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Tests Reveal What's Causing 2006 Saturn Battery to DrainPosted 11/19/2007By John Esau
While I was working for a Saturn dealer, a new 2006 Saturn Ion was delivered from the factory with a problem in the electrical system. When the vehicle sat for more than a day or two, the battery would be dead. I checked for any lights or modules that were operating with the key off, and noted that the radio clock display was lit - though there was no sound from the radio - when the key was not in the ignition. Turning on the ignition switch caused the radio to function normally. When the key was removed from the ignition switch, the radio and clock display turned off. As I continued to monitor the radio display, the clock display suddenly lit up. Was it a problem with the radio, the retained accessory power (RAP) system, OnStar, or was another module causing the radio to wake up by sending serial data on the low speed GM LAN data circuit? The RAP system provides power to the radio, sunroof and power windows so that they can be operated for 10 minutes after the ignition is turned off. Testing the RAP system revealed that when the key was removed from the ignition and the driver's door was opened, the power windows stopped working and the radio shut off. So the RAP system appeared to be working correctly. I disconnected the data circuit terminal at the radio and the symptoms were unchanged. This confirmed that other modules waking up the radio by sending serial data on the low speed GM LAN data circuit were not the problem. This left the radio and/or the OnStar system as the source of the problem. The OnStar system is controlled by a vehicle communication interface module (VCIM). The VCIM contains two modems: one to process GPS data and one for cellular communications. The GPS and cellular antennas connect to the VCIM along with the radio and the OnStar buttons on the rearview mirror. The VCIM has the ability to function even when the ignition switch is turned off, and chooses one of four readiness states based on the following parameters: 1) The type of cellular market the vehicle is in (analog or digital system), 2) Ignition switch status, and 3) RAP system input. Three circuits go from the VCIM to the radio: two voice circuits and the cellular telephone mute signal circuit. The radio sends 2.0 volts to the VCIM on the mute signal circuit. To mute the radio, the VCIM will ground this circuit, pulling the voltage low. This causes the radio to amplify the cellular telephone voice signal, instead of the radio or the CD signals. The VCIM also has the ability to turn on the radio using this mute signal circuit, even if the ignition switch and RAP systems are turned off. Back-probing the mute signal circuit at the radio connector with a voltmeter showed 2.0 volts on this circuit with the radio working correctly. The ignition switch was then turned off and as the radio display lit up, the voltage on the mute signal circuit dropped from 2.0 volts to 0.1 volts. Next, the VCIM was disconnected and the same test was performed. With the VCIM disconnected the radio display no longer lit with the ignition switch off. From our testing, we concluded: 1) The battery was being drained by the radio, 2) The radio was not the problem because the VCIM was turning on the radio via the cellular telephone mute signal circuit, and 3) The VCIM would have to be replaced. The VCIM had to be ordered through the GM Technical Assistance center, not the parts department. It was necessary to contact the OnStar operator to set up the new VCIM using the OnStar system in the car. After all the repairs had been made, the system was tested to ensure that the clock did not display with the key off and removed from the ignition. The system was now functioning perfectly.
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