LoneShark

Just learned that my '87 S4 project car is missing the hatch-mounted 3rd brake light that it would have left the factory with. Instead I have a nice uncluttered rear-view of two short wire stubs. Beyond concerns of annual inspections and the poe-lease, are there any electrical ramifications I should be aware of?

Might this be the source of the "Stop Light" dash warning every time I use the brakes?

If so, any quick fixes to trick the lamp moitoring system? Such as grounding one or the other of the two wires, connecting a small resistor inbetween to mimic the missing bulb, or maybe simply connecting the two remaining wires together?

Thanks in advance!!!

Mrmerlin

those 2 wires are to run a brake light so connecting them will result in a blown fuse or fried harness.

try connecting a bulb to the wires and see if the lights go out on the dash ,
since the system is wanting to see 3 brake lights when the pedal is pushed adding the center light should make the warning go out.
You may have to refit the center light housing,
If the mount tabs are missing then you can do this with some silicone sealant and blue tape till the sealant cures

Lizard928

That is the source of your "stop light" warning yes.

Cover them up and tuck them up behind the covering, or connect a 3rd brake light.

mrdieseldude

Why won't a "dummy load" resister installed in place of a lamp do the trick for the loner? I don't think any system is sophisticated enough on a 928 to tell the difference between these two loads is it? Its sure easier to hide a 2 watt (think that would be more than big enough but someone should to the calculation first) resistor than a lamp.

Alan

You will need an approx 6.8 ohm 21W resistor - this will need to be a power resistor bolted to metal for dissipation - you can do this neatly on the inner skin under the sunroof cover - plenty of room.

There are no other electrical ramifications - adding the 3rd brake light unit is easy too - maybe easier than adding the resistor

Alan

Bart-Jan

and safer...

mrdieseldude

Wow! 21 watts. I really goofed on that 2 watt call. Maybe he can add a hot plate instead???... at least that can keep his coffee hot.

77tony

Am I correct in assuming that a 6 ohm, 50 watt resistor will work ? Superbright P/N: RL-650 ? TIA, T

76FJ55

The system is designed to sense a balanced load on all three lights. Since the factory light is 21W that is what the resister needs to be + or - some small tollerence. If you use a 50W resistor the load will not be balanced and again the warning light will illuminate, unless you increase the wattage of the remaining brake lights to 50W each. This would cause significantly more current draw on the brake light circuit so is not advisable.

77tony

More like the Superbright P/N: RL 2525, 25 ohm, 25 watt resistor ? 77Tony

TheoJ

P=V*A. Power = Voltage*Current (amps) If you were to simulate a 21W bulb, you would be looking at a resistor of about 6,85 Ohms. How much power will it dissipate? 21W. To be on the safe side, a 25 or 30W resistor mounted to a larger cooling frame would be sufficient. Don't make the system different from what was designed Power wise. Use the power spec from the original bulb in the 3rd brake light. It might be less than 21W. I don't know for sure as the 3rd brake light was not equipped in Europe cars.
It may take a bit of experimenting. The Bulb has a non linear curve when it heats up. Go from a higher resistance down until it is not critical anymore.

The bulb controller was changed for a 3rd brake light. The 928.641.603.11 version is for 3 bulbs, the .09 version is for two. I don't know exactly what the MOD was but it might be only a resister value or a different comparator chip. If someone knows, maybe post it here?