Background:
'92 300SE. PO had installed an aftermarket alarm, complete with auto-start. Lots of "changes" (nice phrase for butchery) occurred as a result of this install. For instance, when you'd lock the doors via the remote, you'd get the red light on the doors, but the door locks themselves would unlock. The reverse was true when you'd try to unlock the doors via the remote.

Don't have the factory remote - will get one soon. Maybe this is why the PO went to aftermarket alarm - don't know.

Anyway - I removed the alarm - fortunately there was only one cut wire - all the rest were just taps and were easy to remove. But the doors still unlocked when you'd lock the door with the key. Finally cracked open the PSE unit and found that some bright individual had reversed the leads on the motor!!! Everything on the car worked backwards - closing the trunk extended the closing handle - putting the car in park extended the parking assist masts, etc. Fixing this problem at least cured the backwards behavior.

The BIG problem is that periodically, the car "shuts down". By this I mean that almost nothing works. Locking and unlocking the doors reveals no red/green lights. Door lock actuators don't work - seats don't move, steering wheel adjust - dead - windows - dead. Car will crank but not start. Doors don't pull shut - nor does the trunk lid pull down.

Initially, I thought it was an alarm related shutdown, but locking and unlocking the car should cure that problem. I have verified that when the alarm is set, opening a door through an open window triggers the horn and lamps - but curiously, the car will crank and start while the horn is blaring. Likewise, opening the hood under the same conditions produces the horn and lights. These tests are terminated by the required locking and unlocking of the driver's door.

I suspect I've yet to uncover all of the modifications made to get the aftermarket alarm to work.

Just as abruptly as the problems begin, they end - and for no apparent reason. Recently while working on the car, things quit and remained dead for about 36 hours - after which point they began working again. I'd remove the negative terminal on the battery and reconnect it. When I'd insert the key into the trunk lock and turn to the unlock position - I'd sometimes get a VERY brief green light, and then everything would shut down again. Most of the time, the battery reset method had no effect.

Worst of all is that this can happen while you're out on the road. Fortunately, it's never happened in a bad spot, and never at speed - just while rolling away from a stop sign or making a turn in a parking lot.

I'm sure my mechanic can diagnose the problem if I can get it to stay broken - but the problem is intermittent. I've got the car in the garage while I repair some of the damage from the PO, but I'd really like to drive the sucker - otherwise, it's a 4500# couch.

Any ideas or suggestions would be greatly appreciated!

The particular era you are in with this car has a very suspect ignition switch style. When it acts up again try moving the key around in the switch a bit to try to see if you can get all the contacts in the switch working again. Only real hard part about changing the switch is getting the lower panel off the dash, really over-engineered to protect your knees! That's my thought anyways, that one of the contacts in the switch (electrical part) isn't making contact.
On the alarm-blaring but still starting, I don't think the early ones had driver authorization (or an immobilizer in other words) and will start, but just has ATA-anti-theft alarm.
Gilly

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Thats like saying "here you go, take it, but no stearing or brake assist and I'm telling the world"...

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-Marty

1986 300E 220,000 miles+ transmission impossible
(Now waiting under a bridge in order to become one)

Reading your M103 duty cycle:
http://www.peachparts.com/shopforum/831799-post13.html
http://www.peachparts.com/shopforum/831807-post14.html