On 09/05/2020 13:49, 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?


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.