Door Lock Module - some technical musings...

[Martin Gutkowski <webmaster@xxxx>]

Hi All,

I've spent some of my quiet time this Christmas looking at the DeLorean
door lock module. Having spent time previously examining Dave Swingle's
pain-stakingly documented layout diagram and circuit diagram on the
DMCnews tech page, I've discovered some things and re-engineered two
modules already for reliability and longevity (for at least the next 20
years :-)

This was prompted by Paul Salsbury complaining that his didn't work at
all, and sending it to me to fix.

Firstly, referring to the circuit diagram here

http://www.dmcnews.com/Techsection/graphics/lockcircR1.jpg

some errors:

1) Transistors Q1 and Q11 are in fact BFR39 and NOT BRF39 as stated.
BFR39's are pretty common and still available. I got ten on sample from
www.diamondelec.co.uk

2) R1 and R11 are in fact in series between D2 and the emitter of Q1
(and likewise between D12 and the emitter of Q11). C1 and C11 connect
directly between ground and Q1 and Q11's emitters respectively.

What do we know about the module and it's problems?

1) The relays can stick shut energising the solenoid until it burns out
or the curcuit breaker trips (if present) or the battery flattenns.

2) Referring to Nathan's notes on the module, the transistors are
vulnerable to spikey voltages resulting from intermittent earthing
during the lock or unlock cycle. He also notes that he'd put a diode in
series between point A and output pin 2.

1: The original relays are open to the air and have a pathetically small
contactor. This is a recipie for sparking casing gumming up of the
contacts. Regular cleaning with a switch lubricant like servisol10 is
likely to prevent the contacts sticking, though it's an underrated
device. (A small fact which may be of interested to some is that the
explosion aboard Apollo13 was traced back to the use of an underrated
relay getting stuck closed - you have to read the book to get the
detail!). I found a nice 10A miniature relay made by Siemens available
from Maplin for £1.49. It's almost the exact same size as the stock
relays.

2: When I tested Paul's module, I discovered that one "side" of the
circuit was functioning correctly, but the other was failing to charge
its capacitor (the one that energises the relay). The likely culprit was
the transistor Q1 so I replaced it (I also replaced the two capacitors
on principle. On re-trying the module, I found no change. This was a
puzzle because there's not a helluva lot else on the board that can go
wrong! Remember this board is actually two identical circuits in
parallel - one performs the "lock" function, the other the "un-lock"
function. Paul's would unlock but not lock. It was after a lot of
head-scratching that I noticed something strange... D5 has no "partner"
on the "lock" side of the circuit. Even stranger, my own working module
had D5 mounted by hand to the underside of the board, with the track
cut. Pauls was evidently a later board with a position for D5. NEITHER
board had a place for the same diode on the lock circuit. The lock relay
on Paul's board was sparking badly when manually closed it, so I
concluded that Q1 was being blown due to a spike on the base. I cut the
track from pin 2 to point A and added in a diode (marked "D" on the
diagram). I also added the protection diodes from point A and B to the
12v supply. Hey presto the module works fine, with a lovely blue flash
ever time the lock relay is triggered. So I removed the stock relays and
replaced them with the Siemens ones. It required some creative
additional wiring, but you can use the holes for the original relays to
feed to the underside of the board. It's very easy to solder direct to
the tracks on the underside as it was never coated.