On 09/05/2020 14:39:50, Roger Hayter wrote:
Dave Plowman (News) wrote:

Been fixing the Lucas ECU on an old Jaguar. Similar to the type used on my
Rover.

The fault was on the 5v regulated supply. Being a 50 year old design, all
done with discrete components. Now fixed.

But looking at the design, one thing I'd like explained. The 5 volt rail,
as well as powering internal things, is exported to 'power' the throttle
position sensor, coolant temperature sensor and air flow meter.

After the regulator, and just to this external 5v feed, there is
additional smoothing etc. And between two of the smoothing caps, a diode.
Which reduces the 5v to the nominal 4.3 +/-0.2v of the external rail.

Any guesses as to the purpose of this diode?
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.

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.