Hazard Warning with DB10 relay

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When I visited the Harrogate MG Club event at Pateley Bridge in August '18, I met up with Paul Batho, our Magnette Register chairman. During our chat, I showed him the hazard warning system I had installed in my '57 ZB Magnette. Paul was interested and suggested that I should write a technical piece on how it works. So here goes.

Direction indicators on early Magnettes were Semaphore arms also known as Trafficators. Later cars were fitted with a device named the DB10 Relay, which enabled the rear side-lamps to also function as direction indicators. The twin filament rear lights already doubled as brake lights so this gave them a third function. At the front, either the side lights were modified to also take twin filament bulbs or separate indicator lamps were fitted and the DB10 relay contrived to control brake lights and flashing indicators front and rear. Many early Magnettes were retro-fitted with the DB10.

The triple function of the rear lamps (all red of course) is seen as confusing for today's drivers and consequently many Magnette owners, myself included, have fitted separate indicator lamps with amber lenses which are more familiar to modern drivers.

I decided that I should fit hazard warning lights after a rather un-nerving incident when my Magnette became immobile at dusk on a busy two lane dual carriageway, meaning that I had to adjust the points whilst standing near the middle of the road. I didn't like the switches used in the commercially available solutions so I set to work out how to do it myself.
The internals of the DB10 were a mystery to me and I suspect many other owners but I found help in the UK and USA Magnette web pages https://www.mgexp.com/phorum/list.php?27 and http://www.magnette.org. This enabled me to work out the circuit shown below which would enable all four indicators (and the two side repeaters I also fitted) to flash simultaneously.
My first move was to source a more understated switch which, I think, does not look out of place on the walnut dashboard of the Magnette and I was away!

To get all four indicator lamps flashing at the same time, I have used a second flasher-unit coupled with a 3-pole changeover relay activated by the hazard switch. It took some careful thinking to work out how to achieve this with a single 3-pole double throw relay, but I got there in the end!

The function of the 3-pole relay is to power both left and right coils of the DB10 simultaneously (DB10- 4 and 8) so all four indicator lights operate together, whilst at the same time disconnecting the indicator flasher and connecting the hazard flasher in its place to the input (DB10- 1). This sends the pulses from the hazard flasher to all four direction indicator lamps via DB10 terminals 2 and 3 (LH) and 6 and 7 (RH).

Since the rear lights are no longer going to flash, the connection from the brake light switch to DB10-5 is removed and connected directly to the rear lights as in earlier cars without flashing indicators

In order that the hazard warning system should operate with the ignition system switched off, it is powered from A1 on the fuse unit. However, the indicator circuit is powered from A4 so that the indicators only work if the ignition is switched on. This leads to an interesting situation where, if the indicator switch is operated while the hazard warning system is on, the ignition circuit is activated even though it is switched off! There are several ways to circumvent this problem. An additional set of relay contacts or a pair of diodes might do the trick, but it seems much simpler to just not operate the indicator switch when the hazard warning system is in use!
The circuit is shown in the diagram below.

To reduce complexity, most of the original connections to the DB10 are omitted from the above diagram apart from the indicator switch outputs which are shown connected at terminals 4 and 8. A separate diagram below shows all the original connections. These original wires are left in place with two exceptions, the first being the stop lamp connection at terminal No 5. Since the stop lamp filaments now have only a single function, they can be directly connected to the brake light switch and do not need to go through the DB10 - this leaves terminal 5 unconnected.

The second exception is the output from the indicator flasher-unit which is removed from terminal 1 on DB10 and connected instead to the changeover relay at the normally closed (NC) terminal 3. The output from the hazard flasher unit is connected to normally open (NO), terminal 6 of the relay. Terminal 1 on DB10 is connected to common terminal 9 of the relay. In this way the relay can switch the indicator input between the two flasher units.

All this may seem rather complex and confusing, but it really isn't too difficult - its just a matter of connecting the wires to the right terminals! To aid in this, the photo below shows the underside of the 3-pole relay and the hazard flasher with all the terminals numbered and the wires colour coded as in the circuit diagram.

As part of this modification, I also converted all the subsidiary lighting to LEDs. This greatly reduced the current drawn but, consequently, I had to use LED-compatible flasher-units. If you do not intend to use LEDs, then standard 3-terminal flashers can be used but the choice of hazard relay will be determined by the expected current load.

I have been very pleased with the LED lights especially the front side lights which give a very bright white light for the side lights but which flash orange when indicating.

For convenience during my initial trials, I mounted the hazard relay and flasher-unit together on the inner wing next to the DB10 relay box, but I suggest it would be better to locate them somewhere under the dash as I have had issues with dampness!

 

COMPONENT LIST

Of course, you may wish to choose your own components, but I have listed below the ones I chose. I have checked the availability and all three items were still listed as of 31st August 2018

3-Pole Relay: Schneider Electric/Telemechanique 3PDT Plug in Non-Latching Relay RXM3AB1, 12v Coil, 10Amp. This was available on eBay as item 322656280102 at £7.99. Seller was: Basic Micro UK. David Russell, Unit 1 Park Lane Business Park, Kirkby in Ashfield, Nottinghamshire, NG17 9GW. Also Available from RS Components and Farnell.
LED Flasher:  12v electronic flasher relay LED compatible. This was available on eBay in both positive and negative earth versions from LED_bulbs_for_classic_cars at £11.99. NB Remember to specify 3-pin version in the appropriate polarity for your car. There are many other flashers that claim to be insensitive to polarity, but this was the only one I found that worked on positive earth. More choice for -ve earth cars!
Hazard Switch:  Available from several Chinese sellers on eBay. Mine came from mandyxiaoxiao at £2.35 as item 111798712677.