Cousin gave me a 99 Yukon. Key fob locks/unlocks doors. Even beeps horn once indicating it's armed the second time you press lock on the fob. However, open any door and no alarm sounds. No headlights/lights/security lights come on or flash either when arming, when armed, or when opening doors while alarm is armed.
Notes: It looks like someone replaced lock cylinder and we don't have any transponder keys for it. Engine always starts no matter what you do with the fob/doors.
Any help much appreciated.
fobs, doors, locks, nor anything else for that matter will stop the truck from being started and driven away with a key that turns the ignition. there is no transponder on the 99, the best you will get is if someone broke the lock cylinder and managed to screw up any hall sensor that would then tell the ecm to turn off the fuel pump power, that is the only thing that stops anyone from driving it away, any steering wheel lock is tied into the ignition switch with a pin, turn the ignition and the pin retracts and the steering wheel turns, engine starts and off it goes. we are talking 30 year old technology, pull the ignition, jumper the fuel pump relay, stick a screw driver in and drive it away.
any halfway decent thief just sticks a lag bolt in the ignition with a dent puller, bang out comes the ignition, most older cars have nothing to then stop them from sticking a screw driver in and turning the mechanism behind the lock cylinder and driving it away.
things to check:
if the door "ajar" sensor is not working the horn will never honk if the door is opened, one way to tell is if the interior lights come on or not when you open the door assuming the dome light switch is not turned off or if the radio stays on after the key is removed and the drivers door is opened.
check all your fuses.
bottom line, if you want it to not get broken into invest a little in a aftermarket alarm 2-way, and have the installer place the alarm LED where it has high visibility. that alone deters the common low life scum, that and don't leave anything in the vehicle in plain sight that you don't want to be gone in the morning.