Vehicle telematics unit

[Diver300]

I'm trying to fix a vehicle telematics unit.

What I think was a ceramic capacitor got damp and was damaged. I wasn't able to read any value from it. The circuit board near it got corroded as well.

I traced some of the circuit and it was connected to the mid point of two 4k7 resistors like this:-

I can't work out where the tracks go from there as it is a multilayer board and all the connections to the resistors go to vias and I can't see where they go as they don't go anywhere else on the top or bottom of the the board.

What is odd it the position on the board of the component that I think was a capacitor. Here is a photo of the board:-

That was taken under UV light to show the conformal coating. The failed component was at point A, and the resistors that it connects to are at point B.

The conformal coating has been missed off where there are a test points that are needed after assembly, and that is what has let the moisture get to the failed component.

On both top and bottom of the board, there are spaces for components that are not populated near point A.

The conformal coating makes it difficult to trace continuity.

Does anyone have any idea why the failed component was places so far away from the resistors?

Should I just fit a 100 nF capacitor or are there any better suggestions?

 

[Tony Stewart]

What energy source could blow away capacitance in series with such large resistance may have arc'd over the resistor? Yet it's not in the path of the diff. coplanar tracks on an interface that might experience ESD if it is the CAN bus.


It has 4 nice exposed solder pads PCB-ESD handling on each side of the PCB corner from ground to fingers {to ground}

Above the A circle are pads for the CAN bus terminator(?) CM choke. but no IC.
The test pads are nicely gold plated.

The Resistors may be just pull-ups to this isolated external bus power pin (verify). but surprisingly isolated.

There is an unused 16 pin pad nearby so the function is unknown.
Nor is the defect symptom known, so assumed no communication.

 

[Diver300]

The capacitor wasn't blown apart. The electrode had corroded off the capacitor and the pad had been dissolved away as well. That could have happened over years so it's quite possible the at the power for that came through 4k7 resistors.

I couldn't find any connection to the 16 pin IC, but only to the resistors that are much further away.

The module has two CANbus interfaces which connect to the multi-way connector that uses standard wires. The CANbus wires are likely to be twisted pairs.

There is provision for three connectors at the top right, none of which are populated. They are all connectors for screened cables, so I think that the tracks above circle A are for much higher frequencies than CANbus