S2TGART

I tried in vain to search for other threads about this, but nothing came up. I have been putting up with this issue for a while, but no more. My car's factory alarm system has a mind of it's own. The car is a 1983 SC, all stock, even has the old style push button presets for the radio. The alarm system acts as if it's armed when it isn't. It doesn't matter what door you open, or even if you open the trunk, the alarm goes off. Thankfully the horn aspect isn't working, but you can here the relay clicking away and naturally the engine will crank over for all of a second and then die. Even if I try an avoid opening the door, and reach in to start the car, as soon as the ignition system is powered up, the alram kicks off. Then at a point and it varies each time, she will behave and start without issue. Most recently, it left me stranded at a restaurant for 30 mins. I tried over and over to get it to "clear up", but nothing. My wife picked me up and about 2 hours later we returned, I opened the door and all was good and started her up and drove it home. Once home in the garage, I turned the engine off, open the door and off went the alarm again. There is absolutley no consistency to what makes it go off, or at the same time, clear itself. So, with all that said, I need some major assistance in fixing this or just simply taking the alarm system out of the loop so that I can drive this car with confidence. I have the Bentley 911SC service manual and have looked extensively at the wire diagram, but before I cut any wires I wanted to get other opinions on this. Thank you in advance

theiceman

i would look at all the connections carefully and clean and inspect. I dont have one but most alarms look for voltage drops from a door being opened etc. So with that in mind it could also be a dipping battery voltage. I personally would like to have it working if it were factory, its kinda novel . If it is gonna leave you stranded however .. out she comes.

S2TGART

I know that alarms systems are very voltage sensitive and it typically signals a weak battery as you suggested, or the voltage regualtor is on the fritz. So, trust me I gave that some thought. This Christmas my wife bought me a battery maintainer which I promptly connected to the battery and it was connected and fully charged just before I drove it to the aforementioned restaurant. When I got to the restaurant, I shut the car off, opened the door, no issues/alarm. Come back out after dinner, open the door, tada alarm. As mentioned above, I tried for 30 minutes with a combination of turning the key or opening the driver's door and even passenger door, no go. I have to mention that what I do is wait for the clicking (again the horn isn't working luckily) to stop, then try the key again, or try a door. Each time waiting for the clicking to stop. So what completely baffles me is what changes from the time I stop messing with it, to when I come back 2 hours later and all is good? There have been times where the car sits for 2 weeks and I'll try to start her, and wham, alarm. Other times I can go 5 different times starting/running her and no issue.
I retired from the Air Force and my career was working aircraft avionics systems and engine instrumentation. So I know about electronics and troublshooting. The first step in troubleshooting is finding a common denominator/cause. In this case, there isn't one. I even checked for a short in the wiring from the alram lock mechanism, nothing. My thought there was that when you arm the system by turning the key, you are closing an electrical loop. Since I had not armed it, I figured shorted wires may make the system think it was.

s4magman

Sometimes the simplest of solutions come to light - check the door light switches (all of them including hood and trunk). I replaced those on my 964 which seems to have corrected a similar alarm issue.

1986 Targa

Do you have the Bentley, or the CKT diagram?

If not, here's one from my '86 factory manual.

As per the previous post, the door, hood, and trunk, switches,
or the alarm switch-key slot in the door jamb are the "usual suspects".

Please note that the alarm door switch contains two resistor
which prevent bypassing the system at the alarm switch.

Having helped a few folks on Pelican with factory and aftermarket alarm issues,
my vote is for one or both of the door switches which can easily be pulled,
(taking care to not drop the switch, or connecting wire into the body), and cleaned.

Also, do a search on both this site and the Pelican site.

pm me as needed.

Good luck,

Gerry

S2TGART

S4magman, I am all for simple fixes. What specificily am I looking for with the door/trunk switches? I know they all work because the respective lights come on when opened.

s4magman

I recommend you remove them and inspect for oxidation and clean them. Pop the rubber caps off and simply unscrew. Even the slightest short will excite the alarm. Some people recommend replacing these switches every couple of years (cheap) as part of regular maintenance. I hope the switches will do the trick.

rusnak

When removing the door switch, hold the black plastic plunger and keep it from turning. Carefully pull the wire out through the hole, and tape it down until you are ready for reassembly. If the wire falls back into the doorjamb then you'll have a heck of a time getting it back out.

S2TGART

Well, I would like to thank Gerry, aka 1986 Targa, for going above and beyond in helping me with the problem. Through, his advice and links he sent me via PM, I was able to find the "work around" to the possessed alarm unit. All testing done indicates that the alarm unit is faulty, probably due to age.

While troublshooting and with both of the alarm connectors disconnected, on whim, I tried to start the car, and it did. However, at start up, the battery and OXS light did not come on. this prompted me to check the battery voltage while she was running. The voltage stayed at 12.5, but not for long. Over the course of 5 minutes, the voltage went down to 12.01, that is until I turned on the headlights, it dropped to 11.8.

Through one of the links to factory manual he sent me, I learned of a factory "workaround", to the problem. Though it means no working alarm system, it does allow me to drive the car with confidence that she will start everytime. You place a jumper wire between pins 61 and 61a and all will be good. 61 and 61a, go to the battery charge light and the alternator.

I will continue to look into a physical cause like corrosion, loose wire, etc. However, for now I have the use of the car again without fear of being stranded.

s4magman

I'm glad you found a work around. This forum is a great place. Keep us posted and good luck!

theiceman

I would start with fixing what you can see first ... that charging voltage is not correct. Sounds like you are due for a battery, alternaor or both.

Ed Hughes

IMO, an antique electric alarm is a huge liability to reliability. I'd make the workaround a permanent fix.

rusnak

I use my factory alarm due to it's engine disability function (lol). Much better than the pos aftermarket systems. You might send the old unit in for repair, or buy a new one.

S2TGART

With the jumper wire in place, the battery is charging at 14.3 Though I have noticed that when I rev the engine, the headlights get a littke brighter.

I will continue to check everything out, though through the checks I did do, following a couple different manuals, it's pointing to a faulty alarm unit. Not to go over everything again, but it didn't matter if you opened the drivers door, passenger door, trunk, or turn the key, the alarm would go off as it were armed. The common component to all these is the alarm unit.

Not only did I check the ohms values at the alarm switch, but from the connectors on the alarm unit. From both points, the values were spot on.

The real sticking point to troubleshooting this is, there are times when the system works as it should. So in checkiing the wiring via a meter, things could check out fine with everything, for that particular moment. There has never been a set pattern to it's "acting up", as there was never just one thing to trip it of, ie opening one particular door. There have been times where it would act up, I keep trying, leave and come back 30 minutes later and all is good. Other times I would try every day for a week, with no luck, then suddenly on day 7, 8, 9 or whatever she decides to work.

The car built in Oct. 1982, with many of the components being made earlier in the year. So these parts are essentially 30 years old at this point. I know that many older Mercedes can suffer from minor cracks in the solder in the circuitry, just simply from age/vibration. So, I will take the alarm unit apart and look for that. At least for now, I can drive with confidence that I won't get stranded.

Albie

This won't help you with your alarm problem, but with your avionics/electrical systems experience, it might help with mine. I have a 1983 911 SC Targa, all original.
After being stored over the winter, the horn has started acting up. With the car parked, no ignition, the horn works.....all three contacts.
Start the car, and sometimes the horn works, sometimes it doesn't. Again, all three contacts.
The contacts on the steering wheel have been checked, clean as a whistle.
The horns are securely mounted, connections are all good.
Switched out relay.
The pattern here is............. ignition off, horn works perfectly. Ignition on, horn works perfectly. With the engine running, horn may or may not work.
Any ideas ?