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Adding a solenoid for the headlights - is this correct? - Update!


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Folks, I’ve decided to add a solenoid to the headlight circuit on my TR4A. While I was drawing a proper diagram of wiring mods I’ve made over the years (alternator & electric fuel pump), I added what I think would be the correct wiring for the headlight solenoid. Please take a look and see if this makes sense (or if anything else looks dodgy), or would lead to a gigantic explosion! :blink:

Thanks 

Jim

TR4A_wiring_solenoid.jpg

Edited by Tr4aJim
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Jim,

I would put a seperate fuse in each of the "Beam" circuits, not in the feed to both. Then if something causes the fuse to blow you are not left with no headlights at all.

Charlie.

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seems OK, I would probably put the fuse the 'supply side' of the relay... your design puts all the power thru the dipswitch...but its the only way to do it with one relay...other option would be to put two relays in the UW & UR wires

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+1.   Your single relay protects the lighting switch contacts but doesn't help the dipswitch.  Relays are cheaper than replacement switches ! 

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Agree, When I did it I used 4 relays. 1 for right high beam, 1 for left high beam, 1 for right dip, 1 for left dip.

Locate them in pairs near the wire junctions behind the headlights.

Bob.

Edited by Lebro
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The Works' Competition Department ensured that the wiring for the TR4 Rally cars incorporated separate fuses for each headlamp filament so that if a prang resulted in a short-circuit, as many lights as possible would remain operative - although not necessarily pointing in the right direction!  The spotlamps were fused separately.

All I have done is to improve matters further by incorporating 4 relays in the feeds, and thereby have removed the high current from both the dashboard lamps switch and the dip switch.

Ian Cornish

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Don't want want to hi-jack this thread, but just a quick related question:

I am thinking of fitting relays, but If I moved to LED lighting would it be no longer necessary/worthwhile?

Ali.

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No point with LEDS as the current is much lower anyway.

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1 hour ago, RobH said:

No point with LEDS as the current is much lower anyway.

Also, the circuitry driving the LED bulbs gives the same light output (same current) over a range of input volts, so it would not matter if you were loosing say 1 volt along the wires.

With Halogens I used relays which made a marginal difference, I have since removed them having moved over to LED's.

Bob.

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I published an article in TR Action 296 (March 2017) which you might find helpful.

Can be accessed in the scanned copies of TR Actions available on-line in the TR Register's website.

Ian Cornish

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Posted (edited)

Folks, here is my new diagram based on the recommendations to use two solenoids. Please give it a once over for errors. 

I saw some suggestions to add fuses to each headlight lead, so I may do that as well. I can install a solenoid per headlight as some also suggested.


I do have a question regarding the fuse on the main power line to the solenoids. Some diagrams I saw show a 20 amp fuse, while others had 30. Opinions on which to use? 

Thanks 

Jim
 

65EE6A3D-460F-4E64-AC14-3AA178EBD5A9.jpeg

Edited by Tr4aJim
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  • Tr4aJim changed the title to Adding a solenoid for the headlights - is this correct? - Update!

The circuit is fine Jim.    

60W nominal headlamps draw 5A each at 12V supply, and about 6A at 14.7V so the normal current draw will up to 12A total.  A 20A fuse will be enough to protect the circuit.

The resistance of incandescent lamps increases as they get hot meaning the initial in-rush current to a cold filament will be a bit higher than the rated one.  I think some people like to use a bigger fuse to allow for that but it really isn't necessary when the fuse being used is a modern one rated for continuous current.

(You should be aware that the original glass Lucas fuses were rated for the current they will actually blow at while a modern fuse is rated for the highest current it can pass continuously without blowing. This can cause some degree of confusion)  

 

Edited by RobH
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It would appear that should the fuse between battery and pin 30 on both relays blows, then you would be left with no headlights whatsoever!

That is not desirable, especially if driving rapidly at night down a winding country road.

You need a separate fuse for High and for Dipped beams - just put 2 fuses where you have shown one, with the first feeding the UW relay and the other feeding the UR relay.

Better still, a fuse in each of the connections to the lamp filaments (i.e. 4 fuses situated downstream of the relays), but some would consider that somewhat unnecessary.

Ian Cornish

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3 hours ago, BlueTR3A-5EKT said:

Hello All

             I have used this type of relay for several things (electric fans, Headlights, electric EFI fuel pump etc)

Roger

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52 minutes ago, Tr4aJim said:

Yes, two fuses it is!

”Blue”, is that a solenoid with a built in fuse?

I was looking at something like these:

https://www.wiringproducts.com/bussmann-short-stop-circuit-breakers
 

thanks,

Jim

Yes Jim the relay has an integrated replaceable fuse of the ‘modern’ blade design as found on euroboxes and rice burners.  I do not know what US domestic production vehicles use for fuses.

Peter W

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