Quote:
Originally Posted by SD1too
 In other words, you don't know either! For the benefit of other members, what you have done there is to list some factors associated with the instrument pack and HRW button irrespective of their relevance to my earlier comment.
In fact you are "completely and utterly wrong" Brian because the DS2 (on board diagnostics) bus has nothing to do with the operation of the HRW and neither does the ECM, but then it sounds good quoting those doesn't it.  You have also missed the BCU from your list which actually activates the HRW relay!  Perhaps you didn't think that was important. Maybe you should have added the ICE system as well since that uses the rear window heating elements as an aerial.
Nothing you have said assists with my original statement that I would be surprised if the CAN bus has anything to do with the HRW operation, which I stand by.
Simon |
Of course I know Simon, there is a very good reason I am sole supplier of remanufactured diesel ECM modules to X-Part, and it's not because I'm wet behind the ears when it comes to the electronic systems on these cars
The CANBUS signal from the ECM is sent as high and low signals as opposed phase signals at a baud rate of 500kbit/s to the instrument pack, in this data packet among others are the coolant temperature signal, the engine speed signal, and ambient temperature signal.
This is fed into a multiplexer IC within the KMB, the very same device also interprets the information in and out from the DS2 Bus, Nav Bus, trailer bus, Window bus, in fact the KMB is the central processing area of
all of the data busses.
In the case of the CANBUS, the KMB decodes the data packet, and transmits data on the lower speed K-bus to whichever ECU is prioritised first, and the ambient temperature signal, previously transmitted from the ECM on the CANBUS, is sent to the HEVAC ECU transmitted on the K-Bus, and as there is not a valid signal transmitted forward from the HEVAC ECU onward to the BCU on the K-Bus, heated rear window operation is inhibited.
The CANBUS is always treated as highest priority in terms of data transmission over all other data busses on the car, as it is where the critical safety decisions are made, but failure particular of one transmission line, in this case instrument pack 3, compromises only the recipients of that particular data line.
The CANBUS Messages 1 is reserved for ABS operation and SRS operation, and Messages 2 relates to JATCO EATCU, I think I've already covered the operation of Messages 3
So can you appreciate now, why failure of the CANBUS controller in the ECM has the affect it does on the heater rear screen?
It's a bit like the having a gap in a line of dominoes, if all of the brown "CANbus dominoes don't fall over, the white K-Bus dominoes that follow the brown ones don't either.
Brian
