One for design techs.

[Dave Plowman (News]

I've been asked to repair the central locking controller on an early Rover SD1.

Bit of history. That car was pretty early in the UK in having electric central locking. It uses solenoids, rather than motors which are the norm today. Later SD1 changed to motors.

The solenoid units ain't Lucas. Not quite sure who makes them.

The control unit is pure Lucas, and very odd.

What the control unit does is provide a pulse about a couple of seconds long to operate the solenoids, then back to zero. Even when the switch is still made. Same as all, really.

The Lucas one uses two massive Lucas starter motor relays with 1/4" screw termimals - rated at about 300 amps. And massive caps and a resistor box to time them. The fuse for the entire circuit 20 amps.

I realise the early 70s was a bit early for electronic timers (although the car has points-less electronic ignition) - but why not use the smaller and cheaper horn type relays rated at 30 amps? They have been around since the 50s and likely earlier.

It's not a problem repairing the thing as all bits easily available. Just curious about the design.

[Harry Bloomfield, Esq.]

on 04/01/2021, Dave Plowman (News) supposed :

I have seen large capacitors used as the power source for solenoids on one car. Slow charging via a resistor, then fast discharge via the solenoid when triggered.

[Dave Plowman (News]

Indeed - what I am curious about is the use of massive relays rated at hundreds of amps to switch under 20. Obviously their coils have a much lower resistance than a 30 amp one which would typically be about 150 ohms. So need much larger caps too.

[Jeff Layman]

Maybe Lucas had surplus in stock and decided to "second-purpose" them.

I'm more interested in the large capacitors. I can't think of a more hostile environment for a capacitor than a car engine compartment, particularly if it is an electrolytic to get that long pulse. Or was the control unit in a cooler place?

[Dave Plowman (News]

It's situated inside the rear wing, so temperature depends on the weather.

I doubt you'd use surplus stock on a design hoped to sell many thousands?

[Brian Gaff (Sofa]

This is what happens whe a designer has no idea on how to do something and so decides to try the first thing that comes into their head with no thought to failure modes as the device ages, sticky solenoids, broken springs and any detection of whether the job was done as that will cost them the cost of another bit of wire.

Brian