LED turn signals caused a short circuit code on FEM

Eschmacher · 10-06-2021, 07:01 PM

So the other day, my front right turn signal stopped functioning and gave me a turn signal malfunction warning. I had replaced both front signals with an LED about 2 years ago, and then coded them to disable cold and warm monitoring, and reduced the power by 50% to keep them from being unbearably bright.

When I hooked up ISTA, it went through the reset procedure, but told me the maximum number of short circuits had occurred on that output and it would be permanently disabled and the FEM needed to be replaced.

I ended up paying someone with BMW explorer to remotely connect to the car and they were able to reset it.

I have since replaced them with another set of bulbs and also coded FRA_V_R/FRA_V_L KURZSCHLUSS to "nicht aktiv."

Will this prevent this from happening again? I am slightly worried the issue was not with the bulbs themselves, but with the loom/harness going into the engine bay. There is no corrosion on the pins going into the headlight housing, or the connectors on the FEM.

Billfitz · 10-06-2021, 09:39 PM

It was probably a bulb that went bad. The short circuit counter is a protection that prevents damage to the FEM, forcing you to fix whatever is causing it. If the signal is working then chances are it's OK, although an intermittent fault in the wiring is a possibility.

Eschmacher · 10-07-2021, 10:51 AM

Quote:

Originally Posted by Billfitz

It was probably a bulb that went bad. The short circuit counter is a protection that prevents damage to the FEM, forcing you to fix whatever is causing it. If the signal is working then chances are it's OK, although an intermittent fault in the wiring is a possibility.

That was my thought too, but then I decided to test the old leds on a 12v source outside of the car, and the one that caused the fault still worked!

I'm hoping maybe the old bulb was in fact still bad, just not in a visible manner that the FEM was able to detect. Maybe the resistance changed or there is a short inside the bulb somewhere.

I hope the new bulbs hold up better. Only way to find out I guess - drive it.

Billfitz · 10-07-2021, 12:18 PM

The system detects bulbs via the warm and cold checks. It can't do so with LED because their resistance is too high to be read as a load. That's what can cause an LED to be seen as a blown bulb. Preventing that from happening is one reason why the warm and cold checks are coded to inactive. The other is that the warm check works by sending a pulse to the bulb every thirty seconds or so when the engine is running and the lamp is off. That pulse doesn't cause an incandescent bulb to flash because it's too short in duration to heat the filament. LEDs don't have filaments, they don't rely on heat to create light, and they react to applied voltage so fast that the warm check pulse does cause them to flash.

10-06-2021, 07:01 PM	#1
Eschmacher Car Hypochondriac 335 Rep 901 Posts Drives: 2014 335i M-Sport Estoril Blue Join Date: Feb 2017 Location: Sterling, VA iTrader: (0) Garage List 2014 BMW 335i [0.00]	LED turn signals caused a short circuit code on FEM So the other day, my front right turn signal stopped functioning and gave me a turn signal malfunction warning. I had replaced both front signals with an LED about 2 years ago, and then coded them to disable cold and warm monitoring, and reduced the power by 50% to keep them from being unbearably bright. When I hooked up ISTA, it went through the reset procedure, but told me the maximum number of short circuits had occurred on that output and it would be permanently disabled and the FEM needed to be replaced. I ended up paying someone with BMW explorer to remotely connect to the car and they were able to reset it. I have since replaced them with another set of bulbs and also coded FRA_V_R/FRA_V_L KURZSCHLUSS to "nicht aktiv." Will this prevent this from happening again? I am slightly worried the issue was not with the bulbs themselves, but with the loom/harness going into the engine bay. There is no corrosion on the pins going into the headlight housing, or the connectors on the FEM. __________________ Power mods: VRSF IC/Catted DP - FTP CP/BP - AFE Cat-back - BM3/XHP - Dorch S2 Handling mods: Koni yellow - Eibach prokit - Millway street - M Performance LSD - M3 brakes - EBC bluestuff Exterior: Euro LCI tail lights - Shitty ECS CF spoiler - Chinese M performance diffuser Interior: Bavsound stage 1 and subs - custom trunk sub by Mike1327
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10-06-2021, 09:39 PM	#2
Billfitz Lieutenant General 8243 Rep 16,082 Posts Drives: '15 328iX GT Join Date: Jan 2015 Location: New Hampshire iTrader: (0)	It was probably a bulb that went bad. The short circuit counter is a protection that prevents damage to the FEM, forcing you to fix whatever is causing it. If the signal is working then chances are it's OK, although an intermittent fault in the wiring is a possibility.
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10-07-2021, 12:18 PM	#4
Billfitz Lieutenant General 8243 Rep 16,082 Posts Drives: '15 328iX GT Join Date: Jan 2015 Location: New Hampshire iTrader: (0)	The system detects bulbs via the warm and cold checks. It can't do so with LED because their resistance is too high to be read as a load. That's what can cause an LED to be seen as a blown bulb. Preventing that from happening is one reason why the warm and cold checks are coded to inactive. The other is that the warm check works by sending a pulse to the bulb every thirty seconds or so when the engine is running and the lamp is off. That pulse doesn't cause an incandescent bulb to flash because it's too short in duration to heat the filament. LEDs don't have filaments, they don't rely on heat to create light, and they react to applied voltage so fast that the warm check pulse does cause them to flash.
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