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From digest.v7.n356 Tue Sep 30 22:00:26 1997
From: Jim Cash <>
Date: Tue, 30 Sep 1997 14:35:30 -0400
Subject: Re - E28 Door lock module

Domenic Mirto asks:

> Has anyone documented, (traced), the schematic for the door lock control
> module in their e28...??
> I have an '84 533i and I think the module is KAPUT...
> The module is housed in a rectangular black plastic box located in the
> right front door pillar...

I think I can add some detail to the excellent inforamtion provided by Bob Van Epps.
Some of this has been printed before, and some information is gather from others like
Andres Yver.

FIRST you should be aware of the "CAUTION" notes that are on the drawings:

  • Disconnect Battery before moving or removing the locking module. If power is applied to the control unit when it is not properly positioned the unit will be damaged. ( I think this is due to the "inertia sensor switch" which senses and impact or roll over)
  • Do not operate the system more than 8 times in quick sussession to prevent overheating of the control unit.

Here is how the 1982, and I assume other year's, systems function.

The key locks are mechanical and are independent of the electric locks. But they are physically linked at each door and the trunk so that the actual lock mechanism is controlled by the combination of the key & electrical actuator linked together. When you turn a key you are not only locking/unlocking that door physically, but also moving the actuator for that door - which then sends a lock/unlock "request" to the control unit. The control unit then sends the appropriate lock/unlock "signal" to all the actuators (in parallel), which use a solenoid to produce the same movement in the locking mechanism as does the physical key lock.

Each actuator is actually a double unit.

  • - the internal "request" contacts that send ground to the control

    unit on the "lock request" and "unlock request" leads.

  • - the solenoid (motor) which actually moves the locks electrically.

When all actuators are in the same position (locked or unlocked) the system is at rest. As soon as one actuator is moved (i.e. moving the solenoid from unlocked to locked position) now a ground appears on the lock request lead to the control unit - which reacts and sends a "lock signal" to all actuators. If they all respond and go to their locked position, then their associated request switches are now grounding their "lock request" leads. Since the system is already locked, and since there are no grounds on the unlock leads - all's well.

But if one of the actuators fails to operate, or if one of the internal switches has failed, then you immediately get a ground on the opposite lead. Depending on the nature of the fault it might affect only the locking, or the unlocking operation.(i.e. a switch shorted in the unlock position will allow all the locks to unlock, but will not allow them to lock.) If this is intermittent due to a slow acting actuator, or corrosion in the switch, then you will get both situations, at times.

Contacts & Leads each door actuator are as follows:

  1. - BU - solenoid motor - LOCK SIGNAL from control unit
  2. - WT - solenoid motor - UNLOCK " " " "
  3. - YL/BU - Lock request switch - sends ground to control contact #7
  4. - GN/BU - Unlock request switch - " " " " " #6
  5. - RD/BK - shown "unused" on mine but I think it goes to alarm circuit.
  6. - BR - request switch - ground connection.

The trunk is the same except that the function of contacts 3 & 4 are reversed (this might be a typo).

The drawing shows the lock request leads from the doors, and the trunk connecting together inside the control unit. While this makes sense functionally it does not account for why you can sometimes use the trunk unit when the doors will not work. I suspect that the drawings not completely accurate inside the control unit.

Contacts and Leads on the Control Module are as follows:

Connector Pin - Wire colour - Lead Function

  1. - .75 BU - "LOCK" signal to all 6 lock actuators (includes fuel door).
  2. - .75 WT - "UNLOCK" signal to all 6 lock actuators. The relay feeding these just reverses the battery and ground to the actuators dependingon whether they should lock or unlock. (current sensing will cutout if an actuator runs for more than 35 seconds).
  3. - 1.5 RD/WT - Power - hot at all times.
  4. - .75 BR - ground
  5. - .75 GN - Power to inerita switch (closes at impact of 5 G) to unlock car)
  6. - .75 GN/BU - UNLOCK REQUEST from front doors (actuators).
  7. - .75 YL/BU - LOCK REQUEST " " " "
  8. - .75 GN/BK - LOCK REQUEST from trunk actuator.
  9. - .75 YL/BK - UNLOCK REQUEST " " "
  10. - .75 RD/BU - DS Lead (may not be used) For security system.
  11. - .75 RD/YL - DWA Lead (" " " " ) Security Siren

The control unit also has an internal "automatic unlock" switch that initiates an unlock request should it sense an "impact" greater than 5g. This is what unlocks your doors in an accident. I remember hearing that it also reacts to a roll over - never tried it. That switch is powered in "Run" and "Start" positions on contact #5. It is not powered when car is off so that you can not unlock a car by running into its bumper (impact) - yes some of the older ones work this way.

If you are having intermittant problems with your locks, here are some things to

Fuse # 5 - make sure it is tight and not corroded.

If you can not lock/unlock it from the drivers door, try the passenger door. If that works then I would suspect the actuator in the driver's door. Both these actuators are wired in parallel.

If neither of those work, then try the trunk unit - it connects to the Central locking unit on different contacts (8 & 9). If that works then I would suspect the actuator in the passenger door, or it could still be the drivers door. A clue could also be do you remember one of these operating more slowely than the others. In cold weather my locks would sometimes quit, but I could always get in using the trunk unit.

The actuators are held together with pop rivits. If it turns out that you think it is an actuator (if you unplug one and the problem does not occur - i.e. all the others work) then rather than buying a new one I would attempt to take it apart.

Although I no longer have on to verify my assumptions I would do this.

  • Drill out the rivits and open the unit.
  • clean the contacts and ensure they can operate freely.
  • spray inside with WD40 and reassemble. (use some small screws & nuts etc to replace the rivits - sometimes sheet metal screws will work as well.
  • check that you have electrical contact, or plug in and test before putting door back together.

In case you are interested, I found it very useful to have a toggle switch on the console (just below and to the right of the shifter) to operate the locks. I just drilled through the little plate to the right of the automatic shift - it pops out) and mounted a double throw toggle switch. I connected the centre pole to ground and the other 2 leads were connected to the lock & unlock request leads that go to the control unit. You can splice into them up under the dash a various places - or you can go right to the control unit.
Suggest you mount the switch so that pulling it back causes a lock request. That way you can get in the habit of locking the car as you shift into gear, and unlocking as you shift to Park.

Hope all this helps.


Jim Cash
London Ontario, Canada
E39 540ia

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