I did tons of searches and didn't find another occurance of this problem so I'm hoping someone will have some good advice. The car is an 1988 420sel. I finally replaced my ailing Becker unit with a new stereo and rear speakers (bypassed amps). It was a smooth install and only took about 3-4 hours, though I did blow the radio fuse once (number 18 I think).

Unfortunately when I went to lock up for the evening my alarm went off as soon as I turned the key. This is actually the first time I've ever heard it and until now I had thought that it was inoperable. A few weeks ago I tried setting the alarm off on purpose but wasn't able to. That said, now that I know the thing works (sort of) I would like to get it functioning properly if possible. Clearly something is shorted or tripped but don't know where to start looking. Do I need to short one of the factory connections to fool it into thinking the original unit is still there?

For now I had to pull out the firewall and disconnect the thing just to be able to lock the doors for the night. The only other symptom is the 'light bulb' indicator has come on, on the dash.


Thanks!!

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1988 420SEL 170K

It sounds like you have used the factory alarm trigger wire as the ground for your radio. You are not to hook up the alarm wire or the wires in the 5 pin connector. Just leave them disconnected. This is the brown blue wire and the 2 brown wires and the grn black wire. You get your ground from the power harness or I use the 10mm head bolt / ground point below the coin pocket.

Scott@lajollaaudio

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The Mercedes Specialists Since 1986

Scott,

Thanks so much for the input! I love it when problems have simple answers. I didn't use the 5-pin connector but I did use the brown wire in the 4-pin for a ground. I remember thinking I aught to use the 10mm bolt but the wire was right there and I got lazy... I should've known

Thanks again!

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1988 420SEL 170K

Update:

Well, I finally found a little time to reground the alarm to the 10mm nut. Alarm still goes off. I'll recheck all my connections this weekend.

Is it possible that I've broken a circuit somewhere?

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1988 420SEL 170K

howdy..let me know what you do to fix that..i have a 87 420 sel that i just put a new alpine in..and sets off the alarm when i lock the door..take care

well contrary to other replies.. I went back in and grounded the thief lead to the radio.. alarm does not go off upon locking the car...i read two other posts that say the same thing..makes sense to me...if a crook took your radio..what sets the alarm off if that circuit doesnt need to be connected?...hope that helps

If you have an aftermarket stereo then leave the alarm wires disconnected as they are intended to work only with the oem radio................

Scott

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The Mercedes Specialists Since 1986

Yes they are just for the oem radio..but by leaving it disconnected, tells the alarm system the radio is missing..and..sets off the alarm

I installed an aftermarket receiver in my 190D yesterday.

The first problem was the alarm sounding when the key was turned (I fixed that by grounding the blue theft wire to the back of the radio.

Today I went to lock my doors for the first time and the alarm sounded!

From what I read above there are more than 1 security wire, I will have to look, any other ideas?

Thanks,
John

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1986 190D, 2.5L, 5-speed swap, 180,000 Miles (60K by me).
Jeep CJ-7 with Cummins 4BT/NV4500/AtlastII 4.3.
Grand Wagoneer 4BT project in progress!

Any ideas?

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1986 190D, 2.5L, 5-speed swap, 180,000 Miles (60K by me).
Jeep CJ-7 with Cummins 4BT/NV4500/AtlastII 4.3.
Grand Wagoneer 4BT project in progress!