CEM cannot be diagnosed
 

Hello,

Im having a headache from trying to find out why CEM is not diagnosing on Volvo XC90. All blocks on LS CAN are diagnosing fine, except SRS module, but CAN seem to pass through it just fine. On the other hand everything on HS CAN is not diagnosing at all . Ive read numerous manuals about CAN network in these cars and ive read that LS CAN resistors is located in UEM and REM, both 120ohm. HS CAN resistors are in ECM and ABS both also 120 ohms. So in obd2 port measuring resistance on LS and HS can pins should show 60ohms, and if there is open circuit it should show 120ohms. So my question is does both CAN resistors must be pluged in into network in order for HS network to work at all? But on the other hand when i disconect REM i get 120ohm on obd2 pins as if there is open circuit, but everything else on LS CAN is diagnosing anyway. So it kind of denies my question 😀 but just to make sure, maybe there is something different about HS CAN?

P.s. All the experimenting was done with a new, guaranteed working CEM. I also tried disconecting modules on HS one by one to see maybe some of them is giving problems for the network.

Thanks in advance!

A can bus is terminated in 2x120 ohm resistors

If both terminations are present, you measure 120/2=60 ohms
If one termination is disconnected, you measure 120 ohms
If both terminations are disconnected, you measure open circuit.

This is a simple way to test if the terminating devices are plugged in.
It is purely a passive test. It does not confirm if can bus data is ok

The can bus is very robust and will continue to work under almost any failure scenario.
So one or two terminating devices missing will not kill the bus.

The other passive test is to check can high and can low resistance to ground. Neither should be connected to ground, or to 12V.

If the CEM is refusing to communicate, then check
Power to CEM
CEM ground
CEM can bus connection
CEM Odbii can bus connection.

In addition you can measure DC voltage on the 2 CAN wires , the low should be around 2.2v and the High 2.8 v depending on activity on the network .

As I found on 2009 XC90 manual, CEM power pins are B2,B16,B24,C4,C25.
Acc+ pins C9,B25,B11.
GnD pins B5,D6.

Please correct me if im missing something 🙂

Wiring from CEM to OBD2 is ok.
CAN connections from CEM to HS CAN modules are ok.



Voltage measuring is not the best choice in this case 😀 because voltmeter shows normal CAN voltage, but osciloscope shows literally DC voltage with some weird pulses and not a CAN like signal.