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Parasitic current draw
Posted to Technical Tips Forum on 11/27/2012 17 Replies

This is about current draws on late model vehicles that seem elusive.

What I am starting to find the reason for this problem is the OBDll devises that are used by insurance companies. This problem was addressed by SAE at the last meeting in Anaheim, Ca. as well as with the OE's that are currently struggling to find reasons for batteries doing dead for no reasons.

The issue is the OBD devises. They are classified on the data bus as a scan tool. Their job is to request data and send it out over Bluetooth to a phone. With the vehicle running this is not a problem. Yet when you turn off the vehicle, the devise is staying powered up because of constant power on the OBDll connector. The 'scan tool' is still requesting data. The CAN bus which normally goes to sleep is now woken up. The 'scan tool' does not know that the vehicle is not running. It's just requesting data. If the CAN bus is only to the PCM there is usually not an issue. This problem does not seem to be occurring on the other communication protocol because the PCM's power down when the ignition is turned off.

If the CAN bus is networked so that communications is allowed after vehicle is powered down, this current draw issue is now appearing on these vehicles. The vehicle population that I have seen this problem effect is on vehicles with Permanent DTC's capability, 2010 and later. Also on vehicles with high end CAN buses. Which manufacturer of the devise is to blame? It does not appear to be manufacturer dependent at this time.

The problem is the devise is a 'scan tool' that is requesting data after the vehicle is turned off and is turning the CAN bus on to get this information. The solution: Unplug the devise when you turn off the vehicle.

Hope this helps

Steve Caruso
Westminster, California, USA

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