On Sat, 09 May 2020 15:15:32 +0100, "Dave Plowman (News)"
wrote:

In article ,
Jim GM4DHJ ... wrote:
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?

can't be that old if it has a TPS.....

TPS is as old as EFI.

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.
--
Dave W