbulbs were still good. The bulb holder assembly is a three-dimensional injection molded plastic unit that combines with a three-dimensional formed galvanized steel sheet metal frame that serves both as a structural holder and electrical conductor. The sheet metal frame is swaged to the plastic structure by thermally melting six studs.
Two years in the Texas heat had raised the temperature of the plastic to a level that the creep strength of the plastic was exceeded. The two bulbs that didn't light were in an assembly where the plastic actually flowed between the sheet metal holder and the ground cylinder of the lightbulb. The steel frame had raised about a quarter inch off its intentional mounting and tilted, allowing the ground frame to contact plastic and initiate the plastic melting. That broke the circuit.
The dealer was out of stock on this assembly, which was just as well. The price of each assembly was about $100 with tax. So I decided to break the studs off, then drill pilot holes for #4 sheet metal screws to replace the swage studs. All the lights now work perfectly, and the instrument status error has been eliminated.
While I admire the simplicity of the design, its execution failed under conditions that should have been within the designer's requirements. It isn't clear whether the sub-tier supplier cheated by using the wrong plastic, or if this is a common design flaw, which has gone unresolved over the five-year production run of this model.
This entry was submitted by Kirk Miller and edited by Rob Spiegel.
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