Simple enough at first glance. (You can read more about the fault in an earlier post).
It turned out to be the side light/brake light that was inoperative.
I checked the lamp visually it appeared fine, so I measured it's resistance it showed 0.6 ohms.
This is the bit that got me thinking..............is that right? I compared it with the working side light and it too had a resistance of 0.6 ohms. So it is correct. Or is it?
These cars and many newer vehicles have done away with many of the fuses and use the ECU's to monitor faults, if any faults are detected it simply switches off the outputs. Then a warning is displayed in the dash panel. The joys of vehicle networks.
I set about some calculations to find out if the measured resistance was correct.
I knew the bulb rating 21W 12V therefore I could work out the current flow.
P= I x V
I =P/V = 21/12 = 1.75 Amps
Ohms law states I = V/R
R = V/I = 12/1.75 = 6.9 Ohms
So why was the resistance of the lamp filament so wrong?
Lamps convert heat into light, heating a conductor will increase its resistance. A lamp will show around 1/10th of its operating resistance in a "cold" measurement.
This can be clearly seen in the scope trace above, this was taken after I repaired a high circuit resistance in the wiring to the lamp. (the cause of the lamp failure warning you can read about in an earlier post).
Notice the "cold" resistance is much lower than the "hot" resistance seen as a high current 11 Amps falling to the operating current of 2 Amps. Notice how the Body ECU warms the filament with a short burst of current before switching the lamp on this stops current spikes and prolongs filament life.
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