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Fault finding CAN bus lambda and egt not working


doninphi

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Hi there.

I'm helping a friend on his 350z plug in which has twin turbo. It has two link canbus lambda sensors plugged in as well as 6 EGT sensors one per cylinder.

He says he thinks one lambda stopped working first. Now the lot failed. Or perhaps all at once.

Hes continuity tested all wires and they seem fine.

I'm no CAN bus guru at all. But how can I isolate and test each part or the system? Is there a way to fire it up and test it or find faults in it other then resistance checking? 

I bet you'll say oscilloscope and we dont have one or the knowledge.

Ideas?

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How are the devices connected to the ECU?  On the 350Z ECU CAN 2 does the OEM bus (dash etc) and CAN 1 is not externally available.  Have they added another another CANPCB cable and drilled a hole in the case or were they swapping the USB connector with CAN after tuning?

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6 hours ago, Apex Speed Tech said:

Actually, you should be at about 60ohm if its terminated properly at both ends.

That's the kind of comment I wanted thanks. I guess if its higher then things like unnecessary connections. A partially broken connection or termination issue would cause the resistance to go up? 

Thanks 

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12 hours ago, doninphi said:

From the main plug into the ECU. Getting a resistance value of 100 ohm

Can you tell us or show us how these devices are connected to the ECU?  As I mentioned earlier there is no CAN plug on the 350Z ECU so unless someone has added one, you are possibly measuring something irrelevant?  There is a spare CAN port inside, but when shipped from Link it is not connected to anything external.

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2 hours ago, Adamw said:

Can you tell us or show us how these devices are connected to the ECU?  As I mentioned earlier there is no CAN plug on the 350Z ECU so unless someone has added one, you are possibly measuring something irrelevant?  There is a spare CAN port inside, but when shipped from Link it is not connected to anything external.

Its my car Phil was talking about so to answer your question: I guess the guys that installed the ECU for me must have drilled a hole for the canbus wiring as there is a hole in the front of the case that these wires go into. The USB cable is still there permanently so that hasn't been replaced.

I've taken the ECU case off and done continuity test right from the pins on the circuit board to the ends of the wires where they plug into the little boxes that run the O2 sensors and all was fine. 

FYI here's a screenshot from the link software if that helps.

To clarify, the O2 sensors and EGT sensors that run on this canbus wiring were all working fine for several months. Then one O2 sensor stopped working, then about 2 months later the other O2 sensor and EGTs stopped.

Screenshot_20200316-113653_WhatsApp.jpg

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13 hours ago, doninphi said:

That's the kind of comment I wanted thanks. I guess if its higher then things like unnecessary connections. A partially broken connection or termination issue would cause the resistance to go up? 

Thanks 

Yes.  At the end of the day, this is 2 resistors in parallel, nothing more.  So if you have a CAN bus and its supposed to have a 120ohm (100 works in my experience too but 120 is the spec) then:

image.png.ec3e484a91b5fa87ccbf020f3867e1c6.png

 

So 1/(1/120 + 1/120) = 60.  This means only 1 side of your bus is terminated. 

8 hours ago, ChrisD said:

To clarify, the O2 sensors and EGT sensors that run on this canbus wiring were all working fine for several months. Then one O2 sensor stopped working, then about 2 months later the other O2 sensor and EGTs stopped.

CAN is one of those weird things that, in my personal experience, works sometimes even when it well, shouldn't.  You can "get away" with no terminal resistors or 1 terminal resistor sometimes... then stuff just stops working.  So I'd fix the bus resistance for good measure anyway.

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