Alarm Installation - Wiring - PeachParts Mercedes-Benz Forum

300SD81 · #1 09-23-2006, 12:25 PM

I'm installing a Commando FM-870 remote start alarm in my 300SD. Can anyone tell me what all the wires on the ignition switch do? A good constant power source behind the instrument panel would be useful too. The system has a wire that provides an output while it is in remote start mode, so I think I can use an ALDA switchover valve to disconnect the vacuum shutoff (do the valves just vent the pressure, or disconnect? If they just vent, will a vacuum leak in that line have a large effect on the rest of the system? (Mainly the door locks, since it cannot be driven without the key anyway, so the trans dosen't matter.)). I already installed a door lock motor onto the drivers door, so that the vacuum system controls the rest of the locks. Any other wires from here would be helpful, but I'm only doing one wiring harness at a time so I don't have to take the whole car apart in one weekend. http://www.commandoalarms.com/FM-870install.pdf

300SD81 · #2 09-26-2006, 10:38 PM

I got the remote start section working, still working on the door pins (need to get replacemnts from junk yard) and trunk triggers (anyone know a good way to run a wire from the trunk to behind the gauges?). I'm also having trouble finding the brake light switch, does anyone know where it is? And, how can I tap the door pin wires so that it will trigger the alarm when any of the 4 doors are opened, or wire the dome light output so that all the lights come on when the alarm turns its output on, but still have seperate circuits for the front and rear door pins? Should I just use some more relays?

SD Blue · #3 09-26-2006, 11:14 PM

For places to tap into the wiring, robably the best bet is to go to http://mb.braingears.com for the on-line manual. (If you have DSL or better.)If not then you can get a CD manual at MBCA.org for about $20.

I believe most of the trunk wiring goes along the left side. Check by removing the sill plates.

300SD81 · #4 09-27-2006, 08:08 PM

I'm currently using that online manual for wiring diagrams. The doors seem to be on seperate circuits, and I do not want to short them together, and my alarm only has one input. I was hoping someone had some tips on connecting them without using another pile of relays.

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I got the power locks working today, nice not having to open/close the back door to lock the drivers door

(it won't let me lock with he door open for some reason, and the replacement door lock assembly from the junk yard didn't either. is this standard?) I found the brake light switch in the diagrams, bu am unable to locate it in the car.

deerefanatic · #5 09-27-2006, 09:56 PM

Yes, you can't operate the locks with the door open, it's a safety feature so you don't lock your keys in the car....

As far as the door pins go, you really don't want to use the stock door pins for sensing, been there, tried that, you end up, to overly simplify things, turning on the dome lamps appears as the door opening to the alarm, which just confuses it. Buy some seperate door pin switches from Parts Express www.partsexpress.com for door sensing, then use some diodes to power the front and rear domelights while still isolating them..........

It's tuff work in these old benzes, but worth it when done.....

300SD81 · #6 09-27-2006, 10:17 PM

Based on the wiring diagram, it dosen't seem like the dome light delay relay will cause alarm to go off if the pins themselves are tapped and not the light. The alarm will not go off if its own light output is fed back into its sensor wire. There are also 2 door pins on the drivers door on my car, but only one in the diagram. What does the second pin do?

deerefanatic · #7 09-28-2006, 08:05 PM

Maybe it won't cause a prob for you...............I am just going on what I had which is a cheap chinese piece of crap that they called a car alarm when they sold it to me!

Second door pin is so you can adjust the power seat, even with key off, so long as driver's door is open.......

Really though, if you want to keep the front and rear dome light circuit's seperate, then break down and use a set of dedicated pins.......... easier and more straightforward in my book, and you don't have to botch up the factory wiring........... worth big $$$ in my book.

300SD81 · #8 09-28-2006, 09:48 PM

This alarm way bought for way too much while I was pissed off after school after finding 2 new scratches on the car. Definitely not a cheap piece of junk, I almost bought a Viper sytem for $400 before I found this one with better features and at less than half the price. I personally find splitting the insulation a little bit and twisting a new wire around the exposed wire more comfortable than drilling a hole anywhere in the car. I figured out a way to use 2 relays combine the front and back to trigger the alarm, but still keep them seperate for the lighting. Going to try it out tomorrow after school, or saturday, probably not going to have enough time tomorrow before the school football game.