Re: [DML] Door Lock Module - some technical musings...

[Martin Gutkowski <webmaster@xxxx>]

Hi John

It seems to me that you were working with a working module :-) I had the benefit of
Paul's broken one to play with which resulted in my finding this design oversight of "the
missing diode" causing the transistor on one side to blow as soon as the arcing in the
relay got bad enough. It blew my first replacement transistor on the first go. Without
the addition of the diode to that side of the circuit, this will keep happenning. With
the addition of a further two diodes, you can protect against the other possible source
of popping those all-important trannies.

By replacing the capacitors, working or otherwise, you are safeguarding yourself against
the "phantom battery drain" which afflicts a number of cars (NB you CAN'T test the
current drain of the module by plonking an ammeter between the circuit breaker and the
big red wire - this is purely the supply to the solenoids, and not to the board).

I cannot see how the linkage in the lock mechanism could electrically cause the module to
continually power up the solenoids - this only happens due to the relays welding
themselves shut. You can simulate this by un plugging the module, and locking one door,
with the other unlocked. Then plug the module in and take a voltage across both
capacitors - neither will show a reading greater than a couple of volts. Without a
voltage in these capacitors, neither relay can engage and neither solenoid can fire.

I do not expect my circuit breaker to ever have to work now. I have seen some photos of
your replacement relays, but IMO having open-to-the-air units that size for this
application is asking for trouble. The units I'm using are these:

http://www.maplin.co.uk/products/details.php?cartid=0112301242421456778&moduleno=25603&manufacturer=Siemens

All the best

Martin
#1458