This article is meant to help you make sense of that rats nest behind your dash. We promise you, Volkswagen had a method to their wiring and if you know the basics; it’s not that hard to understand. So, you bought a car, looked under the hood, and saw a mess of wiring. If it’s all chopped up or has colors you have never seen before, it might be a good idea to start over. Here are some thoughts you should keep in mind: A plain red wire should be hot all the time (An exception is the starter solenoid wire, which in most cases is plain red). A plain black wire will only be hot when the key is on. A plain brown wire is the ground. In most cases, if you see a stripe on a black wire it is a function of something with the key on. A stripe on a red wire is typically a function of something without the key. A brown wire with a stripe is a function of a ground. There are a lot of other colors in there, but this is the basics on the main power sides and ground.
All black and red wires in the fuse box will take a 16 amp fuse; all other colors will take an 8 amp fuse. Also, if you look closely at the switches you will see numbers. Here are some clues as to what they are: The number 30 is always red and hot all the time. The number 15 is black and is only hot with the key on. The number 31 is always ground.
Tricks of the trade: Before you hook up your battery, make sure all switches are off. Make sure your dome light is off or it will cause it to show a false short. Make sure the key is also off. Now, hook up the negative cable on the battery. Take the positive cable and just tap it on the positive post. If you hear or see a spark, you have a short. Don’t hook it up.
If you don’t hear or see a spark, then push it on and turn the key to its on position. If things are good, then make sure you’re out of gear and hit the starter. If you’re messing with the wiring and need to have the key on, disconnect the black wire from the positive side of the coil; This will keep your ignition from burning up.
Gauges
“Steve, I want to add gauges” or “Someone added gauges and I want to straighten that mess out, what do I do?” Here is what I do:
I use black for the hot because I know that it will be hot with the key on. I use brown for the grounds. Also, on the grounds do not chain the illumination ground with the gauge ground. If you do, when you hit the lights it will effect the gauge reading. I use gray with a red stripe for the illumination (if I can find some) because that is the color of the stock dash lights. If not, I use white or gray as it’s as close as I can get to the stock color.
Tach: For the, tach I use green wire because if you look at the stock wiring, you will have black wires on one side (the hot side). The condenser wire is Green (the negative side). This way, when you pull your motor out you know that black wires are on the positive side and green wires on the negative side.
Oil Pressure: For the oil pressure gauge I use blue wire because the stock wire for the oil pressure idiot light is blue with a yellow stripe. This way, I will know that it has something to do with oil pressure. On the dual pole sender, which I highly recommend, there are markings on the poles. “WK” is for the idiot light. Use the stock wire - blue with the yellow stripe. Your blue that you ran for the gauge should go to the G. (The way I remember this is WK is work the light. G is Gauge.)
Oil Temperature: For the oil temp gauge I use yellow wire because, again, the stock wire for oil is blue with a yellow stripe. So by using yellow wire I know it has something to do with oil.
Amp Gauge: Amp gauge use should be red wire as that gauge reads the main power, whether the key is on or not.
Volt Meter: The volt meter should be just Black for the keyed power and Brown for ground.
Aftermarket Gas Gauge: An aftermarket gas gauge an exception to the rule. On the gauge side you will have a black for power, brown for ground, and brown for the sending unit. Why? That’s how the factory did it. Keep in mind the sending unit does not know voltage, it’s a floating ground.
Cylinder Head Temp Gauge: For this gauge, you don’t have a choice; You have to use the wire you get with it (red&black). Keep in mind that gauge does not have power going to it.
Stereo Temp Gauge: You’re on your own.
Stereo Amplifiers: Stereo amps want you to have a main wire going to the positive of the battery. If you don’t switch that wire, your amp will have a slight draw on the battery. If you don’t use your car everyday then it will drain the battery. I personally switch this wire so when I shut the car off the amp is off. Back to the beginning of the article, if you have a stereo amp and you tap the positive and see a spark then leave the main amp wire off and you will see what I am talking about.
One last thing: The hardest part about wiring is getting the smoke back in the wire.