Ok guys I need some help here. I just bought a 1987 300SDL. Body etc is in
excellent condition, no prior damage, rust etc. The car had a glitch in that
when you applied the brakes or turned on the wipers , or caused any electrical use the headlights would dim for a second and the after market radio (digital) would cut out for about 2 seconds. This was annoying but livable since most of my driving was on the highway and not city where I would be appling the brakes etc. alot. The radio speakers would also cut out left to right every now and then as well. One day as I stopped the car and got out the alarm went off and lasted for about a minute. The radio stopped working from then on. It would power up but no sound. Then I was driving home one day, put my foot on the brake and the car alarm started, lights flashing the whole bit. and the radio began to work again. After about 20 mins I realized that if I did not touch anything electrical the alarm would sound approx 1 min go silent for 20-30 sec then sound 1 min. then shut off untill I did anything electrical. I was able to drive home like this and now it sits in my driveway. If I so much as open the door the alarm goes through its cycle. Is there a short? If so where would the most likely place be? Is this a radio theft alarm or car theft alarm?
Is there a way of deactivating the alarm (short of disconnecting the battery)?
Any help would be appreciated.

Try this...

Once you have unlocked the car, keep the key in the door....Open the door a few inches and pretend to be unlocking your car again. Do whatever you do after that.
Sounds weird, but it can work in some cases.
Let us know if that works/does anything for you.

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It is a truism that almost any sect, cult, or religion will legislate its creed into law if it acquires the political power to do so. Robert A. Heinlein


09 Jetta TDI
1985 300D

I think you might also check to see if the car is operating with a low voltage situation. Measure the voltage with the car running and see if it is up to spec which would be something over 13volts. The fact that when you draw on the system (brake lights, wipers) it causes problems probably would point to lack of voltage or current available. A bad battery can also cause this problem. My 2 cents...

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83 300CDTurbo 307,000
2005 Honda Element 266,000
56 Nash Ambassador Country Club Special 34,000(under restoration presently)

http://www.peachparts.com/shopforum/showthread.php?t=125099

I'm inclined to agree with a low voltage situation. Check the voltage with the vehicle running and some accessories on. It should maintain 13.5 volts if the rpm's are at 1000 or above.

The dimming of the lights and the loss of the radio is a indication of a sharp voltage drop.

Do you have any starting issues with it? Can it crank for 10 seconds or so? You might never need to crank the 603 long enough to know this.

Don't forget to check your ground cables, particularly for the battery and between the engine and frame. Bad grounds can cause all kinds of wierd stuff.

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1991 M-B 560SEL Arctic White/Grey 99,000 Miles
1987 M-B 300SDL Ivory/Palomino 229,000 Miles (sold but never forgotten)
2006 Volvo XC70 Blue/Beige
1999 Porsche Boxster Arena Red/Savanna Beige
1986 Porsche 928S Goldweiss/Brown

Low voltage or ground issue.

Check the battery voltage.

Unlock and lock the passenger door to turn off the alarm. Sometimes my driver door won't disable the alarm when I open it.

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Michael LaFleur

'05 E320 CDI - 86,000 miles
'86 300SDL - 360,000 miles
'85 300SD - 150,000 miles (sold)
'89 190D - 120,000 miles (sold)
'85 300SD - 317,000 miles (sold)
'98 ML320 - 270,000 miles (sold)
'75 300D - 170,000 miles (sold)
'83 Harley Davidson FLTC (Broken again) :-(
'61 Plymouth Valiant - 60k mikes
2004 Papillon (Oliver)
2005 Tzitzu (Griffon)
2009 Welsh Corgi (Buba)

The original Becker radio was wired with the cars alarm system, it was in my 87 300D, and I had a very similar situation, not to mention my alarm went off all by it's self one night, but also similar things you've mentioned.

You may not elect to do what I did, but I disconnected the alarm system from under the passengers floor mat, tapped another source for power to my new radio, and that solved my situation, but I do not have any longer an alarm and to that I say...........soooooowhat! And if they want to break into it, there going to anyway, or do some damage trying......I can live without my alarm.

Don't know if this could work for you, but it did for me!...........BB

Yeah, if it is the alarm contacts in the doors going bad, pull up the passeger front carpet, youll see two modules. Unplug the two black plugs going to the one box, do NOT touch the red plug.

Mine tends to work when it wants to as well. right now my drivers lock is not locking and unlocking the rest of my doors again. Only once did I have to go outside in 20 deg weather at 2am in my underwear to shut off my alarm

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1980 500SE/AMG Euro
1981 500SEL Euro
1982 380SEL
1983 300TD
1983 500SEC/AMG Euro
1984 500SEC
1984 300TD Euro
1986 190E 2.3-16
1986 190E 2.3
1987 300D
1997 C36 AMG
2003 C320T 4matic

past: 1969 280SE 4.5 | 1978 240D | 1978 300D | 1981 300SD | 1981 300SD | 1982 300CD | 1983 300CD | 1983 300SD | 1983 380SEC | 1984 300D | 1984 300D | 1984 300TD | 1984 500SEL | 1984 300SD | 1985 300D | 1986 300E | 1986 560SEL | 1986 560SEL/Carat | 1987 560SEC | 1991 300D 2.5 | 2006 R350

Where is the connection for the engine/frame cable?

Thanks for the replies. I have had a few mention the alarm disconnect. i will do this when it gets a little warmer. I tried both the door and trunk locks
with no luck. The key will lock and unlock all locks but will not stop the alarm.
Once I disconnect the alarm I will be checking all grounds (if I can find them) especially the main engine and radio ground. Luckily I do not need to drive this until I can find the problem.
Thks Bob

I believe it is connected from the back of the engine near the bottom of the bellhousing to the adjacent frame rail.

Isn't there master and valet keys on that year? One key with the red dot on it activates and deactivates the alarm, while the one without doesn't do anything with the alarm... If you have both keys that is...

-m

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Now:
2018 Tesla Model X
1999 S500 Grand Edition 164k
1992 300D 2.5 Turbo 287k
2005 E320 4MATIC wagon
1991 Alfa Romeo 164L 99k (sleeping for a while)

Then: 96 Lincoln TC, 93 Lincoln TC, 87 560 SEL, 87 300 SDL, 80 300D, 89 560 SEC

Check low voltage. If the ground cable is intact, try disconnecting the alarm system and see if it makes a difference. You probably need to test your altenator, it'd be a good time to do this.