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Author Topic: Setting own DTCs - limp and MIL  (Read 5119 times)
woj
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« on: November 14, 2017, 03:55:37 AM »

The thing refers to ME7.9.10, ST10 based. This is not about removing DTCs, quite the opposite. I am trying to find out at which point the DTC decides to put the car into limp mode or flash the MIL. Is it in the bit field that is passed to the set_DTC procedure (the one that takes a 16 bit field through [r0] and the DTC index through r12), or does the error class define it? Or is it somewhere else? In particular, I have this one huge array of data that seems to be DTC related and I have not yet managed to decipher it.

I am planning to hijack one of the DTCs in the ECU that is not used in my case (electronic clutch engine switch-off), but I cannot get my head around the semantics of bits and data that I need to set for this.

Any help is appreciated.
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prj
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« Reply #1 on: November 14, 2017, 08:01:11 AM »

DTC does not put car into limp this is done in code and DTC has nothing to do with it whatsoever.

MIL on/off is decided by fault class and fault class configuration.
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woj
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« Reply #2 on: November 14, 2017, 09:02:56 AM »

Thanks! That simplifies some searching (but will have to figure out limp activation anyhow). I already figured out that this large array I mentioned is indexed by the error class (not exactly, but in practice), so I am guessing this array defines what happens with the DTC (lamp, recording freeze frame, etc.). I can also see that it might be possible to simply add my own DTCs, there are empty slots, plenty, in the DTC codes table. But I will first try to play with an existing one to see what the bits passed to set_DTC in [r0] mean, before I do something that messes up the EEPROM entries for DTCs.
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prj
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« Reply #3 on: November 14, 2017, 12:53:37 PM »

Limp activation is whatever the code says it is, as I said, this is completely separate from DTC's, no connection whatsoever.
Truly unified diagnosis disabling came with MEDC17, where each DTC is bound to a diagnostic routine, and the entire diagnostic routine can be switched off using a disable mask.
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woj
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« Reply #4 on: November 16, 2017, 12:17:38 PM »

Sorted, all in all wasn't that difficult:

https://www.youtube.com/watch?v=IbbBHgS1Dm8

On the DTC side, that is. One thing I do not know yet if the class that I have chosen flips the MIL on. The status of the DTC says no, but so it does for all the other DTC when I have them enabled and the ECU is on the bench without an actual MIL connected.

Following your hint I looked into the limp mode as well and decided to give up on the idea (to trigger one on the sensor fault). Trying to hijack any of the existing limp modes would result (my slightly educated guess tells me) in the other procedures immediately disabling it back, or freaking out otherwise. Implementing a whole new limp mode is beyond the purpose - with a faulty FlexFuel sensor the car would limp anyhow and the MIL on the dash would tell me to go easy on the engine anyhow. 

To be continued... (I guess I should also move these ramblings about my developments into one place instead of scattering it all over different topics).
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prj
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« Reply #5 on: November 19, 2017, 05:21:03 AM »

Simply add a bitfield and an additional condition for rlmax. Nothing difficult.
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woj
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« Reply #6 on: November 19, 2017, 07:50:54 AM »

This is what previously working only with cable operated throttle body systems does to you - you only think of ignition cut based solutions, while something much easier is in front of your eyes. Thank you sir!
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