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Basic wiring question


UKTJ

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Posted (edited)

As most know I am rubbish at electrics; I have another dumb wiring question.

 

Background

I am installing two A Pillar lights, each comes with its own wiring harness and switch.  The harness is pretty basic, a connector to the tail off the light runs to a relay at which point it branches to ring terminals to connect to the battery (runs through a fuse on the live side wire) and a separate set of wires running to the switch.  The rating on the fuse is 5 amps.  The rating on the relay says 6 amps at 240v and 10 amps at 120V.  No rating I can see on the switch, but I would look to change that to an after market switch anyway.  Lights are quoted at consuming 2.8 amps.

 

Question

Ideally I'd like to have just one switch inside the cab.  Can I splice the wires so that both lights run through the same same relay?  If I splice on the relay side can I just replace the 5 amp fuse with a 10 amp?.  That would then give me one connection to the battery for both lights and one switch inside the cab would activate both lights.  Alternatively if each light needs its own fuse is my only option to splice the wires just before the switch?  Then a single switch will switch a separate relay for each light.

 

Any input gratefully received.

Edited by UKTJ
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lamp-circuit.jpg.463bb02f49d8413c347ddf80405e984e.jpg

 

If the lamps are drawing 2.8 Amps each then rounding up to 3 Amps will be 6 Amps for the pair.

6A x 12V = 72 Watts

 

The relay's continuous power rating for the load at 120Vac is 1200W, at 240Vac it is 1440W.

 

AC and DC behaves differently when it comes to switched contacts, DC arcs five to ten times more intensity than AC so I usually take the Amps for a 120Vac rating as the Amps for a 12Vdc rating, using ten times. If a device is unobtainable at that rating I take a judgement on using a lower factor closer to 5. Using a lower number just erodes the life expectancy on number of cycles of operation.

 

In your case 6A lamp draw is less than the 10A relay rating so your relay will be fine.

 

For the feed choices, number 2 is the safest for forgetting to turn them off or after accidentally switching them on in daytime. 3 enables the Jeep to go fording in deep water with the 'A' pillar lights on and the headlamps off. If you switch the sidelamps off the 'A' pillar lights go off too. Choice 1 gives fully independent control but could result in a flat battery or a traffic violation.

 

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1 hour ago, UKTJ said:

Can I splice the wires so that both lights run through the same same relay?

You can either run a wire from each lamp +ve terminal to pin 87a on the relay with a piggyback connector at the relay.

or

You can run one wire from relay pin 87a to the nearest lamp's +ve terminal and use a piggyback connector or splice the second lamp's feed into the first lamp's connector.

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1 hour ago, UKTJ said:

If I splice on the relay side can I just replace the 5 amp fuse with a 10 amp?

See how you go with the 5A fuse first. Most fuses have a continuous current rating that is lower than the actual Amperage it takes to blow them and the environmental temperature of the location of the fuse can also affect the continuous current and failure Amps.

 

If the 5A fuse blows too frequently, fit a 7.5A instead.

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1 hour ago, UKTJ said:

Then a single switch will switch a separate relay for each light.

You only need one relay for your pair of lamps.

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Car manufacturers have historically made their ignition controlled circuits with sufficient capacity for a few extra relay coils to be added. On a car with more than a couple of extra relays it is worth having just one ignition controlled relay enable a downstream accessory fuse box using a cascade of relays or a single high power relay switching the accessory fuse box feed.

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@V as ever, thank you for taking the time to provide such a comprehensive set of answers.  I am trying to get my head around electrics, but I just seem to struggle.

 

When I connect the harness to the light, put the switch to 'on' and touch black and red wires from harness to the battery the light comes on, as one would hope.  Does this correspond to your option 1?  I can understand the risk of running the battery flat by having the lights on when ignition off, but you also mention a legal risk.  What rule is being violated?  Is this an MOT issue?

 

Now you have confirmed a sjngle relay and fuse will be OK I have found that I can buy a Deutsch connector splitter.  Rather than splice wires I think I will go this route as it avoids the risk of me messing up the wiring and means I have an intact second harness if I ever need one.

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1 hour ago, UKTJ said:

When I connect the harness to the light, put the switch to 'on' and touch black and red wires from harness to the battery the light comes on, as one would hope.  Does this correspond to your option 1?

If your harness includes the relay then it certainly sounds like it is wired as option 1 with the switch feed coming from the fuse.

 

1 hour ago, UKTJ said:

I can understand the risk of running the battery flat by having the lights on when ignition off, but you also mention a legal risk.  What rule is being violated?  Is this an MOT issue?

There are various height limits on forward facing lamps which are in the Road Vehicles Lighting Regulations 1989. On my TJ the 'A' pillar lamps were wired as option 3 with the switch feed from the sidelamp circuit. It passed MOT as the tester considered them to be off-road use only. If they are on while driving on a public highway they have to be extinguished when the headlamp main beam is dipped. The legal problem would be driving on a public road and forgetting to turn them off when dipping beams thus causing oncoming traffic to be dazzled. The same goes for any LED light bar mounted higher than the headlamps or 1200mm with a beam extending higher or further than the headlamps.

 

There is no specific definition for off-road lamps or 'Driving Lamps' in UK law from what I can see. If the beam illuminates further than the headlamps, my interpretation is that they are 'Main Beam' headlamps and have to be extinguished when switching to dipped beam. They also require a tell tale lamp to be visible by the driver when they are on. You can only have one pair of dipped beam headlamps.

 

On my TJ I only wanted to use the 'A' pillar lamps for night wading with the headlamps off to avoid cracking the glass. I should have fitted an extra relay to cut the 'A' lamp's power feed when the headlamps were illuminated but my MOT tester wasn't bothered as long as they worked. Mine were aligned to match the extent of the headlamp dipped beam range.

 

Edited by V
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My TJ was silver like yours. When the 'A' pillar lamps were on at night the reflected light from the bonnet was dazzling to the driver. It didn't matter if the lamps were setup for dipped beam range or long range. They only got used for night wading and for that they were perfect.

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2 hours ago, V said:

If your harness includes the relay then it certainly sounds like it is wired as option 1 with the switch feed coming from the fuse.

 

There are various height limits on forward facing lamps which are in the Road Vehicles Lighting Regulations 1989. On my TJ the 'A' pillar lamps were wired as option 3 with the switch feed from the sidelamp circuit. It passed MOT as the tester considered them to be off-road use only. If they are on while driving on a public highway they have to be extinguished when the headlamp main beam is dipped. The legal problem would be driving on a public road and forgetting to turn them off when dipping beams thus causing oncoming traffic to be dazzled. The same goes for any LED light bar mounted higher than the headlamps or 1200mm with a beam extending higher or further than the headlamps.

 

There is no specific definition for off-road lamps or 'Driving Lamps' in UK law from what I can see. If the beam illuminates further than the headlamps, my interpretation is that they are 'Main Beam' headlamps and have to be extinguished when switching to dipped beam. They also require a tell tale lamp to be visible by the driver when they are on. You can only have one pair of dipped beam headlamps.

 

On my TJ I only wanted to use the 'A' pillar lamps for night wading with the headlamps off to avoid cracking the glass. I should have fitted an extra relay to cut the 'A' lamp's power feed when the headlamps were illuminated but my MOT tester wasn't bothered as long as they worked. Mine were aligned to match the extent of the headlamp dipped beam range.

 

OK, that all makes sense, thanks again.  The switch will have illumination for when they are on.  I have seen on a US forum that in some states they have a similar legal position to the one you describe.  I believe that over there if the lights have covers that are only removed for off road use there is no issue.  I think a pair of covers may therefore be a prudent investment, as my currently my MOT is done up the road to a garage where I am well known, my guess is that would be sufficient, well I hope so anyway.

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Got my switch to replace the one that came with the harness - wanted one that fitted into the blanked off switch housing rather than drilling a hole for the switch that came with the lights.  Wired up this morning, tested it out and all fine.  Then fitted switch into dash and found light not working.  Appears fuse has blown even though just one light attached.

Deutsch connector splitter arrived this afternoon (bonus as it was a two pack of them so have another one to use with the remaining harness for some other lights).  Fitted the splitter and stole the fuse from the spare harness.  All works OK now, so no idea why fuse blew.

Have orders a box of spare fuses in a range of sizes as seems something I should have in my kit bag.

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  • 5 weeks later...
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On 17/08/2023 at 18:28, UKTJ said:

OK, that all makes sense, thanks again.  The switch will have illumination for when they are on.  I have seen on a US forum that in some states they have a similar legal position to the one you describe.  I believe that over there if the lights have covers that are only removed for off road use there is no issue.  I think a pair of covers may therefore be a prudent investment, as my currently my MOT is done up the road to a garage where I am well known, my guess is that would be sufficient, well I hope so anyway.

It turned out that Wilderness (where my lights come from) don't make a cover for the ones I have which include side illumination.  TJ had its MOT today and no issue raised even without any covers.

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