Rick's Free Auto Repair Advice

Automotive LED bulbs cause big problems

There’s no upside to automotive LED bulbs

Here’s why you shouldn’t install automotive LED bulbs for your turn, park and stop lights.

LED turn signal and stop light bulbs are all the rage right now. You LED turn signal bulbmay be tempted to install them. DON’T. They don’t last nearly as long as you think, which means you’ve spent more money for a bulb that just won’t last very long.

Worse than that however is the fact that swapping out incandescent turn park stop bulbs with LED bulbs can cause big problems. This is still new technology and lots of suppliers don’t understand the various “bulb out” systems used by car makers.

Automotive LED bulbs cause problems with auto electrical systems

Many late model cars have “bulb out” light bulb detection system. There are many ways to check the status of an incandescent bulb. One common method is to measure voltage drop or current draw. Since LED bulbs consume far less power, they automatically trigger the bulb monitoring system making it system think the bulb is burned out.The bulb out indicator then lights up on your dash.

LED turn signal bulb manufacturers try to get around that problem by adding resistors to the circuit so the LED turn signal bulb provides nearly the same voltage drop as an ordinary bulb. The downside to this approach is that adding a resistor consumers more energy. So kiss the electrical savings goodbye. And, if the bulb out system in your vehicle doesn’t use voltage drop, the resistors won’t help at all.

LED bulbs cause fast flash

In the old days, car makers relied on current draw to make the flasher turn on an off. It was a simple device that included a bent metal disc that heated up and changed shape as the bulbs drew current through the spring. When the shape changed, the spring metal would disconnect power to the turn signal bulbs. With no power flowing, the metal would cool and return to its original shape. When an incandescent turn signal bulb burned out, less current would flow through the disc, resulted in less heat. Bottom line; the disc would disconnect much faster, cool faster and reconnect faster, resulting in a “fast flash” condition.

When you replace an incandescent turn signal bulb with an LED, you can duplicate the fast flash condition automatically because the LED bulb draws far less power.

LED turn stop park bulbs don’t play well with body control modules

Many car makers eliminated the mechanical flasher unit described above and instead relay on the body control module to control the stop park turn bulbs. These systems occasionally conducts kind of continuity test on the bulb to see if the filaments are intact. If you replace an incandescent bulb with an LED turn park stop bulb in one of those vehicles and you’ll not only get false “bulb out” indications, you can also cause all kinds of electrical problems including a dead battery and no-start condition. See below.

GM issues bulletin on LED bulbs

GM has issued a service bulletin to address dead battery and inoperative start/stop functions on some 2017 Acadia (VIN N) and XT5 models. Installing an LED backup bulb in place of an incandescent backup bulb can also cause an instrument cluster and switch backlighting illumination problem.

In the bulletin, GM explains that the body control module monitors the backup lamp circuit to detect bulb outages by applying a low voltage signal to the bulbs. Replacing the incandescent backup bulbs with LED backup bulbs causes the BCM to see the wrong return voltage.

The cause huge problems

With an incorrect return voltage, the BCM retries the signal over and over, which prevents the BCM from entering “sleep mode.” The result is a drained battery, instrument cluster and switch backlighting illumination problems. So those nifty $15 LED turn signal bulbs can wind up screwing up the electrical systems in your vehicle.

They just not worth it

Why pay a higher price for the LED bulb, use more power and get the same life as regular $2 bulb? It’s crazy! Add in the electrical problems and it’s just an insane move.

Some day the car and LED bulb industry will work this out. But this generation of LED turn signal bulbs isn’t ready for prime time.

©, 2018 Rick Muscoplat

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Posted on by Rick Muscoplat

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