One for the electronics types.

A number of possible reasons. They may have wanted 4.3V for a reason. Most likely, though, in case the supply got connected to 12V either due to a wiring fault or because someone assumed all live connections should be 12V.

They put it to give some union wanker a job.

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I assumed the latter, where either accidentally, or intentionally the pin is touched or strapped to 12V.

I've worked on 24V equipment and the criterion was similar. Where the unit must not be damaged by applying 24V to any pin.

TPS is as old as EFI.

Doubt it, as it merely feeds what are effectively pots.

That does seem the likely explanation. Although there is also a zener to take care of possible over voltage.

The zener will only protect from over voltage if the current is limited to less than the power rating of the device. If the protection diode wasn't in place and 12V from a battery, capable of supplying 10s or 100s of Amps were connected t0 the 5V output the zener would instantly destruct, possibly giving a short circuit.

How big is the smoothing cap after the diode? If large, maybe the diode is to let the 5V source be switched off instantly without being held up by the capacitor until it empties of charge.

Is there anything on it that could feed back a voltage of the opposite polarity to the ecu? Brian

The diode would not help with that problem.

The electrical environment outside the ECU is harsh - my money is on stopping ECU operation being affected by imported "crap" arriving via the power supply lines.

without seeing the circuit, hard to know. I've used diodes before as nonlinear resistors in cap - nonlinear R - cap filters, but that's not a common way to use em.

NT