When replacing turn signal incandescent lamps with LEDs, the typical problem is hyper flashing. This happens due to the car diagnostic reporting a drop in power consumption, and to get attention from the driver to check the bulbs. LEDs typically consume much less power. One easy fix is to add a resistor, to compensate the load. But the value of this resistor must be considered carefully. The most common advice on YouTube videos is to install 6 Ohm 50W resistor, which will dissipate 30W of heat (average will be 15W, due to 50% duty cycle of the blinker, but still it is a lot of heat)… Also, if you replace only one bulb, it will completely defeat the diagnostic. If the LED bulb malfunctions for some reason, there won’t be any notification. I decided to check BCM implementation, to get a better understanding of this issue. The photo below shows BCM module from the Mazda MX-5 2016.
Wires from the front and rear lights are coming to the different connectors, but on the PCB they are directly connected to the same switch (see red circle on the photo)
Main microcontroller is NXP (Freescale) SPC5603B. Other chips there are:
UJA1078A – high-speed CAN/dual LIN core system basis chip
TLE7244SL eight channel low-side switch with SPI interface
TLE72592GE transceiver for the Local Interconnect Network (LIN)
Turn signal switch implemented on Infineon BTS5020 Dual High Side Power Switch. This switch is very sophisticated. It provides all kinds of protection and diagnostics. On the picture below is a typical connection diagram (from the data sheet).
As shown on this diagram, all the diagnostics come from a single IS pin. R-is resistor will define voltage for the ADC on the Micro controller. I did a test with variable load and found that hyper flashing starts when the load drops below 40W.
It is more likely that a threshold is set for 3A current through the switch, generating 1mA on the IS pin.
The ideal option would be software modification, but it’s also possible to simply change the value of the sensing resistor for the new estimated load. By changing the value of this resistor, we can tune the threshold for hyper flashing. The resistor on the board is marked as R108 and has a value of 1.3K, for the maximum load of 27W+21W+5W (53W total).
Off course this modification is not for everyone. I think it is a better solution, instead of adding high power resistors and disabling real-time diagnostic.
To test this approach, I replaced the R108 resistor with a 3.2k value resistor, and replaced the front 27W bulbs with LEDs from iBrightstar (4W power consumption). Without high power resistors, the hyper flashing starts only if I remove the LED bulb.
The BCM module is very easy to access. It is located under the dash on the driver side. There is no single screw, everything is held by plastic hooks. It took me 20 minutes to replace the resistor and reinstall BCM in place.