[T3] HIstory Behind Our EFI

[Phil Hof]

Another note on the Bendix-to-Bosch transition; I can’t care the source but did a deep internet dive and found evidence of the following:

Two of the issues with the Bendix ECU, ones which caused Chrysler to give up on a production car, are the following:
1) The stackup of resistance variations prevented consistent operation without matching parts and adding endless calibration adjustments.  D-jet is resistance driven, and this variation mad consistent A/F ratios impossible.
2) The variation over time and temperature, as installed in a harsh automotive environment.

Bosch solved this by going to US MIL-STD parts for the ECU, at least for the key components.  Although this greatly increased the parts cost, it had several important benefits:
1) The tolerance of the MIL-STD parts was on the order of 1% instead of the common commercial 10%. This is how the stackup problem was eliminated.
2) The variations due to temperature were greatly reduced, as these parts were qualified through environmental lot testing and rated for such environments.
3) Most importantly for us 50 years later is that these parts were much more rugged, especially as exposed to temperature cycling, heat soak, vibration, and long life.

This is why ECU failures are so rare (the only exception being the output transistor legs which rusted due to poor choice of a foam base insulator).

And this is the main reason these cars are still running today!

-Phil