Electrical Possible burn mark on computer motherboard

Gently Used · Apr 1, 2022

I have an engine out of a 93 automatic. This is a california car and everything is stock. I have been running this engine for 24 years with absolutely no problems. Until... I got a 41 code. Come to find out the O2 harness was melted and melted the wires together in a dead short. I fixed the harness but can't get the code to clear. (Well technically it clears, but it comes right back).

I opened the ECU and looked at the board for obvious marks. I did not see anything on the board for Pin 29, but there is a spot that looks like a possible burn mark near it and it's bridging three traces. Before I get out a sharp instrument and scratch off the mark to isolate the traces, can you guys look at my pictures and tell me if what I am seeing is normal? on the back of the board I drew an arrow simulating where the suspect burn mark would run. on the front of the board I circled the suspect mark. I'm curious because I don't see anything wrong on the back. So is this mark normal?

Thank

s

Mustang5L5 · Apr 1, 2022

Does not look burned to me in the first 2 pics. The final pic, i can't really tell. Need a few more views of that.

Did you replace the O2 sensors after the harness melted? I would. For a code 41, the first thing I would try is to swap the two O2 sensors side to side and see if the Code 41 becomes a code 91. If it does, replace the sensor. If it doesn't, then it's a wiring/ecu issue.

Gently Used · Apr 1, 2022

Mustang5L5 said:
Does not look burned to me in the first 2 pics. The final pic, i can't really tell. Need a few more views of that.

Did you replace the O2 sensors after the harness melted? I would. For a code 41, the first thing I would try is to swap the two O2 sensors side to side and see if the Code 41 becomes a code 91. If it does, replace the sensor. If it doesn't, then it's a wiring/ecu issue.

Thanks Mike I'll try to get a better view. The mark is black not silver. But it's under the coating so for all I know it really is silver. As far as replacing parts

I put in a new right side O2 sensor.
I unplugged the left side sensor and it set a code 91 verifying the left sensor is working fine.
I swapped the harness side to side the 41 code remains telling me that my new sensor is working or the code would have turned into to a 91. Also the 41 remaining on the verified working left side tells me the harness or ECU is suspect.
I have not yet run continuity check on the harness to see if there is a second short somewhere else in the harness.
I opened the ECU and the only thing I can find is that black mark that I circled in the third picture.

Mustang5L5 · Apr 1, 2022

The black mark does look suspect but I just can't tell from the photo.

I think you are on the right track in verifying the harness and checking continuity from the ECU to the connection.

Check continuity and resistance on pin 29, (and compare to pin 43) as well as checking for good power on the Gray wire at the sensor, and a good ground on the black wire.

Gently Used · Apr 1, 2022

I dragged my old DSLR camera out of the closet to see if I could get better pictures. When I zoom in I'm starting to think the mark is the coating and not the solder melting and running.

TIGGER · Apr 1, 2022

It is hard to tell from the picture to me but that does not look burned to me from the picture. Those traces look intact. The dot looks like a test point. Only thing I can think of is that someone used a sharpie for something and did not clean it off before the board was reflowed or coated. That black mark looks too crisp to me. If it was a burn, you would see more haloing in my opinion. You should be able to clean the conformal coating off a couple of the pads between the traces to check continuity.

Mustang5L5 · Apr 1, 2022

Not thinking that’s a burn.

Gently Used · Apr 1, 2022

Thanks, unless somebody else comes along with a better answer I'm going to go with it's not a burn as well.

Gently Used · Apr 1, 2022

As I work through this, can you tell me if this is a good source for the HEGO pinout? I got it from one of the sticky's but it appears it's for an SVO. Are all the pinouts basically the same? Page 248

http://rothfam.com/svo/reference/sensors.pdf

Mustang5L5 · Apr 1, 2022

I believe it’s the same

Mustang5L5 · Apr 1, 2022

Here’s your actual 1993 wiring diagram

Gently Used · Apr 2, 2022

I think I'm getting closer to fixing it. The harness side of the O2 sensor plug is a replacement with 4 wires. Apparently when I repaired the melted harness I mistakenly spliced the unused fourth wire from the oxygen sensor plug into the harness. So the sensor was generating voltage, but it was going into the unused wire on the connector. Had I not had the computer out so I could run continuity tests and compare to the known good left side plug I probably never would have found it. The light doesn't come on instantly now but it did still have a 41 in memory. Now I'll probably have to swap sides on the harness and see if it trips a 91.

While pulling the computer was a pain, it was also a good thing. This allowed me to verify that when the wires melted together it did not burn up any traces in the ECU. It also allowed me to see that I have a two capacitors starting to leak. So now I can order the caps and replace them at my leisure.
Thanks for all the help and the diagrams. I'll probably drive it for a while now and see what happens before I do anything else.

Electrical Possible burn mark on computer motherboard

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