Quote:
Originally Posted by Huskymaniac
I think I could whip something up quickly so let me know when you are ready for it. I may find time to monitor the waveform under more conditions but I REALLY hate hanging upside down and combing though those wires.
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Bumping this topic from 5 years ago, as I'd like to add information relevant to the discussion. I've posted this on the SVX Facebook group but wanted to share it here, as well.
The OBDII TCU->ECM link is pretty simple, and not nearly as complex as others in this thread have speculated. Here are the basic facts:
- The link is unidirectional from the TCU -> ECM. It appears to be a TTL-style push-pull signal (i.e. no pullup).
- It is a single 5V digital signal that runs from the TCU to pin 80 of the ECM connector
- It is a serial port stream but NOT a UART-style (e.g. COM port with standard baud rate and start/stop characters)
- The bit rate is 10ms (very slow!) and repeats every 750ms. This means there are 75 characters (bits) in the repeating stream of data
I've observed 2 states with the use of a Digital oscilloscope as well as an Arduino computer setup to log the data stream. As noted above, there are 75 bits (10ms per bit) and they repeat indefinitely. The stream begins the second the key is in the ON position and continues after the ignition is started.
Here is a snapshot of the stream (a few traces of the signal are overlapped; I apologize!):
Park/Neutral sequence:
Code:
111111111100001100100100100100100100100100100100100100100100100100100101100
Reverse/Drive/2/3 sequence:
Code:
111111111100001100100100100100100100100100100100100100100100100100100100101
Notice the small variation in the code above for being in Park vs Drive. Also notice that the pattern in both cases starts with 10 '1's followed by a '0'. We can use this to our advantage in data logging.
YouTube video of digital oscilloscope is here:
https://www.youtube.com/watch?v=1zMiSCoyKzw
I've driven the car while logging the TCU datastream and observed that no change in pattern occurs during shifting, throttle, or steering changes. The same pattern occurs in 1st, 2nd, 3rd, and 4th gears.
Given the simplicity of this, it seems feasible to do a 5MT swap and emulate this OBD2 TCM serial port link. It doesn't seem like the ECM needs it for much. So how would this be done? I used an Arduino Uno to record the TCU stream with the OBD2 automatic tranny in operation. I can use the same Arduino to transmit the patterns noted above into the ECM. This should prevent the CEL light for code for 5MT swaps.