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Any good auto electricians on here?

Posted: Sun Feb 17, 2019 11:04 am
by roymck
I like vehicle wiring and consider myself quite good at it and I also find it very therapeutic but I just can’t get my head round this problem . I need to change the flasher unit on my Coupe for a electronic type with a separate trailer warning light . I think I have found one that would work but a couple of questions please .
On a Coupe when you indicate do both left and right warning lamps illuminate together or do they work handed ?
And on the following diagram for a Coupe you will see the warning lamps , how do they illuminate as the don’t seem to be earthed when you indicate ?
CFB97553-C403-4F3F-8E86-C6AF7E96A97C.jpeg
CFB97553-C403-4F3F-8E86-C6AF7E96A97C.jpeg (247.52 KiB) Viewed 1662 times
On the following SE5 flasher circuit you can see it uses a different flasher unit and the lamps work in conjunction with the indicator lamps .
0976F447-19D9-4149-A62F-221DB4BD1F6F.jpeg
0976F447-19D9-4149-A62F-221DB4BD1F6F.jpeg (235.36 KiB) Viewed 1662 times
Now I could alter my Coupe by doing away with the LGP wires from the warning lights and earthing them direct so changing the indicator wiring to SE5 style , I could but it would help if I could under stand how the Coupe warning lights earth .
The electronic flasher unit I am thinking of using is Vehicle Wiring Products . It has 4 terminals
C2 Trailer Warning light
49 Supply
49a Indicator switch
31 Earth
However they do a 5 pin flasher that has additional terminal C and this operates the Vehicle warning light, but don’t think this will work as the Scimitar has 2 warning light for L and R indicators , hence my thought on converting to a SE5 style . Brings me back to how does a Coupe warning light earth?
Thanks Roy

Any good auto electricians on here?

Posted: Sun Feb 17, 2019 11:24 am
by Cerberus
Is the body of the flasher unit earthed?

All 3 pin flashers I've seen always send a power feed down that third terminal. It must be earthing back up through the indicator bulbs or something, how that doesn't make them light up I'm not sure yet.


On cars with a single indicator warning light, you get a power wire from each side of the indicator switch going to the bulb, and when you indicate it earths back down the other one. Makes it difficult to fit an audible warning buzzer as the polarity changes when indicating left or right. It must be a similar kind of thing just with two bulbs.

Any good auto electricians on here?

Posted: Sun Feb 17, 2019 11:38 am
by roymck
No the flasher unit was not earthed just held on the bulkhead with a self tapper . If it needed earthing then I am sure it would be shown on the diagram and it’s not .

Any good auto electricians on here?

Posted: Sun Feb 17, 2019 12:03 pm
by scimjim
From the diagram, the LGP must be earth - which is most strange!

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Posted: Sun Feb 17, 2019 12:06 pm
by scimjim
Or, it flashes on the opposite cycle to the indicators and the earth path is back through the light units when the flasher is de-energised?

Any good auto electricians on here?

Posted: Sun Feb 17, 2019 12:12 pm
by AJL Electronics
The earth path is through the cold indicator bulb on the other side, which is how one lamp is able to be used. The current of the WL is far lower than the 21W so it behaves like a shunt to deck.

Any good auto electricians on here?

Posted: Sun Feb 17, 2019 12:25 pm
by David Tew
roymck wrote:
Sun Feb 17, 2019 11:04 am
On a Coupe when you indicate do both left and right warning lamps illuminate together or do they work handed ?
I'm not good nor am I an auto-electrician, however I think it depends on whether it's a 4a or a 4b. Mine is a 4b and has a 2-pin flasher unit. it's wired exactly the same as the 5 diagram you show and the two indicator lights work independently.

On a 3-wire flasher there's a power-in, a flash-out and an indicator-out. The indicator-out supplies the warning light(s) which is/are earthed. If the car has two warning lights they'd operate together. Something like this:
3-wire flasher.png
3-wire flasher.png (34.32 KiB) Viewed 1618 times
If the 3-wire flasher-unit is wired as shown in your diagram, the low-wattage warning lights will earth through the other warning lamp but I can't really see any advantage in doing it that way. :-?

On a 2-wire unit you can't have a single warning light without the use of some blocking diodes.

From a safety angle, I think the 2-wire flash-out operates as soon as power is supplied whereas the 3-wire flash-out operates once the bi-metallic unit has warmed-up. The former is safer which may explain the change.

I'd rewire as you suggest. :wink:

Any good auto electricians on here?

Posted: Sun Feb 17, 2019 12:31 pm
by scimjim
The coupe “pilot lamp” isn’t earthed and they’re L & R and work independently, so it doesn’t work like the picture above.

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Posted: Sun Feb 17, 2019 12:45 pm
by David Tew
scimjim wrote:
Sun Feb 17, 2019 12:31 pm
The coupe “pilot lamp” isn’t earthed, so it doesn’t work like the picture above.
... and that's the mystery. I only added the picture as an example of the 'norm'. :wink:

However, having had another ponder I think the wiring is an ingenious way to get two warning lights from a single output. Both warning lights will have a 12v feed on one side but one will be earthed through the indicator lamps on the other side and will light while the other will have 12v on both sides so won't operate.

(EDIT - as Andy suggested. :) )

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Posted: Sun Feb 17, 2019 3:52 pm
by roymck
Ok thanks all for the input , decided that an easy mod would be to change the flasher unit to the electronic style that I want . And the linked 2 Light Green / Purple wires could go to earth and it would work ok . David makes sense that the later Coupe was wired as per SE5 as my long departed V8 Coupe was a later 2.5 car and that was wired as per a SE5 with square flasher unit instead of the round Can style .

Any good auto electricians on here?

Posted: Sun Feb 17, 2019 5:09 pm
by scimjim
I’m going to link this thread to my coupe thread, so that I remember where the info is in a couple of years when I’m struggling to fit an electronic flasher :D

Any good auto electricians on here?

Posted: Tue Feb 19, 2019 8:07 pm
by DARK STAR
I have an electronic flasher unit on mine, with separate warning lights and hazard flashers facility.
It must be easy because I did it, but it was a long time ago ...
I think I even had a piece in Slice about it.