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]

What causes a fuse to blow?

This is an unexpectedly complicated topic.

All electrical connections have a certain amount of resistance. Resistance limits the current that can flow through the wire. The wire itself will have some resistance (usually very low), and the items that do useful things, like lights, motors, and radios, will have a higher resistance. A dash bulb, for example, will be rated at 2W, so about 1/4A will flow through it (2W / 13v), which is a fairly small current. A dash bulb has a very high resistance. A headlight bulb will be rated at about 55W, so about 4A will flow though it, so it has lower resistance. A 2W dash bulb needs a much thinner wire to pass its lower current than a headlight bulb does.

Current through a wire causes heat. Too much current through a wire will melt the insulation, which may catch fire. The heat given off may also set something else on fire, like seat or dash fabric, or fuel. Wires are sized by the amount of current they are normally expected to carry. Usually, the smallest wires possible are used to save bulk, weight, and cost of the wiring harness.

If the resistance in a circuit becomes very low (near 0), then you get a short, and far more current than is desired will try to flow through the wire, which will get very hot, and perhaps start a fire. Simply disconnecting a power wire and touching it to a ground post will cause a short.

To prevent damage to the wire, and esp. stop a fire, most of the circuits are fused. The fuse is nothing more than a strip of wire that's calibrated to burn up when a set current passes through. Most of the fuses in the 814 fusebox are 8A fuses, usually there to protect circuits made mostly of 18-20 gauge wire. The fuse will burn up long before a 20g wire will, so if a short occurs, the fuse will blow, and the temperature of the wire will never get high enough to burn.

The usual causes of a fuse blowing is simply something that's disconnected and dangling. Sometimes, it's caused by a wire that's chafed from rubbing on the sharp edge of a hole. The wire touches the body, and you have a short. Now and again, a device like a switch or a motor will develop an internal short because of some mechanical fault.

The usual divide-and-conquer approach mentioned in the problem solving section will work to find the short. You know which fuse is blowing, so follow all of the circuits attached to that fuse until you find the shorted connection.

One problem common to 914s and many other European cars of the period is that the bullet fuses are made of materials that corrode over time, and that corrosion will insulate the fuse, making it appear as though it's blown. The contact surface of the bullet ends against the fusebox "fingers" is also very small. Rotating the fuse in the holder can often scrape off enough of the corrosion to allow the fuse to work. These fuses can also sometimes blow but still appear to be intact. If you remove the fuse from the holder, the metal part of such a fuse will usually crumble, even though it looked fine in the fusebox.