300SDL trunk lock vacuum issue - need ideas - PeachParts Mercedes-Benz Forum

old goat · #1 03-03-2011, 01:55 PM

My 1986 SDL has started having issues with the trunk lock opening via the door key. Initially I had to wait about 20 seconds to make sure everything had to time to open or the alarm would go off. I have checked the vacuum at the vacuum control unit per the manual and double checked all lines. I can find no issues....other than the alarm going off. I finally figured out that if I use the key in the trunk lock instead of the doors, everything opens fine, no alarm, no long wait period. What is making no sense is the fact that a leak at the trunk lock should have equal impact on actuating the other locks on the car.....in other words they would be slow to respond or not respond at all.

Has anyone experienced this issue? Does anyone have a suggestion for a fix so I do not have to open the truck all the time to get into the car? I would appreciate any help I can get.

Thanks, Frank

sd300td · #2 03-03-2011, 02:38 PM

Quote:

Originally Posted by old goat

My 1986 SDL has started having issues with the trunk lock opening via the door key. Initially I had to wait about 20 seconds to make sure everything had to time to open or the alarm would go off. I have checked the vacuum at the vacuum control unit per the manual and double checked all lines. I can find no issues....other than the alarm going off. I finally figured out that if I use the key in the trunk lock instead of the doors, everything opens fine, no alarm, no long wait period. What is making no sense is the fact that a leak at the trunk lock should have equal impact on actuating the other locks on the car.....in other words they would be slow to respond or not respond at all.

Has anyone experienced this issue? Does anyone have a suggestion for a fix so I do not have to open the truck all the time to get into the car? I would appreciate any help I can get.

Thanks, Frank

Are you saying that the trunk lock is operating like the driver's door, i.e. unlocking the trunk unlocks all the other doors? That is weird.

I'm not familiar with w126 cars, but the trunk vacuum element is probably the last in the line (it is on w123). So all other doors could unlock as designed, from the driver's door. A bad element can cause the engine not to shut off properly. Does your car shut-off properly?

old goat · #3 03-03-2011, 02:45 PM

Yes it is weird. Yes it does unlock everything else if I use the truck lock to open the car. No, there are no shut off issues. For that matter, when I lock the car via a door lock, the trunk locks just fine. It is unlocking that is the issue.

Thanks for the response!

ah-kay · #4 03-03-2011, 03:19 PM

The 300SDL can lock/unlock the car at 3 locations, driver door, passenger door and the trunk.

IIRC, the passenger door has an interlock at the ignition switch which can override the lock/unlock function. If it goes wrong then the passenger door function may not work.

One thing I know for sure is that you have no vacuum issues as you can lock/unlock the whole car from the trunk. It is more likely electrical.

I need more info to help you. Can you describe what exactly is wrong?

old goat · #5 03-03-2011, 03:30 PM

I am not sure what else I can say. The issue is new....well in last 6 months. Before that the door lock opened everything just fine. Then it started as having to wait a while or the alarm would go off. Then it became the alarm going off period unless I used the trunk lock to open the car.

Now, as you suggest electrical, I can also say that when the alarm went off, it turned out the quickest way to stop it was to use the key in the trunk lock. From that point I started just opening it with the trunk lock

Does this info provide ideas?

ah-kay · #6 03-03-2011, 04:38 PM

The alarm goes off without opening any doors or trunk lid? What happens if you unlock the car at any locations but do NOT open the doors or trunk lid? Then lock it back again, any alarm?

I think all the switches at the vacuum pods are the same. It only has 3 wires, blue, red and yellow ( IIRC ). It is used to detect +12v or 0V. +12 would lock and 0V would unlock. The sensors to detect intrusion are at the door hinge, trunk lid and hood. Therefore no alarm should not go off if you do not open anything.

Anyone messed with the trunk light wiring? The sensor circuit is there.
Anyone messed with the door hinge sensor. I think there are 2 at the hinge. one for the courtesy light and one for the sensor. It is possible the sensor switch is faulty.

This is what I would do. Hardwire the sensors at the door to close position and see what happens. Then remove one at the time and you many find out what is the problem.

If all failed then install a Remote Entry to the car like I do with all my cars.

old goat · #7 03-04-2011, 07:56 AM

Sorry, I had a meeting last night so no time to respond.

If I unlock at a door or at the trunk but do not open a door, no alarm. If I then lock it at that point, there is no alarm and all doors are locked. If I unlock it at the trunk, I can open any door without the alarm going off. However, If I unlock it at either the driver's door or passenger door the alarm will go off and it does not matter if I wait 10 minutes to open the door. It appears that I have to manually open the trunk lock in order to make sure the alarm is turned off. But I did not think the lock itself was integral to the alarm circuit.

I have owned the car for 5 years now and just recently ran into this problem. No one, since I have owned it, has touched anything in the wiring, the trunk or anywhere else on the car. I am finicky about anyone working on my cars.

I guess the question is whether there is a direct relationship between the physical trunk lock mechanism and the alarm circuit. Based on the wiring diagrams, the alarm "disarm" mechanism in each front door and not in the trunk lock.....thus using the trunk lock to prevent the problem is not sensible at all.

I would appreciate any ideas.

Mr. Majestyk · #8 04-07-2011, 01:03 PM

Not a big forum techie here, yet very similar issue with my 91 300 TE. When I unlock the driver door, never have tried it with passenger, 3 out of 10 times the alarm goes off, and I have to run around and turn the key in the trunk lock. This disables the alarm, I'm embarrassed, and the car starts normally and I drive off. It might happen 3 times in a row, or not at all for a week. But it has been steadily occurring for the last 4 months. It is seemingly random: temperature, humidity, and if I wait or not all seem to not matter, and I have been tracking it. I have read that there is some kind of mechanism connected to the lock system which is supposed to disable the alarm, so that when the hinge sensor is triggered the alarm knows whether to sound or not. Two things are funny here. The 1st time it happened I was supposed to be following a bunch of people in another car and did not know to disable it yet with the key in the trunk lock, so I got in and was able to start the car and follow them was driving with the alarm sounding full on. Stressful, got a lot of looks. Thought the alarm was supposed to disable ignition. The 2nd piece here is that I have a sunroof on the car. My daughter fooled with it and it was leaking and sunroof motor froze but it looked closed, seatbelt was getting wet and freezing in the cold. I am going to guess water is coming in and corroding something important having to do with the release mechanism which is supposed to tell the alarm not to sound when hinge sensor is activated. I don't want to "hijack this thread" as I've heard it been put, so I will start my own thread, and I'll call it: 91 300 TE RANDOM ALARM ISSUE or problem, am open to feedback and good luck.

woodboat3 · #9 04-08-2011, 11:00 AM

I didn't know that unlocking at the trunk disabled the alarm. I do know on my 1987 300SDL that relocking and then unlocking again at the passenger door always stops the alarm. My car had been sitting for years when I got it. I didn't even know it had an alarm until I set it off

Anyway at first it seemed like it would go off without any rhyme or reason. I noticed many locks were stiff and seemed slow to open. Things are moving better now as I use it everyday and I have lubed a few things. My alarm hasn't gone off in a week. Is it possible that your mechanical pieces need a little lube?