Topic: factory alarm questions
in Forum: C3 Electrical
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Doing some diagnosis on this system and have a couple questions.
1. Are the alarm door switches the forward door switches (courtesy lights) or the rear-ward door switches (door ajar lights)? I think it's the front ones, but want to be sure.
2. If the alarm is activated, should there be a constant 12v between the red and yellow wires at the horn, or should the flasher that's in the circuit be causing that voltage to fluctuate?
I know I have a few problems right now. Finally got the horn out after a bit of a struggle with the bolt that holds it in. Seems like it's bad - no continuity and there is definitely 12v between the two wires that connect to it when the system is activated. Other problem is that 12v - it's always there if the key switch is turned (i.e. horn would be constantly blaring if it worked) . I think that problem is due to the anti-tamper switch not being adjusted quite right, but if that's not the problem I'll have to diagnose the two door switches and hood switch. Something is causing it to go to ground even when all the doors are closed. And a third problem could be the flasher if the voltage is supposed to fluctuate.
1976 Silver/Firethorn. L48, 4spd. Original 2 bolt, vortec heads, 9.4:1 CR, Speed Pro Cam: 224/224@0.050, 112 LSA, Eagle Steel Crank.
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Lifetime MemberModerator
Duncanville, TX - USA
Joined: 11/8/2003
Posts: 20214
Vette(s): #1-1974 L-48 4spd Cp Med Red Metallic/Black deluxe int w/AC/tilt/tele./p/w-p/b/
Am-Fm/map light National/Regional/Chapter NCRS "Top Flight"
#2-1985 Bright Red/Carmine Cp.L-98/auto
Member: NCRS, NCRS Texas, Corvette Legends of Texas
You should have 12v on one side of the connector at the
horn....the other wires there comes from the relay, with the flasher
making/breaking contact from the grounds on the door/hood switches. Try
removing the flasher and see if you still have voltage on both wires @
horn. If ya do, then you have a short in the harness somewhere....
If the horn has no resistance, it's bad. You should NOT have 12v on BOTH wires at the horn, tho...that's just weird.
I know I have a dang good diagram of the alarm system somewhere. I was lookig for it a few days ago, but have yet to find it. I will keep looking, 'cause it's a really good diagram, with an easy to unnerstand explanation of the operation.
I do have this one here, tho....I posted this the other day, too....
If the horn has no resistance, it's bad. You should NOT have 12v on BOTH wires at the horn, tho...that's just weird.
I know I have a dang good diagram of the alarm system somewhere. I was lookig for it a few days ago, but have yet to find it. I will keep looking, 'cause it's a really good diagram, with an easy to unnerstand explanation of the operation.
I do have this one here, tho....I posted this the other day, too....
Joel Adams
C3VR Lifetime Member #56 
My Link 
(click for Texas-sized view!) NCRS
"Money can't buy happiness -- but somehow it's more comforting to cry in a CORVETTE than in a Kia"
here is the diagram from 77 I believe the 76 is the same as the 77 early which has the key on fender instead of in the door lock
Thanks guys. Bob, that diagram is very helpful. Joel, I misspoke in the first post: I did not have 12v at BOTH wires at the horn. Only the pink one. What I was trying to say was that there was 12v BETWEEN both of them all the time when the alarm key switch was activated - regardless if all the doors/hood were closed or not. (Meaning that something in the downstream part of the circuit was going to ground when it shouldn't allowing the horn circuit to be always complete)
I did a bunch more diagnosis tonight and answered some of my own questions, but now have one or two more. Mainly, what the heck is that relay for? The entire system seems to work without it plugged in at all! In fact, I think it's the relay that's causing my problems. I ended up eliminating all four trigger switches (2 doors, hood, anti-tamper) one by one and still had not found what was causing that circuit to be always hot. So I unplugged the relay (it was hot to the touch, BTW) and now I can actually break the horn circuit by ensuring all four trigger switches are open or disabled. I'm no electrician, but I studied those wiring diagrams and have no idea what the relay does. I think of relays as allowing a low power circuit to operate a high power one independently, but it doesn't seem like the case here since all the hot leads in the system are connected together. I know GM wouldn't put a part like that in for no reason -- hopefully someone that knows more about 'lectricity can edumacate me. If I do need to replace it, are they still available? Is there a substitute I can use?
Two questions that I answered on my own:
1. On a '76, the alarm door triggers are on the rearward door pin switches, not the front.
2. The flasher works fine. I was not seeing a fluctuating voltage because there was no load on the circuit. I put a light bulb in the circuit and it started flashing.
Starting to think I'm crazy for trying to get this alarm system to work...
1976 Silver/Firethorn. L48, 4spd. Original 2 bolt, vortec heads, 9.4:1 CR, Speed Pro Cam: 224/224@0.050, 112 LSA, Eagle Steel Crank.
Lifetime MemberModerator
Duncanville, TX - USA
Joined: 11/8/2003
Posts: 20214
Vette(s): #1-1974 L-48 4spd Cp Med Red Metallic/Black deluxe int w/AC/tilt/tele./p/w-p/b/
Am-Fm/map light National/Regional/Chapter NCRS "Top Flight"
#2-1985 Bright Red/Carmine Cp.L-98/auto
Member: NCRS, NCRS Texas, Corvette Legends of Texas
The relay is a "latching" relay, meaning that it will stay ON until you turn the key switch off. Without the relay working like this, the alarm would turn itself Off if you closed the door/hood. With the latching relay set-up, the alarm will continue sounding until either the battery goes dead, or you turn the key switch off, regardless of the door being open/closed.
The relay being hot to touch indicates it has stuck closed(on)...you might be able to clean it up and get it to function properly....but it might also just be a tin can full of rust...
The relay being hot to touch indicates it has stuck closed(on)...you might be able to clean it up and get it to function properly....but it might also just be a tin can full of rust...
Joel Adams
C3VR Lifetime Member #56
My Link
(click for Texas-sized view!) NCRS
"Money can't buy happiness -- but somehow it's more comforting to cry in a CORVETTE than in a Kia"
Thanks, Joel. Makes perfect sense now. I'll take the relay apart and see if I can clean it up. It's nice and clean on the outside so maybe I can get it working again. Also need to repair or (most likely) replace the horn. And the door ajar switches are not engaging correctly again, I think I have the wrong length in there right now. But at least I know what's going on now.
1976 Silver/Firethorn. L48, 4spd. Original 2 bolt, vortec heads, 9.4:1 CR, Speed Pro Cam: 224/224@0.050, 112 LSA, Eagle Steel Crank.
If it's any consolation Dave, I'm learning lots from your efforts! 
2012101154624r.jpg)
Build Date: May 7, 1975. 383 w/267 RWHP/310 RWTQ
Glad you're learning some, Greg! I am too! In fact, based on Joel's last answer, I think the relay is doing exactly what it's supposed to. Since I didn't understand the function of the relay, I wasn't resetting the system each time with the key switch during the diagnosis. I thought the circuit was staying hot incorrectly, but the relay keeps it hot until it's reset with the key switch. I did take the relay apart so I could watch its operation - looks like it's working as expected. One less thing to fix or replace now. I think I'm getting the horn to work again too -- just needed some cleaning around the contacts. Now it's just futzing around with those door ajar pin switches and I'll have a working factory alarm
1976 Silver/Firethorn. L48, 4spd. Original 2 bolt, vortec heads, 9.4:1 CR, Speed Pro Cam: 224/224@0.050, 112 LSA, Eagle Steel Crank.
So if I order the anti-tamper switch and hook it up chances are my alarm may work as well. Hmm...Everything else is in place; the relay, flasher, horn and key switch and my door ajar switches both work. I think my hood switch needs replacing though. Does the alarm just sound the horn or do the lights flash as well. My cousin has an all original '74 that he has owned since new and I remember when he bought it he showed off the alarm to my father and I but that was a long time ago. He still has the car to this day. Only 40,000 miles on it.
Build Date: May 7, 1975. 383 w/267 RWHP/310 RWTQ
Greg, it's just the horn, no lights. I'm 90% sure the problem with mine all along was that anti-tamper switch coupled with a horn that was sucking power somehow, but not sounding. The anti-tamper switch is just another path to ground in the system - if it's plugged in the button needs to be pushed constantly to keep it open - natural state is closed. It can be disabled by just unplugging the connector if you want to test.
1976 Silver/Firethorn. L48, 4spd. Original 2 bolt, vortec heads, 9.4:1 CR, Speed Pro Cam: 224/224@0.050, 112 LSA, Eagle Steel Crank.
in Forum: C3 Electrical
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