Coverting OEM auto bulbs to LED

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tweybrew

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I want to convert 3157 amber and red bulbs with LEDs. I'm a complete non-techie with anything electric, but I have seen videos and read articles telling me I need a resistor so that the LED will simulate the OEM incandescent bulb. By far the most available resistors are the 6Ω/50W self-install. I have also seen the plug-and-play connectors, most of those being 8Ω/50W. I have tried both and neither prevents the hyper-flash I am getting. Since I have no idea what these specs mean, are either of the resistors I've tried correct for my need? If not, what do I need? And I've also seen that the resistor needed for the amber bulb may be different from the one I need for the red bulb. Can someone lead me through this situation to a solution, please?

 
LED lamps take a lot less current to light them than filament lamps.  Most vehicles have lamp failure detection systems to tell the driver he has a fault.  These detection systems work by monitoring the current flowing to each lamp, and when the current falls below a certain level will trigger the warning.  In the case of trafficators it's usually by rapid flashing of the remaining lamps.  For brake lights its usually a dashboard warning indicator.

The purpose of the resistors is to defeat the warning system by taking approximately the same current as a serviceable lamp. 

Now some basic figures:  The wattage of a lamp is its voltage multiplied by current.  e.g.  A standard 21 watt indicator lamp on a 12volt supply takes 1.75 amps.

So, to use a resistor as a "pretend" lamp you need one which will take that current.

Ohms law tells us that Current equals voltage divided by resistance, or the other way resistance equals voltage divided by current. Hence about 6.8 ohms in this case.

Note that there is quite a wide tolerance band on these fault detection systems to accommodate several variables. A vehicle 12 volt system actually varies between below 12 to over 14 depending on battery condition and whether charging from the engine.  Lamps deteriorate with age, etc  Finally the led lamps you are putting in still require some current; I would guess about a tenth of the filament lamp.  A bit of trial and error is probably needed.

I hope that helps.

 
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I'll just throw this in the mix and it is only a SUGGESTION.    Could this possibly alter/affect/invalidate your insurance as you have made an alteration to the spec of the vehicle?

as I said it's purely a thought BUT you know what these insurance companies are like

And don't forget the "men in strangely peaked white caps " that keep thinking you are Lewis Hamilton!

how DO they get those peaks like that coz they sure don't look like that when issued!

 
I'll just throw this in the mix and it is only a SUGGESTION.    Could this possibly alter/affect/invalidate your insurance as you have made an alteration to the spec of the vehicle?

as I said it's purely a thought BUT you know what these insurance companies are like
Fair point. I hadn't thought of the insurance aspect but I was trying to  point out that you are disabling something intended to help keep you safe.  Personally  I don't think there is a lot to gain in many cases with trafficators or brake lights as the filament lamps last ages anyway,  (but the OP didn't ask that).

I always change all the interior lights of my vehicles to LED, as they give a better light and don't run the battery down if left on!  I also used them in the side lights of an old Volvo where they were on all the time, but that didn't need any fiddling because the monitor system worked by comparing current to the two sides.

 
Another interesting point....I wonder if it will affect the CPUs fitted to most cars nowadays?

cant recall which manufacturer but there is one where if you swap  the radio you have to tell the CPU or it sulks


ha ha ha.

don't most of the modern cars monitor the resistance / load so they know when a bulb fails? Fitting LED's could mean your dashboard may light up like a christmas tree.

 
Yes, that's what the resistors are added for, but they need to be just right then the system, however clever, can't tell the difference.

Lots of newer cars have LED lights as original in many positions. 

Unfortunately, (in my opinion), the manufacturers have taken the opportunity to dispense with lampholders, so when anything fails it is going to cost £££ instead of 50p for a bulb.

 
CPUS on cars ar a strange beast

i live close to a Radar station

if you drove past at the wrong time in its rotation it would kill the engine dead....trick,was to have radio on, if you heard a click when you passed the road sign you were ok and stayed at a constant 30.  If no click then you either had to stop OR gun it at 70 ( dead straight country rd)

this was in a Montego estate   It also affected TR7s, Renault Fuegos and oddly it would,reset the digital dashboards on Astra GTEs to,German

its all been up graded now but it was quite funny watching people's cars just cruise to,a halt

 
I'm sure I read somewhere that changing exterior lights on cars to LED was not allowed. Something to do with non conformity.

Why do want to do this anyway? If you have to add resistors you are not saving any battery power. I very rarely have to change such lamps, they usually last longer than I keep the car.

 
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