Hi George
Diode relays were introduced to help prevent back EMF to the ECU. The switched Earth effect can be seen when you connect a battery or jump leads. Much smaller spark from the earth side of the battery than the feed.
At Volvo we stopped teaching wiring colours years ago and worked from pin numbers because of wiring colour change issues (we had the advantage of also having a BOB - Break Out Box- fitted between the ECU and multi pin connector which helps massively).
Not what I joined the trade to engage with but just what the industry evolved into, give me a gearbox to overhaul any day.
BR
Colin
