Voltage Leak on LED Brake Light Conversion on Automobile

I am trying to convert the brake lights on my 2013 Fiat 500 from incandescent to LED. It is (or should be) a simple bulb replacement. The OEM bulb is a dual filament (running and brake lights) with an 1157 base. The new LED is dual power with the same 1157 base. When I put the LED in the socket, with the lights off the brake light will not illuminate - and with the running lights on, the LED is almost up to bright brake light brightness - without applying the brakes. Step on the brake and the LED brightness does not change.

I measured the voltage at the contacts in the socket with the running lights on and NOT applying the brakes. I have a little over 12 volts on the running light contact, but I have a little more than 5 volts from the brake light contact - and that is with the brakes not being applied.

Apparently, the 5 volts on the brake light contact is enough to fool the LED into reacting as though the brakes are being applied.

Two questions. What would cause the apparently stray 5 volts in the brake light circuit? And what can be done about it? Is there a way to fix it so that there is zero volts when the brake is not applied - or maybe wire in some sort of relay that won't be tripped by the 5 volts, but will be tripped when the brakes are applied?

Thanks!

Terry near Tampa Bay, Florida

Please tell me that you're using a Philips conversion and that your car is on their approved list... otherwise you're wasting your money

N8N said:

Please tell me that you're using a Philips conversion and that your car is on their approved list... otherwise you're wasting your money

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And making your vehicle unroadworthy by modifying an essential safety feature of your vehicle.

Norm

It is overwhelmingly likely that the "LED conversion" you're using is not a legitimate (effective, safe, legal) product. The only LED bulb that stands a chance (and only just a chance, not a guarantee) of working in an effective, safe, and legal manner is the Philips item. Even if it's optically compatible with your Fiat's brake lights, which it might or might not be, that 5v you're seeing on the brake light contact may well be a brake light filament preheater designed to heat the brake light filament to just below the threshold of incandescence so that the brake lights light up faster (almost as fast as an LED) when you step on the brakes. In that case, an LED bulb conversion will not be possible.

You can safely and easily make the brake lights 35% brighter, though, by installing these fully compatible special Honda bulbs.

http://download.p4c.philips.com/files/1/12899rx2/12899rx2_pss_aensa.pdf

http://www.philipsxtremevisionled.com/

Doesn't appear that any FIAT models are on the approved list...

That's not actually an "approved" list. It's a list of applications Philips (USA) has tested the bulb in and found them to work in a manner that complies with the photometric requirements. A vehicle not being on this list is, mathematically/numerically, more likely to mean Philips hasn't tested it than to mean Philips tested it and it doesn't work properly. But there's no way to know for sure. This is kind of an edgy game Philips is playing, because the whole point of a standard bulb type such as 1157, 3157, etc. is that it fits and works in any application that calls for it. No amount of "Check our website and see if your vehicle is on the list" types of statements on the packages exempt Philips from their legal responsibility for selling an item which makes (at least some) vehicles noncompliant with applicable safety standards.

But for now that's what they're doing and for now NHTSA hasn't told Philips to stop, so for now the options are either stick to the list or assess a non-listed installation on its merits. This is actually easier than the basically impossible job of visually assessing headlight beams.

to answer the op's question on the 5V... some European cars use LED bulbs are specifically designed with supervision circuit to allow them to be used with CAN (Controller Area Network) Bus equipped vehicles without triggering the OBC (on-board computer), this feature enables the car to notify you if a light bulb is burned out. While unlikely to be the culprit, this might be what you ended up measuring... a logic level sense voltage. There is nothing on an automotive chassis lighting that uses "5V" that I am aware of

I agree with Illum and Virgil.

Also, I have to admit that even I would be sorely tempted to try those Philips bulbs on a vehicle not on "the list" if I had one on which they would work (well, I suppose I *could* try them on the Heep. But I don't drive it enough to really get the benefits from them.)

I think the FIAT takes a Euro sized tail light bulb anyhow. 2507 maybe?
The pins look different; I had to R&R the bulbs on my wife's car and there weren't many bulbs local to fit.

quick search says P21/5W which is pretty much (not exactly, but they interchange) the same thing as an 1157.

I have responded to a couple posts on this thread - but I haven't seen any of them appear. Perhaps I'm clicking the wrong icons. Has anyone seen them?

Oh crud - now I see - I hadn't been typing in the image verification....... I'll try to redo them later. I do appreciate the input so far.

Thanks!

N8N said:

http://www.philipsxtremevisionled.com/

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I may have to try this since we have a 2013 RAV4 and, Philips did tested it on this car.