Quote:
Originally Posted by cb750chris
Is it definitely the missing ambient air temperature signal that inhibits the HRW?
... would the more likely data between ECU and HEVAC unit be the engine RPM?
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That's a very good question Chris.
RAVE does say that the ATC [HEVAC] ECU receives "vehicle speed" data from the IPK and "main" K-bus. That seems to be misleading Brian because the IPK sends "Engine speed/road speed" to the HEVAC & BCU on the K-bus (Body Electrics Workbook 1999 page 60) which it gets from the ECM on the CAN bus. "Ignition switch status" is also sent by the IPK on the K-bus to all ECUs.
I have also been puzzling over this part of Brian's explanation:
Quote:
Originally Posted by marinabrian
... and the ambient temperature signal, previously transmitted from the ECM on the CANBUS ...
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As the IPK receives the ambient temperature signal directly from the sensor (C0233-4) why would it need it again on the CAN bus?
Significantly, RAVE says that the ATC ECU receives
"Ambient temperature from the ambient temperature sensor in the bumper via a hard wired connection to the instrument pack and the main K bus".
Any comments on that Brian?
The second thing that is interesting about Brian's account is this:
Quote:
Originally Posted by marinabrian
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.
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Brian seems to be saying that the K bus route of the exterior air temperature data is:
IPK > HEVAC [ATC] ECU > BCU.
The wiring diagram and RAVE explain that the ATC [HEVAC] ECU uses a special K-bus line called "K-bus 2" for communication with the BCU. The control signals listed are:
1. Fresh/recirculated air intake flap servo motor.
2. Air conditioning compressor.
3. Diagnostics and ...
4.
HRW!
It would appear that the purpose of this is when defrost mode is selected and the ATC ECU instructs the BCU to activate the HRW.
Simon