The sophistication of the electrical systems on vehicles such as your GM W-Body car as covered by this guide, means that care and forethought need to be used when working on the electrical system. While it is true that for many years, the first step on most repair procedures involved disconnecting the negative battery cable, some caution must be used. The reason to disconnect the negative battery cable is to prevent short circuits, sparks and voltage spikes that can damage the vehicle and even harm the technician working on the vehicle. To a great extent, this rule is still valid. When working on any electrical component on the vehicle, it is always a good idea (and in many cases, mandatory) to disconnect the negative (-) battery cable. This will prevent potential damage to many sensitive electrical components such as the Powertrain Control Module (PCM), radio, alternator, etc.
However, some W-Body vehicles may be equipped with audio systems that have a Theft Deterrent Feature. The radio has this feature if it shows THEFTLOCK- on its face. THEFTLOCK® is designed to discourage theft of the radio. It works by using a secret code to disable all radio functions whenever battery power is removed (as when a battery cable is disconnected). If THEFTLOCK® is active, the THEFTLOCK® indicator will flash when the ignition switch is in the OFF position. The THEFTLOCK® feature for the radio may be used or ignored. If ignored, the system plays normally and the radio is not protected by this feature. If THEFTLOCK® is activated, the radio will not operate if stolen. When THEFTLOCK® is activated, the radio will display "LOC" to indicate a locked condition anytime battery power is removed. If the battery loses power for any reason, you must unlock the radio with the secret code before it will operate.
So think before disconnecting the negative battery cable. It is still important to do so for many procedures, but make sure any necessary codes are available or that the THEFTLOCK® feature has been disabled. More information on THEFTLOCK® can be found later in this information. The following is GM's recommended procedure.
- Record all of the vehicle preset radio codes (if applicable).
- Record the THEFTLOCK® code (if applicable).
- Disconnect the negative battery cable first.
- Clean any existing oxidation from the contact face of the battery terminal and battery cable using a wire brush before installing the battery cable to the battery terminal.
- At installation, the retaining bolt should be tightened to 11 ft. lbs. (15 Nm). Do not overtighten, or the threads in the battery may strip.
- Unlock the THEFTLOCK® radio (if applicable).
- Program the radio stations back into the radio as recorded at the beginning of this procedure (if applicable).
Before you disconnect the cable(s), first turn the ignition switch to the OFF position. This will prevent a draw on the battery which could cause arcing (electricity trying to ground itself to the body of a vehicle, just like a spark plug jumping the gap) and, of course, damaging some components such as the alternator diodes.
When the battery cable(s) are reconnected (negative cable last), be sure to check that the lights, windshield wipers and other electrically operated safety components are all working correctly. If your vehicle contains a THEFTLOCK® Radio, and it has been activated, it will be necessary to unlock the radio with the code before it will operate.