Technical Fiat Panda 312 BCM issue – DRL and brake lights not working, found damaged high-side switch

[Nik01]

Hi everyone,


I’m working on a Fiat Panda 312 BCM (Body Control Module) that’s presenting some issues. The daytime running lights (DRL) are not working, and neither are the rear brake lights (the two 21W bulbs). However, the third brake light (the center high-mounted one) works perfectly.


In diagnostic, the only fault codes I see are related to the front DRLs – nothing else.


What’s strange is that both main rear brake lights are not working, yet the third brake light is. This makes me suspect a hardware issue inside the BCM.


After opening the BCM, I found a damaged high-side switch (not exactly a MOSFET, more like a smart switch). There was a visible crack underneath the component, and the output pins were purple, clearly overheated. Unfortunately, there are no designators on the PCB to identify it.


I've removed the component and confirmed that the outputs were shorted to 12V, which might explain why the DRLs stayed permanently ON before removal – likely a short to battery. Even after desoldering the component, the output pads are still shorted to 12V, which seems very odd.


Has anyone experienced something similar?
Does anyone have a schematic or BCM pinout for this module?


I’m fairly confident that this component controls the DRLs, because now, with it removed, the DRLs no longer turn on. I’ve also managed to source a replacement BCM from another vehicle, but I’m not sure how to bypass or remove the immobilizer in order to test it.


Any help, advice, or shared experience would be really appreciated. I’ll attach some photos of the damaged component below.


Thanks in advance!

 

[Herts Hillhopper]

I didn’t think the brake lights are controlled via the BCM....

But then I found this on eLearn (which sometimes opens!). For some reason it won't show me the circuit diagram, but this descriptive text might help you. And it seems to confirm a different connection from the BCM to the LED centre brake light:

FUNCTIONAL DESCRIPTION​

The brake pedal switch I030 (pin 4) receives an ignition-operated supply (INT) from the line protected by fuse F37 of the Body Computer M001 (pin 7 of connector C): when the brake pedal is pressed, the "N.O." switch (pin 1 for left hand drive versions; pin 2 for right hand drive versions) closes and sends a signal to the Body Computer itself (pin 27 of connector B).

The same signal is sent to engine management control unit M010 and, for the versions where provided, to robotised gearbox control unit M054.

[links to dialogic gearbox systems removed]

Body Computer M001 controls the power supply to the brake lights in the left F030 (pin 4) and right F031 (pin 1) rear light clusters, from pins 60 and 8 of connector F respectively.

The central brake light LED device F040 is supplied by pin 16 of connector F of M001.

The rear left and rear right lights cluster and the central brake light LED device are connected to the rear left earth C030 (connector A).

In versions available from 09/2012, on the other hand, rear left light cluster F030 and car brake LED device F040 are connected to rear window earth C032.

Body Computer M001 receives a direct battery power supply (pin 1 of connector A) protected by fuse F01 of engine compartment junction unit B001 (pin 14 of connector B). It also receives an ignition-operated power supply (INT) at pin 11 of connector E.

The Body Computer M001 is also connected to the driver's dashboard earth C015 via pins 9 and 10 of connector E.

Via the B-CAN line, the Body Computer M001 (pins 18 and 20 of connector D) is connected to the instrument panel E050 (pins 5 and 6) to manage the general failure warning light and the message in the display if there is a failure detected in the circuit or the fuse.