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Bob, Auto Service Technician
Category: Car
Satisfied Customers: 4087
Experience:  40 year GM Tech. +30 Certified. ASE Master Tech.(expired, retired) Medium Duty, Heavy Line, Retired
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I have a 1999 GMC Sierra and im having an issue with the Security

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I have a 1999 GMC Sierra and i'm having an issue with the Security Light which makes it where my truck wont start. I replaced passlock sensor key cylnder ignition switch and coded it. it will start but looses the code over night.Turn the key on and let it set for a while security light go out some times and it will start never know when it will start.
The ignition lock cylinder is most often the problem with these. But because there are so many other components in the system that can also fail, it would be best to have the system scanned for codes and function tested with a class 2 scan tool to find out why you keep having the problem repeat itself. If the system does not have a current ode stored, it should have one stored in history. Once you know which code is stored, you will be able to see what other components are not with in normal operating perimeters and will know what if any other diagnostic work needs to be done to verify and correct the system. Thanks
Customer: replied 8 years ago.
Ive already had my GMC hooked up to a diagnostic scanner and it shows no error codes for the security system in the vehicle. Thanks

Wow, that is unusual. But, considering how complex these systems are today, nothing really surprises me. You would think that it should at least show something in history in the way of a code, but anything is possible, I guess. On the older pass lock systems we used to bypass the ignition lock cylinder and the password XXXXX soldering a resistor into the line so it would always start. I have not tried it myself and I am not sure which of the other two wires are involved, but I am going to pass a note on to you that I received from another technician who is very knowledgeable and I trust what he says completely. It would just be a matter of removing the covers to get to the harness and then trying one of the other 2 wires and if it doesn't work, that would only leave one other wire as a possible. I will post it below, just in case your interested in giving it a try. I am going to try it the very next chance I get, but so far this week have not had an opportunity. If you do, let me know the results. Thanks


You wire a 2200 ohm resistor across the 5 volt reference and return on the sensor wires-there are 3 wires-power ground and return-the yellow is the return and that is the wire you jumper to the 5 volt reference.Then relearn the theft deterrent using the 30 minute procedure of going to start-leaving the key on until the security light stops flashing-turn the key off for 20 seconds and repeating the procedure until it starts,

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