1984 300DT truck lock acting up.........

[HuskyMan]

The trunk lock has been acting up for some time. Right now, I can't push the lock in no matter how hard I push. In the past, I've pulled the cable release under the rear seat which opened the trunk. I then sprayed a special lock lube in the lock and on the latch which has always worked to loosen it up and the lock would return to operating smooth as silk.

I'll probably do the same thing; that said what could be going on with this trunk lock?

[SD Blue]

Do you have a factory alarm?

If so, it will lock you out of the trunk and you will need to ground a certain pin, at the controller to get the alarm trunk lock to release.

[HuskyMan]

Where is the controller located?

[HuskyMan]

I located this chart for all things factory alarm. My problem is I have a limited background in reaching electronic schematics. I'm trying to learn, though....

https://www.startekinfo.com/StarTek/outside/12253/program/Chassis/82-0980.pdf

and this:

http://www.peachparts.com/shopforum/attachments/diesel-discussion/141003d1487185399-source-used-w123-door-alarm-switch-alarm-switch-left-door.jpg

any ideas where the controller is located and which pin I need to ground?

[Alec300SD]

The easiest way to navigate the ETM is by using Internet Explorer as your browser...the embeded links are then active.

These, I believe, are the pdf links you need.
https://www.startekinfo.com/StarTek/outside/12265/disc_2/program/ETM/123%20turbodiesel_2.pdf

https://www.startekinfo.com/StarTek/outside/12265/disc_2/program/ETM/123%20turbodiesel_comp2.pdf

[HuskyMan]

Thank you alec300SD, now all I need to do is buy a book or take a course in basic automotive electrics/electronics.

[Alec300SD]

Here's my take on the diagrams:

On page 8 of the second pdf link (Page 213 - Component Locations) Figure 213.1 shows a grainy image of the Anti Theft Control Unit (C306)
as being located under the LH side of instrument panel.
It has 8 pins.

On page 26 of the pdf (Page 139 - Anti Theft Alarm System), the trunk lock has three wires.

The three wires go to the connector (C307) on the left hand side of the trunk and then continue on to the Anti Theft Control Unit (C306).

The red (power) wire goes to fuse 2.
The Yellow/Green wire goes to Pin 7 (Ground when system is armed)
The Black/Green wire goes to Pin 4 (Ground when system is disarmed)

Hope this helps.

[HuskyMan]

Thank you for the summary; I believe that will help me understand the system better. German electrics seem to be challenging and yet there is a method to their madness.

[Alec300SD]

Just unplug the 8 pin connector at the anti theft control unit.
Connect pin 4 to a chassis ground and this should disarm the trunk lock.

Let us know if this works.

[HuskyMan]

Will do; I won't have access to the car until later today....will get back to you after I've had a chance to try my hand at diagnostics. Several years ago I pulled the horn part of the alarm because the alarm was acting up....I cannot remember if I disconnected anything else.....

[HuskyMan]

Alec300SD, If I remember correctly, I believe the problem with the car's alarm is the driver's door lock. At the time I was short on money and didn't have the funds to purchase a new door lock. So, I chose to take the horn out because the horn was sounding intermittently depending upon which way the key was turned in the lock.

So.....it looks like I need to replace the driver's side door lock to make the alarm system work correctly. Feedback is appreciated....

[Alec300SD]

My cars are pre-1984 so I don't have alarms on them.
Based on your last post, regarding how the alarm system would behave erratically,
leads me to believe that the microswitch located in the DS door lock has probably failed.

Replacing the microswitch with a new or good used microswith will probably restore the
alarm sytem functions (along with reconnecting the horn, of course).

The microswitch has three wires that end up at the anti theft control unit.

The brown wire is the ground. (Pin 8 on the anti theft control unit).
The green wire disarms the alarm. (Pin 5 on the anti theft control unit).
The yellow wire arms the alarm. (Pin 6 on the anti theft control unit).

Bridge pin 5 and pin 8 and this should disarm the system and hopefully allow the trunk lock to open.

You can also access the wires at the 3-pin connector (C302) located at the driver's door lock and also at the 3-pin connector (C303) located in the driver's door.

Best of luck.

[CTD]

I have another odd (but different) issue with the trunk latch on my 1984 300D. It'll open with the key, but I can't turn the key to a position that keeps the trunk unlocked. Perhaps I should disassemble the latch or replace it?

[Alec300SD]

Originally Posted by CTD

I have another odd (but different) issue with the trunk latch on my 1984 300D. It'll open with the key, but I can't turn the key to a position that keeps the trunk unlocked. Perhaps I should disassemble the latch or replace it?

If the central interlock system is active (DS door locked), you can open the trunk with the key but when you close the trunk lid, the interlock will again lock the trunk.
This is by design.

If you unlock the DS door and the trunk lock does not disengage (when the trunk lock key slot is horizontal), then you have too low a level of vacuum in your system.
Most llkey culprit is a failed/leaking trunk or fuel door vacuum actutator or failed/leaking vacuum connector.

My 78 W116 300SD has a leaky vacuum reservoir.
Verified with MityVac, reservoir leaks down very quickly even with a new rubber vacuum reservoir tank seal.
Found a good used vacuum reservoir...install is on the "to do" list.
Once the car is started, the central locking system operates all the door locks, trunk and fuel door locks properly.
DS door locked ==> all locks engaged.
DS door unlocked ==> all locks disengaged.

About ten to twenty minutes after shutdown the central locking system does not operate properly because the residual vacuum in the system is too low.
DS door unlocked==> trunk lock does not disengage==> unlock trunk with key==> close trunk==>remove key==> trunk lock is still not disengaged.

Hope this helps.
BTW on my 83 W126 300SD, when the central locking system is activated, if the tunk is unlocked with the key,
the trunk will remain unlocked when the key is removed with the trunk key slot in the horizontal position.