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]

Fuel gauge specs

The stock fuel level sender is on the top of the fuel tank, under the expansion tank surrounding the fuel filler. There are three connections: brown, which grounds the sender (should connect to the body); green, which is for the fuel level resistance; and black, which is for the low-fuel light (connects to the brown wire when the fuel is low, is open when the fuel is high).

The sender has a float that shows very low resistance (0-10 ohms) when the tank is full, and higher resistance (70-90 ohms) when the tank is empty. There's a separate set of contacts for the low fuel light. +12 is expected to appear on the black wire, and when the low fuel light switch closes, it grounds this connection.

The resistance 75-0 ohms, empty to full, matches the resistance for the early 911 fuel gauge, and the air-cooled VW gauge. VDO sells aftermarket gauges that match the VW resistance.