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Same baseline voltage in CAN H/L
Same baseline voltage in CAN H/L
Good evening,
Mercedes E -2005 with CDI engine and Bosch EDC16C2 does not recognize the ECU. Other modules are communicating with the tester but the ECU can not be reached by the other modules or a diagnostic tester.
Can H/L seems to have the same voltage but a differential signal. Bus resistors were 120 ohms from the loom and 120 from the ECU pins. Trace is backprobed from the ECU with both grounds in the same place.
What can cause the H and L-signal to be 1 volt too low and what effect does it have? Delta is still 1V in both conductors, so shouldn't the signal still be readable?
Or did I do something wrong? ECU's supply and GND is good. I did not measure the ECU's transreceiver from both pins against ground. Usually when contact or wiring resistance affects the signal, it tends to lose its peak-to-peak value and I don't see it here.
Mercedes E -2005 with CDI engine and Bosch EDC16C2 does not recognize the ECU. Other modules are communicating with the tester but the ECU can not be reached by the other modules or a diagnostic tester.
Can H/L seems to have the same voltage but a differential signal. Bus resistors were 120 ohms from the loom and 120 from the ECU pins. Trace is backprobed from the ECU with both grounds in the same place.
What can cause the H and L-signal to be 1 volt too low and what effect does it have? Delta is still 1V in both conductors, so shouldn't the signal still be readable?
Or did I do something wrong? ECU's supply and GND is good. I did not measure the ECU's transreceiver from both pins against ground. Usually when contact or wiring resistance affects the signal, it tends to lose its peak-to-peak value and I don't see it here.
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ben.martins
- Pico Staff Member
- Posts: 543
- Joined: Tue May 16, 2017 1:02 pm
Re: Same baseline voltage in CAN H/L
Hello kukushka,
Thank you for your post. I think we need to double check our connections and our measurements before condoning any components.
When you were taking the CAN H & L measurement at the ECU you mention a ground, where was the ground on the vehicle?
The resistance measurements you have recorded, did you have all the ECU's connected or were some removed from the network?
Can you try using different leads for the same measurement to ensure the leads are ok? Or even just swap the leads to the other channel and see if the voltage concern follows the lead?
I see you have a 4425, can we also repeat the test using channels C & D?
Sorry for all the questions but whenever I see an odd reading I will always blame the measurement first. Just making sure that the measurement is good could save you unnecessary time chasing of a fault that isn't really there.
Let me know how you get on and I'm sure we can help you with this going forward.
Kind regards
Ben
Thank you for your post. I think we need to double check our connections and our measurements before condoning any components.
When you were taking the CAN H & L measurement at the ECU you mention a ground, where was the ground on the vehicle?
The resistance measurements you have recorded, did you have all the ECU's connected or were some removed from the network?
Can you try using different leads for the same measurement to ensure the leads are ok? Or even just swap the leads to the other channel and see if the voltage concern follows the lead?
I see you have a 4425, can we also repeat the test using channels C & D?
Sorry for all the questions but whenever I see an odd reading I will always blame the measurement first. Just making sure that the measurement is good could save you unnecessary time chasing of a fault that isn't really there.
Let me know how you get on and I'm sure we can help you with this going forward.
Kind regards
Ben
Re: Same baseline voltage in CAN H/L
Hello Ben,
When you were taking the CAN H & L measurement at the ECU you mention a ground, where was the ground on the vehicle?
At the spring turret bolt (vehicle chassis) with a crocodile clip, both ground's connected to the same lead.
The resistance measurements you have recorded, did you have all the ECU's connected or were some removed from the network?
ECU was disconnected and others were plugged in. 120 ohms at the ECU multi-plug from harness side and 120 from the ECU pins.
Too bad the ECU is now sent to testing. If it come's out alright, I will re-do the measurement as instructed. Had to work in a hurry which is never good. Might be the measurement.
Leads were pierced through the conductor so it was not the most ideal thing to do, but in other measurements I made -H was always precisely 1 Volt too low.
When you were taking the CAN H & L measurement at the ECU you mention a ground, where was the ground on the vehicle?
At the spring turret bolt (vehicle chassis) with a crocodile clip, both ground's connected to the same lead.
The resistance measurements you have recorded, did you have all the ECU's connected or were some removed from the network?
ECU was disconnected and others were plugged in. 120 ohms at the ECU multi-plug from harness side and 120 from the ECU pins.
Too bad the ECU is now sent to testing. If it come's out alright, I will re-do the measurement as instructed. Had to work in a hurry which is never good. Might be the measurement.
Leads were pierced through the conductor so it was not the most ideal thing to do, but in other measurements I made -H was always precisely 1 Volt too low.
Re: Same baseline voltage in CAN H/L
To sum this up, the original ECU was repaired, CAN voltages were back to normal, but it was still broken beyond repair. Other modules could not communicate with it. Another ECU solved the issue, and the car runs well.