Dear Shatz,

May this find you well. We're conditioned to think of electricity as being "on" or "off". Most of the time, that's right. However, we often find failures evidenced through "intermittent" operation.

I've never opened up the central locking system relay. It could consist of more than one relay, as, I believe is the case with the fuel injection relay. Inside the 940 power seat computer, there are half a dozen "slave" relays, controlled by a "master relay". If the "master relay" fails, none of the "slave" relays work. The reverse is not true.

Relays are electrically-actuated switches. Like all switches a relay can fail as a result of the contacts wearing out. When the relay opens and closes, electricity briefly jumps between the contacts. That arcing will erode the contact material. Eventually, all of the contact material is burned up, and the relay fails. Most relays - and switches - have springs in them. Spring failure - typically a weakening of the spring - also causes relay failure.

I suspect that when the driver's door key is turned, that lock operates directly and sends a signal to the locking system relay to unlock the other doors. If the locking system relay is weak - failing - it may not work at all, only intermittently, or it may make only partial contact. Some of the power may be dissipated in the form of a spark, between the relay contacts. As a result, the relay does not allow enough power to pass to the other door lock "motors", the solenoids that open the locks. Those locks will not open.

I am pretty sure that if the locking system relay is replaced, the system will work normally. This relay can be taken from a car at a salvage yard, for little money.

Hope this helps.

Yours faithfully,

spook