I was awaken this morning a dawn by a car alarm going off. I was sure it was the guy across the street with a car phone and someone calling him at 6 am, but it was my FEH and it did not go off untill I put the key in and turn the ignition on. It would not shut off with the key fob. Did not repeat, but the day aint over yet.

 

When I drive by other cars and key up my ham radio, I can trigger car alarms.
Perhaps some emergency services type vehicle went by and was on the radio at the time.

Duncan

 

If that is the case, you would think it would turn off with the keyfob with no problem........

~John

 

...not if the microcontroller was scrambled!

 

If you were nearby I could try it and see.
Usually I think it just trips the sensors and triggers the alarm, I have not caused trouble to my own FEH while driving.

Duncan

 

My 05 Escape "non-hybrid" alarm also went off, when my wife was at home alone and she couldnt get it to shut-off. It kept honking for about 30min. She called me and by the time I got home the horn had burned out. The alarm finally shut-off too.
Took it in and the dealer couldnt find anything wrong?? They replaced the horn and hasn't happened since. That was about 6 months ago....

 

Here is what could be happening if the alarm is controlled by a microcontroller:

The microcontroller executes a series of instructions is a prescribed order. These instructions take data from inputs, make decisions, and latch outputs on, or off. The output stays in the last state.

Sometimes external signals, especially radio signals of a sympathetic frequency, leak into the microcontroller circuitry. This can disturb the process so the instructions are no longer being executed, or allow them to be executed in the wrong sequence. Other times, the disturbance upsets an internal function register, so the microcontroller continues to run, but without all of its functions. Because of the fault, the microcontroller does not recognize the cancel signal from the remote. The output latch stays set, and the horn continues to honk!

Most of these occurrences are caught by an internal watch dog program that recognizes that certain instructions were not executed, so the processer resets. This does not catch every fault unless the programmer is extremely careful, and program testing is especially detailed.

 

Actually I drive down some white rock roads and get ALLOT of dust in the car so one of the door switches might be getting dusty and breaking contact thus setting off the alarm.