mikex7

So I'm having a lighting issue on my 2000 a4 coupe, I'll just bullet point the issue to make it as clear as possible.

• Both left rear brake lights do not work (brakes or hazard lights)
• Left blinker works perfectly (same bulbs???)
• When car is idling (and probably when driving, I just can't hear it) there is a "whirring" sound that is hard to describe coming from behind my drivers seat somewhere between me and the back of the car.
• Whirring sound changes pitch when I push anything electrical in the car, like the automatic window UP button.

Because of that weird whirring sound that is affected by my using any electricity in the car, I am thinking it might be a wiring problem somewhere? Anyone ever experienced anything like this before and where the heck should I begin?!

65GGvert

The whirring noise is probably the fuel pump. Not a problem, just annoying. Turn up the stereo or rev up the engine. The light problem is probably the hazard switch. Try pressing the hazard button on and off several times then try the brakes to see if you see any change. You can do a search on hazard switch and get info on symptoms and pictures of the easiest way to replace. Post what you find.

mikex7

That's really interesting. How come any electrical usage would have an effect on the sound of the fuel pump if that's what it is? I did what you said with the hazard light to no avail. Turned it on and off a bunch of times and still no dice. I'll look in to replacing it in the near future. Do you recommend taking a look at anything else before I dive in to that? Maybe checking for loose/corroded wiring or something to do with the bulbs?

lucky131969

PM sent.

65GGvert

Lucky131969 can help. I wish there was an easy way to bypass or substitute the flasher to troubleshoot. It could be other things, multifunction switch, grounds, fuse. Lucky and a couple of others on here have technical info to help. The change in speed of the pump could indicate a loose or corroded ground. I hoped you could find it the easy way, but looks like not. Sorry. Good luck, post what you find.

dents

Fuel pump activity varies based on voltage (that's how boost-a-pumps work), so using anything electric can be causing a voltage drop and making your fuel pump spin slightly slower. I have an extra working hazard switch you can borrow if you want. You would have to pull your dash to change it, but that's less work than it seems. Did it a few weeks ago when I threw in a HUD, definitely doable in half a day.

lionelhutz

Typically, the signal working but brake not working is the signal switch or it's also called the multifunction switch. The signal switch changes the wiring connection from the brake light power to the signal power when it is activated. So, if the brake light contact is broken you get no brake lights for that set of bulbs.

If I recall correctly, the brake lights are the only ones that are switched by the hazard switch. However, this is power for all the brake lights before they are split for each side by the signal or multifunction switch. Contrary to popular belief, none of the individual signal wires which power the different bulbs go through the hazard switch. In normal operation, the hazard switch only functions as the flasher and the feed through for the brake light switch. Conclusion - your signals work perfectly and presumably your right side brake lights work so it's not the hazard switch.

I just wish I could double check the schematics right now but the computer I was using for that died.

Peter

byronhunter

OK, this what you want? I agree, his problem is the Multifunction Switch.
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lionelhutz

Yup, that's the one.

The hazard switch passes power for all the brake lights - if at least one brake light works then it's not the hazard switch.

Peter

wrieck

Mike, did you ever solve your brake light issue? I'm having the same exact problems on my '98 C5. Any help would be greatly appreciated!