What does the TA8903SN part located at IC17 actually do?
There’s a lot of false information about this online – actually basically all of the information available about it is incorrect with the exception of the occasional Facebook comment from another well known company within the Honda tuning space.
Do a Google search for this part and you won’t find any datasheet. We can find other datasheets with similar numbers belongings to a class of integrated circuits known as power supervisors – ok, let’s invoke spatial locality here and say that this part probably does the same thing. By the way, the “SN” at the end means lead-free. Without a datasheet available, this part needs to be reverse engineered by black box attack. This is a type of attack where you study the inputs and outputs and how these two coorespond – essentially how the input effects the output. Then you can model everything as a state machine and create an equivelant. What you created between the input and output is probably vastly different from the original chip, but functionally it works the same. The nuclear option would be dissolving the chip in fuming nitric acid and studying the die of the chip, but this is way beyond the scope of what we did.
Often you’ll see people claiming that the TA8903SN controls the fuel pump, since when it fails people notice the fuel pump doesn’t come on. This is just a side effect of the part failing.
The TA8903SN at its core controls the sleep and wake cycles of the microcontroller (CPU). When the ECU loses ignition voltage, there is an auxillary bias connected from the battery to the IC17, with a resistor in between to limit the current. We don’t want too much parasitic draw. The IC17 senses this condition and puts the CPU into a sort of sleep mode to limit its power consumption, so that the bias provided by this battery backup pin is sufficient to keep the CPU alive. The reason we want to keep it alive is because we want to keep any learned data (like long term fuel trims) while the vehicle is off. This data is stored in RAM which is a form of volatile memory, so it is not saved when the ECU is fully unplugged.
When this part fails, not just the fuel pump stops working – most of the ECU shouldn’t run. For example, if you have a datalogger or an emulator such as S300 with datalogging, you likely won’t be able to see any live data if you try to connect. If you are in a situation where the ECU actually seems to still be working with the exception of the fuel pump, then you can look at our STA413 product.
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