Reverse lights not working

[DefiantS4]

Hopefully somebody can give some advice or explain how the reverse light circuit works, because I've been tinkering with this thing for many nights and can't seem to figure it out.

Starting a couple years ago, fuse #31 in the fusebox would pop every once in a while taking the reverse lights with it. I didn't feel like dealing with it completely so I simply spent the few dimes on a new 15A fuse every time it happened. Now it's not as simple anymore. The reverse lights simply won't come on, even with a good fuse. I tested the fuse socket itself with a multimeter with the ignition on and shifter in reverse, and nothing. I just now dug down through the engine bay to the reverse light switch on the passenger side of the transmission. It appears intact. I unplugged the connector and reseated it. Still no reverse lights and the fuse remains intact.

Any ideas? Will a VAG-COM diagnose the circuit?

[The_Jerbel]

Try unplugging the connector and putting a voltmeter across the terminals. See what kind of voltage runs through with the tranny in reverse and the ignition on.

[DefiantS4]

Thanks for the tip.
Here's an update...
I did just that, unplugged the connector and managed to get my multimeter's leads onto the switch terminals. No voltage at all, but then I realized that there shouldn't be any since it is merely an on/off switch activated by the reverse gear. Tested instead the resistance through it in both reverse and neutral, and verified that the switch itself is good. Reconnected everything and went to the fusebox. Yanked out the #31 fuse and stuck the multimeter leads in its place. Voltage was 0 with the car off. 10.8V with the ignition on, 12.0V with the ignition on and reverse gear selected. OK so now I'm actually getting somewhere. The fact that the voltage through the #31 fuse changes between neutral and reverse must be a good thing. Still no reverse lights though. Is there possibly a relay somewhere down the line after the fuse that I don't know about? Thanks in advance.

[DefiantS4]

Some more discoveries:
Voltage at the reverse light sockets is 12V with the ignition on and shifter in reverse... and without bulbs in them. If I'm measuring the voltage at the socket on one side of the car and put the bulb back into the other side, the voltage goes to 0. I revisited fuse #31... yanked it out, stuck a pair of wires into the socket and touched them to a bulb, and no light. Same story as the sockets in the very back of the car. Voltmeter would read 12V, but stick anything with resistance like a lightbulb into the circuit and the voltage falls to 0. I'm no electrician but I seem to remember "current follows the path of least resistance"... more than likely a short somewhere? This is gonna be loads of fun.

Over the past 4 years I've slowly been discovering what the previous owner did to this car. The wires on both front amber lights were cut and taped up with masking tape. The independent foglight mod and 2nd rear fog light were done, half-assedly... the lights only work 75% of the time. I can only guess that they had a strobe light module installed, and if the reverse light circuit was tapped for it that explains everything.

[The_Jerbel]

Hey I'm glad you're making headway.
Sorry for the confusion, I was tired when I wrote that. What I meant, was to run 12 volts THROUGH the switch with the tranny in reverse and see if there's continuity.

I've dealt with reverse switch problems on multiple VAG cars, so I've gone through this "reverse-lights-not-working" troubleshooting process many times before. Stupid things always have issues on german cars.

Anyways, you are correct in your assumption. It could be a short. I would look at both bulbs and make sure that the filaments are intact. If one of them is burnt out, neither will light up IIRC, as they are wired in series.

If nothing else, even replace both bulbs just to be sure that isn't your problem (they're so cheap that you might as well). If that's not it, then start looking into a short.

[DefiantS4]

Well I finally figured it out. After tearing half of the dashboard and trunk apart to inspect the wires, I revisited the reverse switch. I realized that my earlier conclusion was wrong... that if it truly is just a switch, it shouldn't have been showing the amount of resistance that it was when closed. I took the connector going into it and bridged the terminals with a piece of wire... the reverse lights lit right up. I removed the switch from the transmission and disassembled it to see what was going on. It turned out that plunger wasn't reaching far enough to trip the switch. A tiny spacer was all it took to get it working again. I was amazed at the unnecessary complexity of the internals of the switch... I'm surprised we don't see more posts here about reverse lights being stuck on or not coming on at all. Thanks for the help!