Fitting a lights left on alarm buzzer

To my mind its surprising that a luxury car from 1984 wasn't fitted with a lights-still-on alarm system at the factory. It wasn't particularly high-tec and other cars of the period were already so-equipped. However, my Excel doesn't have a warning buzzer and I've already left the lights on several times and run the battery down once- even in the 300 miles or so since it was recomissioned! I have changed the panel-lights, sidelights, number plate, interior and door-edge lights for LEDs which should help preserve the battery, and leaving the headlights on isn't really a problem as the pop-up action is an obvious reminder, but even so leaving the side-lights on is a bit of a bind.

I decided to fit such an alarm. There are several ways to do this- the neatest is to pick up the relevant feeds behind the dash and at the rear of the lighting switch and route them to a new lights warning relay mounted somewhere out of sight behind the dash... Something like the RLYBUZ from Car Builder Solutions. A cheaper alternative is to use a simple buzzer (obtainable from eBay for a couple of quid) to let you know if you've turned the ignition off but left the lights on. Again there is a neat and a quick and dirty way* (*actually 2 ways at least) of wiring this in and it isn't as good as the real above.

However... I am strapped for both cash and time at present and I don't want to strip out the dash again- in fact I don't want to have to dismantle anything if I can help it, so the fitting needs to be quick and simple- so dirty ways it is!

I bought this buzzer from eBay- there are a lot of these out there and I think they are all pretty good. I have been warned that some can be drowned out by the sound of a gnat farting in a hedgerow, but both types I bought were pretty loud enough. Anyway check the buzzer function and volume before fitting. This is a simple buzzer-only kit and its wired in simply as well- I tried it first, but to cut a long story short it wasn't completely successful, so scroll down for the system I eventually fitted.

This is the cheap buzzer I bought- its pretty loud and even comes with a sticky pad on the back for fitting

These buzzers draw a very low current and being made to a price, have been fitted with the cheapest and thinnest wire possible. Its far too thin to handle conveniently so I soldered some "normal" wire on so I could make the connections conveniently.

Soldered joints to normal wire- covered in heat shrink

The buzzer will sound only when there is a voltage difference across the terminals and a current can therefore flow. If both terminals are at 0V or at 12V then there is no such voltage difference, no current and so no sound. The idea is then to connect the positive lead of the buzzer to anything that's live when the lights are on, and the negative lead to anything that is live when the ignition is on but earthed when the ignition is off. Again I am sure that really clever connections could be made behind the dash but the dirty ways of making these connections is firstly to connect by wrapping wires around the relevant fuses (Noooo!!!! even I baulk at that!) or as a second choice, use one of the light feeds themselves. I opted for that choice and picked up my positive connection from the sidelight power feed in the right headlamp pod. Make temporary connections just to be sure it works before going ahead.

My headlights are out at the moment as I'm still midway through replacing the bumper, so access here was easy. As the sidelights have been removed I had a loose bullet connector (red/orange wire) which I could use so I fitted a bullet connector to the end of the thicker positive wire from the buzzer...

Bullet connector (Lucas) attached to the buzzer positive feed.

 I will use a double bullet connector to join this wire to the sidelight feed.

Working from inside the car I threaded a section of curtain wire through the bulkhead grommet and into the engine bay, pulling the wire through. I then routed this along the RHS of the engine bay and attached it to the existing run of the loom along the side bulkhead.


Before passing beside the radiator and into the headlamp pod via a hole drilled in the plywood crash panel.

Wire routed through plywood crash panel

Double Lucas bullet connector for sidelight connection.

At first I found a ignition controlled live/earth by trial and error with a voltmeter. I found all the green wires entering the two multi-connectors just above the fuse box would be suitable.

Buzzer connected. Note the interruption block in the green ignition fed wire at the top which feeds power to the buzzer via a blue wire

However, connecting the buzzer in this way wasn't satisfactory. The system did work in that the buzzer sounded when lights were on and ign off, but there was a lot of "leak-through" interference. The buzzer sounded during normal driving- very faintly and in pitch with engine speed and this was clearly not acceptable. I tried moving the switched live/earth connection to the red switched power feeding the radio, and which already had a spare bullet connector vacant (see "Why am I going through batteries"). Sadly this made no difference. Its possible that a suppressor capacitor might have helped, but a better solution  is to mimic a standard lights left on relay and the wiring for this is available on the web. Using a standard 12V SPDT (normally closed) relay its fairly straightforward. The relay with base and pre-fixed wiring is available on Ebay for £2.50, a dedicated integrated relay and buzzer is around £15!

Relay and base unit (pre-wired)

This is the base of the relay (wiring base removed) 

Terminals 86/85 energise the internal solenoid.

Terminal 30 is connected to  87a when the relay is not powered, but switches to 87 when the relay operates.  

Terminal 87 is used as a parking position and will not be connected to anything.  

Wiring was obtained from the web, taken from here- 

For operation  Pin 85 is connected to switched live/earth and pin 86 to earth so that the relay will operate whenever the ignition is on. Pin 30 is connected to the lighting circuit (lights on power) via the lead wire I had already installed from my sidelights. The live buzzer connections are then made  to terminal 87a and the negative buzzer fed to earth by piggybacking onto pin 85. 

 When the ignition is on, pin 87a is disconnected and parked to 87. Power entering at pin 30 from the lighting circuit can go no farther, so lights on, ign on- no buzzer. Should the ignition be turned off pin 87a is switched back into power supply from pin 30, and if the lights are still on, then the buzzer will sound until the lights are turned off and power supply to terminal 30 is interrupted.

In the base received with the relay pins 87 and 30 are fed by green wires, pin 86 by white and pin 85 by yellow. I wired the buzzer between 87 (green, buzzer +ve) and 86 (white, earth, buzzer -ve). I soldered all connections and covered them with heat shrink tubing and zip tied the wires and buzzer to the relay  to avoid stressing the connections.

I attached terminals compatible with those already in place and fitted the assembly into the car. I used the radio switched live as the live feed . There was a handy hole already in one of the under-dash panels that I could use to mount the assembly.

Overall this was much more successful. The wiring is a bit chaotic since I have already fitted the fan bypass switch. I'm forced to use non-standard colours as I don't have the appropriate cable colours and  pre-wired components are joined to my wires often resulting in a colour change...nightmare! However I defaulted to red for anything that can supply power and black for any earths. I will make and fix some labels to these wires as I can't guarantee I will remember what is what in a few months time!

However, overall dead pleased and I will make up a similar component for my MGB.