This article is one in a series that have been released in conjunction with Wayne's book, 101 Projects for Your Porsche 911. The book contains 240 pages of full color projects detailing everything from performance mods to changing your brake pads. With more than 650+ full-color glossy photos accompanying extensive step-by-step procedures, this book is required reading in any Porsche 911 owner's collection. See The Official Book Website for more details.
One of the most common problems with older cars is the existence of intermittent and annoying electrical problems. Example: a dashboard light goes on when you hit the brake, but only when the rear defogger is on, or the radio only works when you are in reverse. As bizarre as it sounds, electrical problems like these are very common on these older cars, and unfortunately, they can be quite difficult to fix.
What typically goes wrong with the wiring on these cars? There are several things that can happen. First and foremost, every time the car is sold to a new owner, there is a high chance that they will do some modification to the wiring that only they will know about. The installation of a new stereo, power mirrors, a radar detector, or the worst of all, an alarm system, can seriously mess up your wiring configuration if not performed correctly. You're left holding the mess, armed with only a few sporadic clues as to what is causing the problem. Troubleshooting electrical problems is a tough chore, and one that most automotive repair shops will not perform without telling you they will charge you their $80 an hour diagnostic fee.
This project will give you some tips for troubleshooting your electrical system, but it's not meant to be a step-by-step guide for fixing all of your problems. That would take almost as many pages as are contained in this book!
The first step in troubleshooting is to make sure that you are armed with all the information that you can get. Namely, the most important item you need to get your hands on is a copy of the electrical diagrams for your year car. At the time of this writing, they are available from a few sources. Check the information sources section in the front of this book for more details. These diagrams are essential for troubleshooting electrical problems.
One of the most common problems is the continuous drain on the battery. One example would be a situation where you leave the car sitting for a week or two, and the battery becomes completely drained. This means that something is "on" inside the car at all times bleeding the battery of power. Start by disconnecting your battery ground, and connecting an ammeter to the battery negative and to the chassis. The ammeter will show the amount of current that your electrical system is draining from the battery. When you hook the meter up, it will most likely show that there is some small current flowing through the system. Don't try to start the car or turn on too many electrical accessories, because this might blow up your meter. Disconnect the front luggage compartment light before beginning any testing.
Now, move to the fuse box in the front trunk, and start removing fuses. See Project 26 for a picture of the fuse box located in the front trunk. Carefully watch the ammeter to see if the current drops to zero when a particular fuse is pulled. If it does, then you have successfully isolated the electrical circuit with the problem. You are more than halfway to solving your problem. Look at the electrical diagrams and you should be able to tell what components are located on that circuit. Try disconnecting each one, while watching the ammeter, and you should find the culprit.
If pulling fuses doesn't reveal anything, try pulling out the various relays. Sometimes a relay will be powered on, but the device it controls will be disconnected. This may lead to gradual battery drain. One example of this is the heater levers in-between the seats that control the blower motor in the engine compartment. If the blower motor is disconnected, but the levers are pulled up, the relay will be activated, and the battery will slowly drain down.
While we're on the subject of wiring diagrams, in 1975 Porsche switched from using typical voltage wiring diagrams to using what is called a current flow diagram. How do you read a current flow diagram? It took me a little time to figure it out. Look at it this way. Imagine that you are looking at a diagram that shows waterfalls emptying into a lake. At the bottom of the page (electrical ground), it is similar to a large lake that the waterfalls empty into. At the top of the page (high voltage potential) it is similar to the top of the cliff, right before the waterfalls off the edge. Each path that the current diagram shows can be interpreted as a separate waterfall that turns a small turbine and generator as it falls down into the lake. The electrical accessory can be seen as the generator. The battery of the car is similar to a pump that pumps water from the lake back up to the cliff.
After you get used to them, you will undoubtedly find that the current flow diagrams are much easier to read, primarily because they separate circuits from one another. You don't need to look at one circuit that you don't care about, just to find an electrical fault in another. The current flow diagram can tell you everything that you need to know right away.
Another common electrical problem is the device that just won't work. If you carefully look at the electrical diagrams, you will notice that there are actually six points of failure for most electrical devices on the 911. For lack of a better example, we'll use a blower motor to explain and demonstrate the electrical troubleshooting process.
Starting from the rear of the electrical chain, the first point to be concerned about is the actual device itself. You can start the troubleshooting process by testing the motor. Unplug it and apply 12V DC to the blower motor to see if it will turn. It if doesn't then you have a problem with your blower motor.
While the motor is unplugged, another excellent test to perform is to check the electricity in the wires leading to the blower motor. If all the switches are on, and there is no power going to the blower motor, then the problem lies somewhere else.
Next spot to check would be the relay for the blower motor. Consult the electrical diagrams to determine which one is the correct relay. When the switch is pulled for the blower motor, the relay should make a slight clicking noise on and off. Swapping out relays with one that is known to work is a good method of checking the proper operation of the relay as well.
If the relay checks out, then you want to make sure that the fuse is still good. Identify the proper fuse, and check to make sure that it has not blown. Also, the long tubular fuses that the 911 uses are susceptible to corrosion building up on the terminals. Make sure that the fuses are clean and securely seated. If necessary, check the continuity across the two points that hold and mount the fuse. Photo five in Project 26 shows the location of the fuse box in the front trunk, on the left side.
Many electrical components on the car are also switched through the ignition switch: allowing them to be turned on and off with the ignition. If your blower motor doesn't work along with a host of other equipment, then you might have a faulty electrical ignition switch. One clear symptom of this is the blower motor turning on and off as you jiggle the key back and forth. For ignition switch replacement procedures, see Pelican Technical Article: Replacing your Ignition Switch.
The switch for the device itself may be faulty. In the case of the rear blower motor, the switch is connected to the red handles on the floor of the car. Check the switch with a continuity tester to make sure that it is working properly.
Finally, if all the other tests fail to locate the problem, the wiring may be at fault. Especially on older cars, the wires have a tendency to become brittle, and sometimes break, even if the outer insulation is intact. Using a continuity tester, check each of the wires in the harness that powers the blower motor to see if any have lost continuity.
If all of these steps fail to pinpoint the problem, then you probably made a mistake somewhere along the line, or there might be a short circuit somewhere in the switch or the wiring of the car. Only painstaking testing using a continuity tester will be able to locate such a problem.