Electrics FAQ Options

[lapuwali]

General problem solving

A few things to remember:

All electrical circuits start at the battery + post and end at the battery - post. A particular circuit may take a very convoluted route to get from one to the other, but they ALL do this.

The battery - post is directly tied to the body, so the entire body acts as the "ground" for the system.

Paint is a great insulator. Even a small amount of paint will completely break the circuit.

Air is also a good insulator. Connections can't be loose.

Corrosion and dirt are good insulators. Connections have to be clean.

90% of the time, a problem is caused by a bad connector, not a bad wire.

The car runs off the battery. The alternator is just there to keep the battery charged.

The 914 (and most Porsches and VWs) follow a few simple rules in coloring wires. Brown wires are ground wires (from an item to ground, usually the body). Red wires are unswitched power wires (wires that don't go through the ignition switch). Black wires are switched power (wires that do go through the ignition switch). Note that some combinations don't follow this pattern. Red wires with a white stripe, for example, provide switched power to the gauges.

Divide and conquer is generally the best strategy. If something isn't working, start at the item and check the connections directly at it. Then follow the wires in each direction (one to ground, the other to + [perhaps through a switch]), checking each intermediate connection. Just looking at connections often isn't enough, as connections can frequently look fine, but not actually be properly connected (this is esp. true if the problem is intermittent). Try flexing any connections to see if insulation is broken, or if the problem item starts/stops working. If the wire disappears deep into a wiring bundle, just look at the other end of the bundle for that wire. As stated earlier, wires in the middle of a bundle are almost never the cause of a problem. On the 914, most of the wiring is fairly well exposed, and can be inspected all along its length.

If there are no problems with connections to be found, then the problem may be in a switch in the circuit. You can check to see if the switch is getting power by disconnecting the power in wire (there will be one, perhaps red, perhaps black, perhaps some other combo) and see if a voltmeter shows 12v between that wire and ground. If the switch is getting power, then check to see if the switch is passing power through it when turned on (check the power out terminal on the switch for +12 to ground, same as the power in). If it's not, the switch is the problem. If the switch isn't getting power, then the problem is "upstream", closer to the battery, so keep travelling up the wiring chain. This may lead to the fusebox.

Wiring diagrams can be hard to follow, but the divide and conquer approach works here, too. Just look for the item you're interested in (there's a key, all of the items are numbered), then follow the connections on the diagram to see where they go. A typical chain is:

battery + to fusebox to light switch to dash lights to dash gauge body to ground wire to ground stud

Once you find the dash lights on the diagram (crossed circles), you can follow the wiring from battery + to ground and see all of the intermediate connections. It may help to photocopy the diagram and use a highlight marker to follow the wires in one circuit.

If several things fail at once, look for common connections. For example, the dash gauges share a ground between the lights and the gauges themselves. The tach and the fuel gauge need a ground to work (the speedo is mechanical). There are two power connections, one for the tach and fuel gauge, one for the lights. If the tach and the fuel gauge and the lights don't work, it's more likely to be the ground, as there's only one ground, but two separate power connections. If the dash lights don't work, but the gauges themselves work, then it can't be the ground (or the gauges won't work), so it must be the power to the lights. If the headlights work, then headlight switch must be getting power, so the problem has to be inside the headlight switch itself.

[lapuwali]

Basic theory

Current is the "amount" of electricity flowing, and is measured in amperes (A or I).
Voltage is the "pressure" of the flow, and is measured in volts (v).
Resistance is, well, the resistance to the flow, and is measured in ohms.

There's a simple relationship here: V = IR. Volts = Current multiplied by resistance.
Overall power is measured in Watts (W), and is volts times amps. A 12v, 10A is 120W.

A "12 volt" car circuit is actually more like 14 volts. A healthy car battery will show 12.5v across the terminals, and the alternator is regulated to actually charge the battery at 13.5-14v. So, the typical voltage in a car with the engine running will be over 13v, not 12v.

Resistance in a wire includes the length of the wire as well as the diameter of the wire. Resistance to current flow shows up as heat in the wire, and the more current, the more heat is involved. A longer wire for the same current and wire diameter will also get hotter than a shorter wire. So, you have to take wire diameter (aka the wire gauge), wire length, and the expected current to determine what's safe in terms of preventing the wire insulation from melting and starting a fire.

Wire diameter is measured in "gauge", where the smaller the number, the bigger the wire. A typical wire in a 914 will be 18 gauge. The smallest will be 22g. The largest will be about 4g.

The largest amount of current flow in the 914 is between the battery and the starter motor. A typical starter motor is about 1.5hp. HP and Watts are both a measure of power, and there's 741W per HP, so a 1.5hp starter is 1100W. At 12V, that's 91A of current (1100/12 = 91). Typically, the starter has such a huge draw that the voltage is actually dragged down to 9-10v, so it's actually more like 110A-120A. If it's very cold outside (way below zero), and the engine is stone cold, it will take even more power to turn the engine over. Batteries are typically rated at 400-600A.

Wire of a certain gauge is rated in ohms/ft. 6g wire is rated at 0.00048 ohms/ft. This seems really small, until you intend to run 100A though it. Remember V=IR, so V = 100 * 0.00048, or V = 0.048v. This is the amount of voltage "lost" due to the resistance, or the "voltage drop". The rule of thumb is the voltage drop should be no more than 2% of the original voltage (12v, more or less). If the battery to starter wire is 3ft long, and the ohms/ft is 0.00048, the total resistance is 0.00144 ohms, so the voltage drop for a 100A current over three feet of 6g wire is 0.144v, or 1.2% of 12v (or 1.4% of 10v). So, 6g is plenty for this purpose.

Headlights draw about 55W (low beam) each, 110W for both. The wire bundle from the headlights to the battery is about 20ft long. An 18g wire over 20ft has 0.155ohms of resistance. For 9A (110W/12V), this means a voltage drop of 1.4v, which is way more than 2% of 12v, so, an 18g wire isn't enough for that amount of current over that distance.