Jules 75ZT

Continuing with my ESM P1925 Drive Authorisation fault
What should the waveforms look like?
There seems to be very little info on this.

I have a dual trace oscilloscope and have done some probing on the brown CAN pairs and the Wakeup Green pairs.

DC volts checks are pretty normal and can be seen changing after couple of minutes to the sleep state.

Also if I were to power up the ESM on a bench would I expect to see CANBUS waveforms on pin 6 and 7 of the ESM PCB or is that comms only generated from the EIS key module?


Thanks in advance

bobterry99

Check out pages 11-14 of this CAN training document for details on the CAN message wave forms, Jules.

The ESM is on the drivetrain CAN, and any module can put a message onto the CAN for other modules to read.

Jules 75ZT

Thanks for that.
Now I'm not sure if I've found something odd with my CAN B (Brown pairs) in that I've always assumed a CANBUS network should have termination resistors at either end.

My CAN C (Green pairs) measure 60 ohms but the Brown Pairs measure infinity between LO and HI.

Can someone confirm if this is correct?
I've measured at the Rear SAM and passenger sill X30/15.
Same infinity reading at both ends 🤔

Rear battery disconnect of course

pmercury

iTrader: (2)

No, you should have 60 ohm as well, or 120 Ohm between

Jules 75ZT

Double checked again and brown pairs have no network resistance ie open circuit.

Measured at the EIS N73 key switch pins and open circuit.

My virtual Star diagrams don't indicate which modules the resistors are in? Anyone know ?

Like I say CAN C measures 60 ohms
CAN B is open circuit.
Or am I on a goose chase here?!

bobterry99

In engineering parlance CAN is a transmission line. In practice, generally a terminating resistance is required at each end of the line. With CAN the value is 120 ohms. The need for resistances is dependent upon both the length of the line and the communication rate, with longer lines and faster rates requiring the resistances.

Looking at how the CAN is wired in an R230, I can not discern where there is a beginning or an end to the line. So I do not know the answer to your question. But in the case of the body CAN, your measurement suggests to me that there are no resistances; because if there were, then this would require two to be missing, and that seems very unlikely to me.

At any rate, if there are communication issues on the body CAN, then I don't see how this would have any bearing on your issue with the gear selection module.

Jules 75ZT

That's my thoughts exactly it's unlikely 2 resistors are missing.

However according to CAN networks principles of operation they require resistors to reduce signal bounce and interference, and to "prevent the signal getting damaged" was in one article.

I'm amazed this hasn't been raised before with the 230?!
But it sounds like Mercedes hasn't put resistance in CAN B, or have they?

Unless someone can confirm this on their working car for me , we'll never know?

It would take 5 minutes to access the rear SAM and take a measurement for me 😎

Maybe someone has got their 230 already in pieces ?

bobterry99

I know. You simply remove the access panel from the bottom of the storage compartment located behind the right-side seat. I'd check, but at the moment I can only see well with one eye, and simply guiding the tip of a screwdriver to the slot in a screw is a challenge.

This thread hasn't got many people reading it, so there may be no answer for a while.

pmercury

iTrader: (2)