300E Alarm Wire to the HU

[Strider]

During my recent install of my new Blaupunkt head unit, I learned something about the alarm wire from the old Becker unit. On my car (91 300E), the wire has to be connected to a ground in order for the alarm to function. Without it connected, the alarm goes off whenever I try to lock the door.

Luckily, on my Blaupunkt, the case has a tab that can be bent out on the rear of it, making a male spade connector. The stock alarm connector (female spade) fits right on this and will function just like it did with the other radio, disconnecting when the radio is pulled out and setting off the alarm if the system is armed.

In other posts, people have written that you don't need to connect this wire....which leads me to two possibilities: A. The people who wrote this don't have functioning alarms, or B. The alarm system has changed over different model years. In the factory service manual CD, there is practically no information at all about the alarm system.

Thanks

[Uncle Acky]

Hi there, I too need info on the alarm wiring of a 1992 300E

To your point of not much info on the CD, I learned something today too, for alarm system info:

load disk #2
go to Chasis and Body
go to Electrical Systems
go to 82 body
go to 0980 Function information
or
go to 0983 alarm system testing

The testing procedure is a must for troubleshooting faults or problems.

Unfortunately I have not yet found the alarm system wiring diagram. If you happen to find it please let me know.

Hope this helps,
Acky

[anthonyb]

I think the alarm has changed over the years. I installed an aftermarket head unit in my '87 300E, and at the time the information I found said that the "alarm" wire was basically just a low-current power line that told the radio when to go into theft-protect mode. I don't believe it was otherwise dialed into the alarm. I currently don't have it hooked up to anything, and the alarm still works.

[SCLJA]

The alarm wire does not have to be hooked up to anything. It is merely a trigger wire that tells the radio to go into code mode. I have installed hundres of stereos in 124 cars and never had a problem.....

[Strider]

Originally Posted by SCLJA

The alarm wire does not have to be hooked up to anything. It is merely a trigger wire that tells the radio to go into code mode. I have installed hundres of stereos in 124 cars and never had a problem.....

Without that wire hooked to a ground in my car, the alarm would go off just as soon as I locked the car with the key. I went for a couple of days locking the car with the passenger side door lock and closing the door so as not to set the alarm off. (The car doesn't let you lock the door on the driver's side by pushing the lock down, I guess to prevent you from locking the keys in the car.)

Bottom line is that on my 91 300E, that wire had to be grounded for the alarm to function properly. The Stock Becker radio went into code mode whenever the radio was disconnected from the main connector, or the battery was disconnected. In other words, if it lost power.

I wish I could find the wiring diagram for the stock alarm system...

[SCLJA]

That is quite strange as I have a 92 300e sportline and I don't have to ground the alarm wire.... Never seen it