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1995-2002 Range Rover P38A
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121 Posts
Discussion Starter #1
We know that the engine management ECU can display vehicle speed and fault code P1590 via the OBDII compliant K-Bus and that this data is known to originate in the ABS ECU. My question is - how does this data get there? It would seem obvious to use the K bus to send this data to both the BECM (for the speedo display) and the engine management ECU, as Rave shows that they are all connected to the K Bus, but if I monitor the K bus I can't see any traffic on it other than OBDII data. I read that the vehicle speed goes from the ABS to the BECM and then from the BECM to the Engine Management ECU via a PWM signal. Rave shows some 'undocumented' wires between the ABS and the BECM so maybe this is correct, but what about P1590 surely this must come over the K Bus? I have been having fun with ABS faults (due to a bad battery I think) and it has been setting P1590 but I can't see the communication. (I'm using the AT MA command on an ELM327).

I can sort of see why LR might not want to use the OBDII K bus for running data - so that a dodgy OBDII tool can't interfere with the running of the vehicle, but CAN based systems mix OBDII and live data, though CAN is more robust against collisions.

I'm sure some of you can guess where this is going and I know several have tried before and not succeeded. Information on non OBDII use the K Bus is very hard to find as many people are trying to make money from keeping the info secret. In contrast the BMW I Bus (very similar to the K-Bus but used for 'infotainment' systems) is well documented and I was able to build a gizmo to fool the Alpine head unit into thinking there was a CD player active and hence use the CD input for Aux In.

Of course If anybody knows how to talk to the ABS unit over the K Bus and is willing to divulge then I and quite a few others would be eternally grateful.
 

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LEGACY VENDOR
1995-2002 Range Rover P38A
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4,198 Posts
There is a fair bit if info out there on the ABS road speed signal. It's sent from the ABS ECU as a square wave at a rate of 8000 pulses per mile.

As for wiring - the speed pulses are transmitted on the Yellow/Green wire from the ABS ECU to the BECM on a Yellow/Green wire. This gets to the BECM on pin 11 of the 20 way Green connector on the front edge of the BECM.

Most of the road speed wires to other ECU's - HEVAC, Radio, Navigation system, EAS ECU, Engine ECU etc.

Apparently a good multimeter will show an average voltage value.
 
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