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Originally Posted by SD1too
My research was based on my KV6 which, of course, uses a different ECM to Edward's M47R. Is there a different mode of operation at work here between the two?
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There is no difference in the operation of the KV6, Diesel or 1.8 ECMs they all use the Can-bus to input the ambient air temperature to them.
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As I indicated earlier, the exterior temperature signal bound for the BCU doesn't go anywhere near a CAN bus on the KV6.
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I think that you haven't quite grasped the way the data bus systems work it's not layed out in a wiring diagram.
The ambient air temperature signal goes to the IPK multiplexer IC where it's converted to Can-bus and K-bus signals. Consider it if you will as a connection.
As we both explained above Can-bus messages always take priority so if ECM loses the Can-bus signal then the IPK multiplexer IC effectively blocks the ambient air temperature signal from being transmitted on the K-bus.
Fortunately the Can-bus controller on the Siemens 2000 ECM seems to be more reliable, but the same symptoms can happen if the Can-bus wiring is damaged.
On the Diesel models it's nearly always the Can-bus controller in the ECM that's failed.
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I have to agree with Colvert that it is absurd, on the diesel at least, that the HRW cannot be operated manually in the absence of an exterior temperature signal. It's interesting that TOAF can apparantly disable this restriction. Can T4 do the same?
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Nearly modern cars have multiplex wiring systems that incorporate data busses and they can be a real problem when you're presented with faults, seemingly unrelated systems can and do effect others
The wiring and data systems on our cars are down to BMW a decision was made at some point as to how the vehicle systems would operate and unfortunately for us we're left with this absurd operation of the HRW. They didn't think it was necessary to give the option to change this so T4 doesn't have this option in it's software.
TOAF probably recodes the BCU to ignore the ambient air temperature, they seem to have included some other stuff as well such as one shot up on all windows with no anti trap Ouch!!!
It would be interesting to see if this affected anything else on the vehicles systems.
Russ