Question about 1991 Cadillac DeVille
The security light is a visual indication that something in the car's antitheft system is failing. In most GM cars and trucks there is a hidden system that most people are unaware even exists. From the moment you put your key in the ignition, there begins a constant line of communication between your key's security chip or transponder, to the ignition's lock cylinder, and then to the car's TDM. (Theft Deterrent Module) The TDM is what tells your computer that the correct key is being used and that it is "OK" to allow the vehicle to start and run. These 3 parts must be in CONSTANT communication. If the key is in the ignition, then these parts are sending important signals to each other. The security light illuminates when that signal gets broken, even just for a second. When the signal breaks, (normally due to a failing/broken part) the car goes into antitheft mode. The car misinterperates this and thinks its being stolen or hotwired because it is NOT getting that signal from YOUR specific key. There is where the security light turns on and shuts down your vehicle. So basically, if that light is on, your system has some sort of failing part. This can start as an intermittent "no start", to your vehicle not running for days at a time, to the vehicle NEVER starting again. The problem is electrical and computer based, so don't count on an easy reset button, or the problem fixing itself. The hard part is isolating which part is the problem. You can spend a thousand dollars through a mechanic very easily. I have had great luck with bypassing the entire system. This way the problem will never return. It actually CAN'T ever return because it is eliminated. This can also be done yourself at a much cheaper cost. Read through this link very carefully. You'll find alot of information and pictures, along with ways to help you fix this.
Posted on Feb 18, 2012
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Posted on Jan 02, 2017
sounds like a 12V drain on the system, like a trunk or glove box light that won't shut off.> Could be many. many other reasons why. The way I would find the draining circuit would be to remove the neg cable of the battery. I then would connect a 12V test light between the neg battery TERMINAL and the neg battery CABLE. With a 12V drain the light will shine. I would install into the light a 9V battery powered recepticle to hold computer and radio station memory. Then I remove fuses 1 at a time, then reinstall, at the same time looking for the 12V light to go out. Hope this helps.
Posted on Apr 05, 2009
SOURCE: Battery drain
When the car is shut off, there should be a very small load on the battery to keep the clock running, the security system on etc. But the amount of current is really small. If the battery is dying overnight, something is loading it;
Charge the battery right up.
Have ignition off, doors and trunk closed, no interior /exterior lights on and leave the driver door window open.
Take a fairly large automotive bulb like an 1157 or even a headlamp.
Disconnect the negative terminal on the battery.
Connect one wire to the negative terminal of the battery from the light, and the second light terminal to the negative battery cable you diconnected; connecting the light in series. Don't put the negative battery terminal on the battery.
The bulb should not light if all your ignition system is off and you have no lights turned on.
Turn on the lights or blow the horn by poking through the open window without opening the door, the test light should light and the horn sound.
If the light lights without anything being on, then you have something constantly draining the battery.
Go to the fuse panel and remove and re-install each fuse one at a time from every circuit. You may have to close the interior light door switch by pushing it with something while you are getting to the fuses so the circuit is in the off state all the time. Everything has to remain off.
When the light goes off when you removed a fuse, that is the circuit which is loading the battery. Check the fuse description for the circuit affected.
My guess is a trunk light that stays on even when the trunk is shut. If it is a lighting system fuse causing the problem, remove the trunk bulb and replace the fuse. If the light goes off, the trunk switch is defective keeping the light on all the time.
You can also place a battery charger on the battery (connect the negative back on the battery) and go and check the car in the dark. Is there a light on like a floor lighting in the interior? Do you have a light on under the hood that can't be seen during the day?
Always make sure you have a good charged battery, and that the terminals are really clean before you start analyzing the problem.
Does the alternator charge the battery? Check by measuring the volatge across the battery engine off, no charger connected; should be 12V or close.
Start the car, the meter should read 14V or so, but more than without engine running. If not, the alternator/charging circuit may be the real problem.
Posted on Aug 24, 2009
Anytime I get a too lean DTC I first change the fuel filter if it hasn't been changed in awhile. You'd be surprised how many times this has helped.
Good Luck with it.
Posted on Oct 22, 2009
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