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The case of the automotive Chia Pet

This Hummer is one vehicle that really stands out in my collection of automotive oddities
Friday, July 1, 2016 - 07:00
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I was called to a shop on a 2003 Hummer H2 with a 6.0 L engine with about 120,000 miles on it (Figure1). The complaint was a no start condition. The owner of the vehicle said that he had parked the truck to go into a store and when he came out about an hour later the truck would start up and then immediately cut out. The shop could not communicate with the Engine Control Module but they were able to run the truck with carburetor cleaner. The truck had spark & fuel pressure but was losing fuel pulse. This led the shop to believe that the vehicle was immobilized preventing the vehicle from being started. The shop knew that this vehicle had the Pass Lock 2 system on board and that the Body Control Module played a role in starting the vehicle.

Figure 1

The Pass Lock 2 systems are used on many GM vehicles from 1995-2005 and can be identified by a security light on the dash and by a key with a "PK2" stamped on the metal part of the key where it meets the plastic end. There are some keys that may not have the "PK2" stamped on them and are also used for this system. These keys do not incorporate an internal chip but only function to turn a lock cylinder that houses a magnet in the center of the key cylinder assembly. When the lock cylinder is rotated in the crank mode it will travel far enough for the magnet to go past a 3-Wire Hall Effect sensing device located in the lock housing allowing the sensor to pull the 5 Volt Pass Lock signal from the Body Control Module to a certain predetermined level. It is this specific value that is learned within the BCM's memory. If the vehicle battery was to go dead or a new Pass Lock sensor installed in the vehicle you would have to perform a 30 minute learn procedure to allow the vehicle to run.

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