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MG MGB Technical - Toggle switch for hazard lights

I have fabricated a GRP dash for a 1969 MGB and use toggle switches instead of the awful plastic rockers. I had to use a standard DPDT which require diodes to facilitate the hazard lights. Does anybody have a source for a toggle switch that will function like the original rocker switch.

Joe G
J. E. Guthrie

A DPDT switch does!

Connect common (centre) terminals to the L & R light circuits.

Connect the up position terminals to the circuit to the indicator switch.

Bridge the down position terminals and connect to hazard flasher unit.

Then in the up position the lights are connected as if the switch was not there and the hazard flasher is isolated.

In the down position the lights are connected to each other and to the hazard flasher unit and the indicator switch is isolated.



Chris at Octarine Services

Just a reminder that it *is* essential to isolate the indicator switch when the hazards are on, or you can get voltage feeding back to the ignition and fuel pump even when the keys are in your hand.
PaulH Solihull

Thanks guys, but the DPDT is working very well. I have isolated the two circuits by only using two source lugs. I also have used diodes on the light circuits for the hazards because there is only one lug for each light and the dash light. UP turns on the hazard light circuit and turns off the turn signal circuit. DOWN turns on the turn signal circuit and turns off the hazard circuit. I would like a toggle that would work like the original rocker switch without the requirement for the diodes.

Joe G
J. E. Guthrie

Here is a sketch of DPDT wiring and what I would like to do.

Joe G


J. E. Guthrie

Hi Joe,

This is the circuit that Chris suggested.

Herb


Herb Adler

Herb's diagram is correct but doesn't show the hazards tell-tale that was sometimes provided and is shown in Joe's diagrams. You can't simply connect the light-green/purple to the hazard flasher light-green/brown as it will be drawing current all the time, even when parked, and may be enough to start the flasher going anyway. You *may* be able to implement it by wiring the bulb across the hazard flasher, where it would flash in anti-phase to the corners of the car (rather than in-phase with them as normally which seems of no consequence).

UK cars didn't have a hazard tell-tale as far as I can tell, maybe it is an American requirement. The factory schematics for North American cars for 68 to 72 show a 3-pin hazard flasher unit (not to be confused with the early 3-pin indicator unit used on all cars from 62 to 67), with the third pin driving the additional tell-tale lamp for the 68 to 71 years. For 72 and later the tell-tale is connected to the sixth pin of the hazard switch - unused in UK cars. That 3-pin flasher would work as normal with the DPDT switch mentioned/shown by Chris and Herb, and there do seem to be a number of references in Google to this type of hazard flasher unit being used on British cars of the era e.g. http://www.rimmerbros.co.uk/Item--i-C28520. It needs to be capable of driving at least four 21w lamps, and
may be marked 'heavy duty'.

But why one needs an additional tell-tale is beyond me, you will already have the green tell-tales for left and right on the dashboard flashing regardless of whether you are using hazards (both together) or indicators (just one, of course).
PaulH Solihull

Paul,

You are right the tell-tale is redundant and I think I will eliminate it. I would still like to find a toggle that will connect a source to a load(UP)and a another source to another load (DOWN). It would also need two load connections for one source to eliminate the diodes. It is not a major problem if such a switch does not exist since using the diodes works well. It is just a more fussy wiring condition behind the dash. I probably could mount the diodes on the firewall which would simplify the wiring art the switch.

Thanks guys
Joe G
J. E. Guthrie

I doubt you will find a switch to do that other than a purpose-made hazards switch. You would be able to do it with a standard DPDT switch plus a relay with *dual* normally open contacts, *if* it has two terminals labelled 87 which are isolated when the relay is released. Some relay schematics show a single moving contact with two normally open contacts (terminal 87) linked together which would be no good. Others show two moving contacts but a *single* 87 terminal which would also be no good, even though both types are described as dual normally-open.
PaulH Solihull

Joe,

You are switching in the wrong place with your set up - the standard hazard switch (which is only a DPDT anyway) switches on the lights circuits AFTER the flasher units.

The circuit I suggested and Herb drew out will work as the original switch did.

If you want a hazard tell tale light then just wire it across the hazard flasher unit.
Chris at Octarine Services

The standard hazard switch is *not* a DPDT. It has six terminals it is true, but two entirely separate circuits - one a simple on/off and the other a 4-way on/off. In the 'hazards off' position the two terminals for the green wires for the indicators (the simple on/off circuit) are connected together and the other four (the other circuit) are all isolated from everything else. In the 'on' position the two terminals for the green wires are isolated and the other four are all connected together. Those four are *never* connected to the other two.

The standard hazard switch doesn't make and break the lighting circuits, they are permanently connected to the indicator switch. In the on position it joins the two sides together and connects the output of the hazard flasher to them as well as to the tell-tale.

The circuit you quite rightly said can be implemented with a standard DPDT switch involves interrupting the wires from the indicator switch to the lights, i.e. the wires to the lights in that case *are* switched, either to the indicator switch (hazards off) or the hazard flasher unit as well as being connected together (hazards on).
PaulH Solihull

I am not sure what the original problem was, but I also tried to fabricate a replacement switch when the hazard flasher switch on my '72 MGB failed when one of the terminals broke off. I just could not find anything with a similar function. Finally I got a friend of mine to soder the piece back on the switch and it has been working fine since then.

I noticed a month or so ago that Moss is now carrying a replacement hazard warning switch for my '72B. They also have a replacement switch for your '69B on page 93, item #49 part # 140-540 for $31.95.

If you want to try to find an inexpensive replacement, good luck. If you just want a new switch, do like I did and order one from Moss while they are available.

glq Greg

This thread was discussed between 04/10/2010 and 15/10/2010

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