Troubleshooting Electrical
Problems on Your BMW
Wayne R. Dempsey

Difficulty Level: 4
Difficulty scale: Adding air to your tires is level one
Rebuilding a BMW Motor is level ten

[click to enlarge]

     One of the most annoying problems with older cars can be an intermittent electrical problem. 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 have become more and more common as cars have become increasingly complex. Unfortunately, they can be quite difficult to fix.

     What typically goes wrong with the wiring on these cars? Several things can happen. First and foremost, every time the car is sold, there is a big chance the new owner will do some modification to the wiring that only he or she will know about. Installing a new stereo, European headlamps, a radar detector, or—worst of all, an aftermarket alarm system—can seriously mess up your wiring configuration if it’s 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 most automotive repair shops will not perform without telling you they charge an $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 you are armed with all the latest and greatest information available for your car. Obviously, the most important item you need is a copy of the electrical diagrams for your model year. At the time of this writing, these diagrams are only available from a few sources. BMW has published books with extensive electrical diagrams in the past, but these can be difficult to find. Bentley Publishers prints manuals for most BMW 3 Series cars with summarized electrical diagrams in the back; these are available at PelicanParts.com. They are definitely a good starting point for troubleshooting.

     One of the most persistent of these nagging problems is a continuous drain on the battery. Say you leave the car sitting for a week or two, and when you come back, you find the battery completely drained. This means that something is on inside the car, bleeding the battery of power. Start your troubleshooting process by disconnecting your battery ground (see Project 84) and connecting an ammeter between the battery negative and 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. Warning: Don’t start the car or turn on any electrical accessories, because this might blow up your meter.

     First, disconnect the trunk (E36) and engine compartment (E30) light before beginning any testing. Now, move to the fuse box in the front engine compartment, and start removing fuses. Carefully watch the ammeter to see if the current drops to zero when a particular fuse is pulled. If it does, you have successfully isolated the problematic electrical circuit. You are more than halfway to solving your problem. Look at the electrical diagrams, and see if you can 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, however, try pulling out the various relays. Sometimes a relay will be powered on, but the device it controls will be disconnected. This may also lead to a gradual battery drain.

     Another common electrical problem is the device that just won’t work. If you look carefully at the electrical diagrams, you will notice that there are actually six points of failure for most electrical devices. For lack of a better example, we’ll use a horn 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 horn. Unplug it, and apply 12 volts DC to the horn to see if it will make a sound. It if doesn’t, then you have a problem with your horn.

     While the horn is unplugged, another excellent test to perform is to check the electricity in the wires leading to the horn. If you press the horn button on the steering wheel and there is no power going to the wires that power the horn, then the problem lies somewhere else.

     The next spot to check would be the relay for the horn. Consult the electrical diagrams to determine which one is the correct relay. When the horn button is pressed, 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, you want to make sure that the fuse is still good. Identify the proper fuse that powers the horn circuit, and make sure it has not blown. Also, keep in mind that the spade-type fuses that BMW uses are sometimes affected by corrosion building up on the terminals. Make sure the fuses are clean and securely seated. If necessary, check the continuity across the two points that hold and mount the fuse.

     Many electrical components on the car are also switched through the ignition, enabling them to be turned on and off when you start your car. If a device such as a window motor doesn’t work, along with a host of other equipment, you might have a faulty ignition switch. One clear symptom of this can be seen when the dashboard lights and other equipment turn on and off as you jiggle the key back and forth. Then the switch for the device itself may be faulty. In the case of the horn, the switch is mounted in the center of the steering wheel. Remove the wheel, and check the switch with a continuity tester to make sure it is working properly.

     Finally, if all the other tests fail to locate the problem, the wiring itself may be at fault. Especially on older cars, the connecting wires tend to become brittle and sometimes break, even if the outer insulation is intact (see Photo 2). 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 none of these steps succeed in helping you pinpoint the problem, then you may have made a mistake somewhere along the line—or there might be a short circuit hiding somewhere in the switch or the wiring of the car. In that case, you’ll need to continue with more painstaking tests using a continuity tester in order to locate the source of the problem once and for all.

     If you would like to see more technical articles like this one, please continue to support Pelican Parts with all your parts needs.  If you like what you see here, then please visit our online BMW catalog and help support the collection and creating of new and informative technical articles like this one.  Your continued support directly affects the expansion and existence of this site and technical articles like this one.  As always, if you have any questions or comments about this helpful article, please drop us a line.