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Ray White
Joined: 02 Dec 2014
Posts: 7452
Location: Derby
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 Posted: Tue May 12, 2026 7:54 pm Post subject: Hazard warning lights question. |
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I am considering adding hazard warning lights to my car.
My first question is : does a three pin flasher unit utilise one pin for the tell tale light?
If the answer to that question is "yes" then my second question is: how do the two pin flasher units work to operate the tell tale? |
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ukdave2002
Joined: 23 Nov 2007
Posts: 4320
Location: South Cheshire
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| Posted: Wed May 13, 2026 12:28 pm Post subject: |
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Yes, 3 terminal units such as the Lucas FL5 use terminal P for the warning lamp, B is battery and L the switch.
2 pin flasher warning lamps are simply wired across the Right and Left terminals of the indicator switch, so when turning right the warning light gets its battery supply from the switch and chassis/earth via the left hand repeater bulbs (cold bulbs = low resistance), vice vera when turning left. |
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Ray White
Joined: 02 Dec 2014
Posts: 7452
Location: Derby
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| Posted: Thu May 14, 2026 2:12 pm Post subject: |
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Another question relating to this subject is :
Flasher units of various kinds seem to be referred to as relays.
Does this mean they will protect the switch from damage? |
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ukdave2002
Joined: 23 Nov 2007
Posts: 4320
Location: South Cheshire
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| Posted: Thu May 14, 2026 2:19 pm Post subject: |
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| Ray White wrote: | Another question relating to this subject is :
Flasher units of various kinds seem to be referred to as relays.
Does this mean they will protect the switch from damage? |
No because the switch is in series with the flasher unit. |
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Ray White
Joined: 02 Dec 2014
Posts: 7452
Location: Derby
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| Posted: Thu May 14, 2026 4:31 pm Post subject: |
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| ukdave2002 wrote: | | Ray White wrote: | Another question relating to this subject is :
Flasher units of various kinds seem to be referred to as relays.
Does this mean they will protect the switch from damage? |
No because the switch is in series with the flasher unit. |
Thank you so much for your advice, Dave.  |
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MikeEdwards
Joined: 25 May 2011
Posts: 2796
Location: South Cheshire
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| Posted: Thu May 14, 2026 6:02 pm Post subject: |
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Doesn't the term "relay" just mean that it's not the older-style bimetallic strip?
_________________
1976 Vauxhall HP Firenza, 1976 Vauxhall Sportshatch (x2), 1986 Audi coupe quattro, 2000 Audi TT |
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ukdave2002
Joined: 23 Nov 2007
Posts: 4320
Location: South Cheshire
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| Posted: Thu May 14, 2026 6:46 pm Post subject: |
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| MikeEdwards wrote: | | Doesn't the term "relay" just mean that it's not the older-style bimetallic strip? |
I think because some flasher units physically resemble automotive relays, folk started calling them relays!
Technically a relay is an electrically operated switch that uses a low-power signal to control a high-power circuit. |
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Ray White
Joined: 02 Dec 2014
Posts: 7452
Location: Derby
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| Posted: Thu May 14, 2026 10:12 pm Post subject: |
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Since I have a spare 4 pin relay I think it would be only sensible to use it to protect my rare (and expensive) Lucas switch.
I plan on connecting the output side of the hazard flasher unit to the trigger wire from the 4 pin relay (85... as + earth uses 86) with 87 connected to the switch. From the switch two wires go to the left and right indicator lights utilising the spare terminals in double "bullet" type connectors. A separate wire can go to the tell tale light.
The only thing that worries me is if I need a separate dashboard light. I have an idea that current from the original indicator switch could find its way back down the warning light wire to the hazard switch and onto all four lamps!!
I suppose I could put a diode in the wire? |
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ukdave2002
Joined: 23 Nov 2007
Posts: 4320
Location: South Cheshire
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| Posted: Fri May 15, 2026 6:51 am Post subject: |
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Really need to see a schematic, but it sounds as if you are connecting the Right & Left hand indicators together (from the relay) in which case when making normal turns both sides will flash?
I think adding a relay adds unnecessary complication, however if its about protecting the switch then the relay should be added before the Hazzard Unit. Given the limited amount of times (hopefully!!) the hazard warning switch will be used, is it really worth the additional complication by adding another component that can fail? |
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Ray White
Joined: 02 Dec 2014
Posts: 7452
Location: Derby
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| Posted: Fri May 15, 2026 9:52 am Post subject: |
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It is just an idea at the moment. I had hoped to be able to tidy up the loose ends as I have a spare switch on the dash, a spare relay fitted with wires ready to connect and just a hazard flasher unit to fit.
But as you say it is adding another complication, so I may not bother...it's not important.
P.S. I see what you mean about where the relay should go. |
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ukdave2002
Joined: 23 Nov 2007
Posts: 4320
Location: South Cheshire
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| Posted: Wed May 20, 2026 8:16 am Post subject: |
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| Ray White wrote: | It is just an idea at the moment. I had hoped to be able to tidy up the loose ends as I have a spare switch on the dash, a spare relay fitted with wires ready to connect and just a hazard flasher unit to fit.
But as you say it is adding another complication, so I may not bother...it's not important.
P.S. I see what you mean about where the relay should go. |
My background is electrical & electronic engineering, as such I have always taken an interest in automotive electronics as it developed over the years, its both fascinating and surprising that these days even a simple switch to control an electric window is no longer a single wire, the signal from the switch will be converted to a packet of data, transmitted over the vehicles bus (network) recognised and interpreted by the window controller as something it has to act on and it opens or closes the window accordingly.
Over the years friends and family have asked for help when an engine management or airbag light appears, or some other random fault. I have fixed many of them, and its rare that the electronics has failed, much more frequently a poor connector , bad earth or supply.
My point being that every time we add any type of electrical connection, or component like a relay that effectively triples the amount of electrical connects in a circuit, we are significantly increasing the chances of a fault. The question about the design is a balance; does the additional protection or other features justify the potential increase in unreliability? |
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Ray White
Joined: 02 Dec 2014
Posts: 7452
Location: Derby
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| Posted: Wed May 20, 2026 11:23 am Post subject: |
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| ukdave2002 wrote: | | Ray White wrote: | It is just an idea at the moment. I had hoped to be able to tidy up the loose ends as I have a spare switch on the dash, a spare relay fitted with wires ready to connect and just a hazard flasher unit to fit.
But as you say it is adding another complication, so I may not bother...it's not important.
P.S. I see what you mean about where the relay should go. |
......
...
...My point being that every time we add any type of electrical connection, or component like a relay that effectively triples the amount of electrical connects in a circuit, we are significantly increasing the chances of a fault. The question about the design is a balance; does the additional protection or other features justify the potential increase in unreliability? |
My main reasoning for trying to upgrade the electrics on this car is basically fear of fire. On acquiring the TC I began to hear about how they seem to be hell bent on self destruction. One story after another told of dashboard fires that spread quickly and resulted in ruin.
Subsequently, it has dawned on me that these stories mainly seem to have been in happening in America. Could it be that they are more inclined to overload their car's electrical circuits than us? Here the bi word is originality so perhaps restorers tend to keep things simple?
I also learned that Americans have a different fuse rating system ...so perhaps that is another factor?
Either way, I think I have spent enough time and money on this particular project and once it has been properly tuned on a rolling road (next month) I will probably move it on.
. |
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Ray White
Joined: 02 Dec 2014
Posts: 7452
Location: Derby
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| Posted: Wed May 20, 2026 4:04 pm Post subject: diode question. |
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I had to re solder the diodes to the wires that go to the flashing indicators. The problem is that now they won't work.
I checked the wires for continuity but can't get my multi meter to buzz in either direction.
I presume I have spoilt the diodes with heat from soldering? Is this likely ? They have an operating temperature of up to 150 deg C. |
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