Where do I begin to trouble shoot the alarm system on my 70 coupe. It used to work for years, but today it won't sound when I set it and open the doors or hood. I removed one door switch and jumpered the wires, still no alarm. Do the door switches make a connection to ground? or do they complete a ciruit giving the horn power? If I jump power to the horn tang after removing the plug in socket with wire, should the horn sound and remain on steady like a regular horn? Where is there a good article on fixing these?

 

I don't have experience with a 1970. All of the following is based off my 1976 so wire colors may vary. You should start by making sure all connections are clean. In the storage compartment behind the passenger seat there is a relay and a flasher. Both of these are used to activate the horn when the alarm is tripped. The door switches do make a connection to ground. There are three wires going to the door alarm switches. The yellow with black stripe is the ground, the light blue wire is the ground path for the door ajar warning lamp on the dash, and the white wire is the ground path which activates the alarm. Jumping the white wire to ground will trip the alarm. If you apply 12V to the horn where the pink wire is and ground it, it will sound like a regular horn. When the alarm is tripped the horn will switch on and off, because of the flasher.

 

thanks tom76. On the 70 there are two door switches, one on the hinge pillar and another at the rear of each door below the striker bolt. I assume the one below the striker runs the alarm only. It seem to have only 2 wires, a black and blue. The switch on the hinge pillar runs the interior lights when door is opened. They seem to work fine. So I will try to ground the wire at the switch to see if the alarm comes on. I presume the black is permanent ground and blue goes to the alarm relay or flasher or the like. I will let you known how it goes to complete this thread. If anyone knows more, chime in.

 

Yes, you want the one at the rear of the opening (below the striker). One wire runs the alarm and the other runs the door ajar warning light on the dash. When you unscrew the switch from the body two of the wires will come out with the switch, the third is attached to the threaded piece of metal that the switch screws into. On my car the ground wire had broken off the metal plate. To fix this I had to drill out the rivets holding in the metal plate, reattach the wire, then rivet the plate back into place. It sounds easier than it is. To check for this, take an ohm meter and check for either 0 or ver minimal resistance to a good ground.

 

thanks tom76. I am going away for a week and can't continue to do any checks until next weekend. But I will post my results until I find the problem. I will check the switch retainer plate for ground. Maybe mine broke off like yours.

 

This is a really basic alarm system. While I don't use it much, I wanted my car to be fully restored including the alarm. I just resorted my alarm system and walked through most all pieces. Your problem sounds like the same problem I had. $1.98 flasher. Its under the console in front of the shifter towards the passenger side. The circuit for the alarm is very simple. All switches are in series. They are normally closed and when a switch breaks the horn is grounded. You can ohm out the switches; tie the horn to 12V to test. The horn on my '78 must have 12V to work. I was thrown for a loop when I measured 8v at the horn, so I assumed I had voltage (tested with just a test lamp at first). I also noticed that the voltage to the horn did not flucuate as a pulsating horn should. I pulled the flasher and temporarily installed one I had on hand (not of proper value). The horn then sounded continuousally, measuring 12V. Went to AutoZone, spent my hard earned $1.98, and the problem was solved. There are some postings of the alarm circuit on the net. It helped me figure out this cicuit, I would reccomend finding it. If I were wat home, I would can it and email it to you. If you have the electrical manual for your year it should be inthere as well. good luck...it's not too difficult! :smash: :mad :smash: :hurray: