75ZT Community

There is no doubt that speed locking is controversial in the 75/ZT cars. Some people like it, some hate it... and others like it in principle but would rather the central locking unlocked automatically when the ignition was switched off as is seen in Range Rover etc.

There has been some interest in setting up auto unlock elsewhere recently and a couple of people have asked me if it could be done. I'm sure this isn't the only solution but it's what I did and it works. The only caveat being that the centre console locking switch has to be the rocker switch type as fitted to the earlier cars. I'm not saying it couldn't be modified for the later type of switch, just that I haven't given it any thought as VeeKay is "old skool"

My design was for the locks to automatically unlock when the ignition switch was set to "Accessories" position. This has the effect of allowing you to switch off the engine without unlocking the doors if the situation warrants. Once the ignition switch is switched fully off, the doors will unlock.

The mod is easy to do and requires :-

2 x SPDT relays (for example, the sort used for adding electrical accessories like fog lamp kits etc.)
1 x Resistor (I used a 5 watt 150 Ohm)
1 x Capacitor (I used a 2200uF 40V electrolytic.)

You will also need access to:-

A 12 Volt permanent supply.
A 12 volt supply controlled by the "Accessories" position on the ignition switch (I used a supply to the Head unit)
The unlock wire from the centre console switch (either spliced in at the BCU (Body Control Unit) C0660-17 OR at the centre console switch if you prefer)
A good earth point.

The diagram below shows the theoretical circuit and its operation is perfectly straightforward. In practical terms I used a small plastic project box to contain the components. I ended up with a little plastic box with four wires coming out via a grommet. Mine is situated behind the glove-box but position is not of the essence.

Notes:-

Using the component values provided, RLB operates for a period of roughly 0.5 seconds, ample time to switch the BCU.

PLEASE do not omit the resistor R1. A capacitor initially presents as a short circuit to a DC voltage and the resistor allows for some initial resistance and prevents possible arcing and sparking.

The capacitor value was just one I had in my box of bits. By all means have a try with a different value of capacitance but please ensure the working voltage is at least 20 volts. Exploding electrolytic capacitors are not nice and will make a right old mess

This little mod draws NO quiescent current and will not drain your battery.

This works for me. Try it on your own car at your own risk

Please disconnect the battery before you start making connections to the car's wiring.

My sincere thanks go to Duncan and David for allowing me to bounce ideas around - thanks guys

Glad you've got it working! Neat little mod.

I made one up for a friend last night and using some Veroboard and PCB (Printed Circuit Board) mounted relays was able to build the whole thing into a box the size of a cigarette packet. The biggest thing to find space for was the electrolytic.

This arrangement can be used with the later type of switch but it has limitations.

The mk2 switch is a state changer. When pressed it changes from locked to unlocked and vice versa. So, if the doors are locked when you switch the ignition off it will unlock them. However, if they are already unlocked, switching the ignition off will lock them. Up to you

The new Ultimate Remote addresses this problem in that it will recognise states so the potential problem will not occur.

Has anybody actually tried this... except me?

Not yet

The best mod that nobody ever bothered doing

Our Rexton unlocks when the ignition goes off, which is good, but then it doesn't lock until you hit 30mph, which is less so.....