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Old 07-13-2009, 12:31 AM
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Location: Simi Valley, CA (SoCal)
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84 300SD Trunk Won't Unlock With Key

Trying to follow the electrical diagrams for the central locking system...

The Central Locking System section (80-800) starts 09/84 (Model Year 85). The Electrical Troubleshooting has a diagram that seems to be more applicable. My central locking pump has a single three prong connector on it. The diagram shows a single electrical switch on the driver's door that actuates the pump to unlock all doors and trunk. And this is basically how I understood my system to work. However...

My owners manual says I should be able to unlock my trunk with the square key when my doors are locked. I can't. Here's why. When the driver's door is locked with the key, there is a tiny motor (part of the trunk lock mechanism) that moves a mechanical lockout so that, when I turn the key in the trunk lock and push, it won't go in. If I lock the doors with the interior lock, the motor doesn't actuate and the trunk lock works as advertised. So the door key switch actuates the pump to lock/unlock the doors, and it also actuates a lockout for the trunk.

I've disconnected the motor, and I don't think it's affected my alarm, and the door lock doesn't actuate the trunk lock motor anymore. My question is what kind of bastardized system do I have, has anybody seen this before, is there an electrical diagram that tells me how its all wired together, etc. Okay that's more than one question. Anybody? Anybody? Beuller?

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'84 300SD 119KMi (Liesl der Diesel)
'84 300D 326KMi when the oil left (former parts car)
'82 300SD 253KMi (new parts car)
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Old 07-13-2009, 01:00 AM
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Join Date: Feb 2004
Location: 32(degrees) North by 81(degrees) West
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It's been much too long to rely on memory...

AND I don't know if any of this is pertinent.

Two possibilities:

1.Do you have a Factory Alarm?
(Red Dot on the Master Key)
and if you have a Factory Alarm AND no key with a red dot...
You MAY not have the Master Key to that Chassis.

With the Master Key in the Trunk Lock,from vertical key position:
One click Counterclockwise is UnLock.
One click Clockwise is Lock
Two clicks Clockwise is "Lock Out" the Valet Key (AND maybe the "Headless Key")

2.My one thought is that you don't have a "Bastardized" system...
BUT, that the Trunk Lock (MIGHT? We hope) should have an Electrical
Switch that OPENS the little Electric Motor that's driving you crazy,
when you operate the key in the Trunk Lock.
ONLY IT'S BAD AND NOT WORKING???
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Last edited by compress ignite; 07-13-2009 at 01:09 AM.
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Old 07-13-2009, 05:26 PM
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Location: Simi Valley, CA (SoCal)
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Factory Alarm, Red Dot Master Key, No electrical component on the trunk lock cylinder.

Looking into this a bit more, this may be normal operation of the alarm system. In theory, if the trunk is opened when the alarm is set (i.e., locking the driver's side door with the key) the alarm should sound. Barring the trunk lock cylinder from opening the trunk in this case makes sense since the system cannot sense when the trunk lock cylinder is turned. But my cursory look into the Alarm System circuitry in the electrical schematics doesn't show a motorized mechanism that locks the trunk. Yet. I'll look again tonight.

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'84 300SD 119KMi (Liesl der Diesel)
'84 300D 326KMi when the oil left (former parts car)
'82 300SD 253KMi (new parts car)
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