My '91 FC has been setting off its alarm again. For a long time at midnight and starting back periodically just now, according to the police officer who was at the door.
Not full-on like when I first bought it, but the periodic beep...beep...beep burglar alarm, somehow.

Last time I had this problem, the car was running and it still happened. Is there any way I can disable the subcircuit the alarm is on without taking out the horn to narrow down on the issue? I can't find many references to this particular issue, but it could be because I'm half awake.

 

Your alarm circuit (S5's) is found within the CPU so it is part of a circuit board which likely needs replacing. Do a search using the advanced search feature and compare your problem w/those of others to help remedy your situation.

 

There should be a negative trigger coming from each key cylinder which sends a disarm pulse to the alarm when the car is unlocked with the key. I don't have my Haynes or AllData in front of me right this second otherwise I'd help you identify the correct wire(s), but once the correct wire is identified @ the CPU you can ground it to disarm the system. Personally, I would instead disable the input which sends the arm signal; that way once you disarm the system it will not rearm.

 

Mine is doing the same thing, i just unplugged the horn, how do you disable the input?

 

When the door cylinder is turned (or driver door locked manually with door open) a negative pulse is sent down the arm wire to the alarm brain/CPU. By interrupting this wire the brain will never get the arm signal, hence, will not arm.

 

I search for a few variations of horn, beep, security, and alarm, and didn't turn up much. Actually, Google's indexing of this site is better than the site's own search.


I made some progress yesterday afternoon. Fired it up and drove it, and the alarm started going off middrive. I pulled over and went to pluck out the horn fuse. In front of that footwell fusebox is what I assume is the integrated ECU box. I accidentally brushed the harnesses going into the bottom of it, and that seemed to reset or stop the alarm. I tried to push some harnesses more firmly into the box, and that got the alarm to quit. I drove around some more and it didn't act up. Hopefully it's as simple as a flaky connection at the box, not wires leading to there.


The harness I touched the most is to the right of the metal tab that sticks out at the bottom of the box labeled J.


That purple wire is for the PO's audio setup. Not the most encouraging of signs, but it doesn't like it hacked up anything around it.

When I parked, I decided to unplug the horns, so that if it did go off, I could come back to the car and maybe hear the relay ticking. I found one horn in front of the coolant overflow tank. Took the tank off and got it unhooked. Are there two stock? Cuz I hit my wheel horn and it beeped a single note still. So I gave up and unhooked the battery. I threw a tub of DampRid in the driver's footwell to try to dry it out more. It's been snowing/raining the past couple weeks, so I'm not surprised grounding issues have arisen.

Hmmm. The car was sitting unlocked when the alarm first started going off. When I was driving, would the car have been in armed mode? If not, then the electrical issue would have to have put it into armed mode somehow, right?

 

Yes there are two horns, wish i was home to post pics... and to interrupt the signal, would grounding it cancel the signal? Ive read the above posts before regarding the cpu might needing replaced, but since the car is sitting on four jackstands being converted to manual, i can live without the horn, i just have to get that baby to crank and im happy...

 

Can anyone point me to a diagram of where the second horn is? Or tips on how to access it?

 

Theres one thats visible above the oil cooler, right under the coolant overflow tank. Guess mine was easier to see since all the plastic trim is off close to the front end. If you remove that overflow tank the other horn should be behind it.