1994 124 wagon central locking problem - PeachParts Mercedes-Benz Forum

Maxbumpo · #1 01-20-2020, 08:48 AM

The central locking system on my $500 124 wagon, 1994 model E320, needs attention.

The driver's door does not activate the system. It will lock / unlock with the key, but no response from the central locking system.

The front passenger door, when locked, will activate system and lock all doors, and then immediately unlock them all.

The tail gate will start an endless do-loop, lock / unlock, until I unlock it.

I suspect the driver's door is the issue, giving a continuous unlock signal, so I'm planning to start the diagnosis there, but if anyone has experience and can offer some insight, I'd love to hear it.

dieselbenz1 · #2 01-20-2020, 09:01 AM

I would start at the pump under the rear seat you can check the signals you are getting at the electrical plug. If I remember correctly +12 to one terminal pressures the system to open while another terminal +12 creates vacuum to lock the doors.

Maxbumpo · #3 01-20-2020, 09:09 AM

That's a good thought, although rather tough to get to that pump on a wagon, as the carpet set cover's that up and I think some interior trim needs to come out as well as the back seat bottoms.

jay_bob · #4 01-20-2020, 09:14 AM

That sounds like a good theory.

Pump is located beneath the seat in the second row on the right side.

There is a 3 pin connector at the pump with a blue, yellow, and green wire. These are the signals from the door switches.

Blue = driver door
Yellow = tailgate/trunk
Green = passenger door

These are fed from a double throw switch in the lock assembly. It applies +12 to unlock and ground to lock. The pump figures out the signals and does either a vacuum (to lock) or pressure (to unlock). On the 124 there is just one pneumatic circuit for everything (all 4 doors, tailgate/trunk, and fuel door).

On the pump there is also a 3 pin connector with red and brown wires. This is the power for the pump, unswitched power from fuse C. If you have ortho seats the pump will have a 3rd black wire in that connector for the ortho pump feature and a second pneumatic circuit for the seat cushions.

A 94 will have the convenience control module so this will be in the mix as well (the function for turning the key in the drivers door to open/close all the windows).

Also I would look at the wires crossing the hinge, these were a notorious failure point in the 124s where the wires would break from constant flexing and in the worst case cause a short.

jay_bob · #5 01-20-2020, 09:18 AM

Quote:

Originally Posted by Maxbumpo

That's a good thought, although rather tough to get to that pump on a wagon, as the carpet set cover's that up and I think some interior trim needs to come out as well as the back seat bottoms.

Not too bad.

Remove the right rear seat cushion (raise seat, pull the red lever upward while pulling up the seat cushion to remove from the hinge pins).

Remove the right rear door sill (pry up carefully).

Remove the cover plates around the right rear hinge pins (pry carefully).

The carpet can now be peeled back enough to get to the pump. It's in a foam box to keep it quiet.

Maxbumpo · #6 01-20-2020, 10:36 AM

Jay Bob, thanks for all that info, most helpful.

I think I'm going to start by checking out the driver's door lock and see if there isn't a problem with the 'unlock' signal. I'm suspecting that there is a constant unlock signal given to the system from the driver's door, so trying to lock the system from any other door results in the odd behavior.

Maxbumpo · #7 01-20-2020, 10:38 AM

Quote:

Originally Posted by jay_bob

A 94 will have the convenience control module so this will be in the mix as well (the function for turning the key in the drivers door to open/close all the windows).

Hmmm, did not know about this feature. Is that standard on all 124 cars for certain years, or is there an option code I should look for on the data card?

jay_bob · #8 01-20-2020, 12:21 PM

I believe they made that standard (well probably in the US market at least) starting I think in 1990 or 1991. Had to compete with the newly introduced Lexus/Infiniti and all their electronic gee-whiz stuff.

ah-kay · #9 01-22-2020, 02:46 AM

Quote:

Originally Posted by Maxbumpo

Jay Bob, thanks for all that info, most helpful.

I think I'm going to start by checking out the driver's door lock and see if there isn't a problem with the 'unlock' signal. I'm suspecting that there is a constant unlock signal given to the system from the driver's door, so trying to lock the system from any other door results in the odd behavior.

x2. I think it may be the switch inside the driver door.

Maxbumpo · #10 01-22-2020, 12:22 PM

Looking at the wiring diagrams in the FSM, the diagram for the alarm system seems to have good info. The alarm system module is in the passenger foot well, and gets either "arm" or "disarm" signal from each of the front doors and from the tailgate. This is the same signal that goes to the CLS pump to open or close the locks. Checking for this signal at the alarm system connector is one way to see what the driver's door switch is signaling, and much easier than digging down to the pump. I will also be able to put a signal (ground) here for open or close, depending on which wire is grounded, and check that signal to the pump. On the door switch side, check to see if that signal indeed is grounded continuously, and then move the wires at the according joint for the driver's door to see if I can make that signal change, which would point to a broken wire.

Maxbumpo · #11 01-22-2020, 12:24 PM

Quote:

Originally Posted by tgun84

Find electrical drawing on the web. It is fairly easy to troubleshoot yourself. The most tricky part is removing the door panels without damaging them.

I have the diagrams, bought the 2-CD set for 124 a long time ago.

The front door CLS / Alarm system connectors can be accessed from the trailing edge of the door, no need to remove the door panel.

Maxbumpo · #12 03-15-2020, 06:08 PM

Quote:

Originally Posted by dieselbenz1

I would start at the pump under the rear seat you can check the signals you are getting at the electrical plug. If I remember correctly +12 to one terminal pressures the system to open while another terminal +12 creates vacuum to lock the doors.

Finally had some time to diagnose this issue, and used this method. Got to the pump, pulled the connector, and verified that each of the three locks (RF door, LF door, and tailgate) all gave the correct signals, which means the problem is in the pump circuitry.

I've also determined that all 124 models used the same pump, and I've got a spare somewhere in the garage.

I'd like to learn more about the circuit in the pump, and see if that is repairable.

jay_bob · #13 03-16-2020, 08:08 AM

If I had to guess, there is a simple logic circuit with a couple timers. Probably done with integrated circuits.

Not sure if they are reversing the motor or switching the inlet/outlet around to get the vacuum or pressure as needed.

There were 2 variants of the pump, one with the extra port for pneumatic seats and one without.

paul roberts · #14 03-16-2020, 12:06 PM

Sounds like it may be the micro switch in the driver's door lock. A quick simple diagnosis would be to swap the door lock from the passenger side to the driver's side to see if it activates. It will be reversed since it will be upside down, but will still give a signal.

Maxbumpo · #15 03-16-2020, 02:01 PM

Quote:

Originally Posted by paul roberts

Sounds like it may be the micro switch in the driver's door lock. A quick simple diagnosis would be to swap the door lock from the passenger side to the driver's side to see if it activates. It will be reversed since it will be upside down, but will still give a signal.

I've already tested the signals from the door locks, getting good signals, so the issue is in the pump control circuit.