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Mr Diagnostic

The leader in mobile Automotive Diagnostics in New England

I was called in to a local used car dealer for a 06 Mazda 6 BCM burning up. When they said burning up I never expected flame burning up. Usually it means it keeps dying. Well you can see in the picture below what burning up meant.

Mazda bcm problem found by Mr Diagnostic

    The picture above was the first module that was originally in the vehicle. The shop went and changed the module and re drove the vehicle and the second module ended up the same way. When I got there I had two burned modules to investigate. There were well over 100 wires going to this module so I began by checking all power and ground leads to and from the module for shorts. After that I started checking command lines to the module for proper voltages or grounds. Everything was checking out ok. I advised the owner to order another module so we can see what is causing the heating of the circuit. There were 9 plug connectors to this module as seen below. Upon further investigation I found the plug in the lower left of the picture was getting the hottest.

Another view of the body control module from the Mazda by Mr Diagnostic

    I was very happy to find these boards are mounted in a plastic case vs a metal case. I then started looking for the most melted part of the case. Once I found it it confirmed my suspicions of the plug getting the hottest was in fact the lower left in the pic above. With luck I also found the pin that got the hottest burned a deep pin hole almost through the case. I went to the diagram and found that pin went to the brake light circuit. Once we got the new board in I removed it from the plastic case and plugged the plugs in. While I was holding the board in the palm of my hand I had a tech connect battery voltage. after ten minutes of sitting there with no heat generation I started the vehicle. I sat there another ten minutes with no heat generation while cycling all controlled circuits of the module except for the brake lights. Everything stayed cool. I then hit the brake pedal and instantly my finger got burned. Followed the wiring from the module to the trunk and found a splice connector at the left brake light. The wries lead through a drill hole in the trunk to a third brake light mounted in the tail someone had put onto the trunk lid. The wire was pulled nice and tight against the drill hole and it chaffed through the insulation causing a dead short to ground when the brake was applied. In turn causing the BCM to go up in smoke.

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