Electrickery question - double relay usage

[spider]

Well sort of. The 'ticky-tack' aka tachometric / double relay. I remember the ones on the 505 (or 605?) were on the column, for 'testing' you'd just go straight down the middle set of pins to bypass it ( ! )

Why are these fitted to some mechanical diesel units ?

I can see that it would be useful with a petrol or HDi unit to cut the in-tank pump off however they appear to serve no purpose with an indirect injection engine at all, unless I've missed the point ?

I can see they are going to control the ECU power (as mine does) however a single relay would suffice for that without an in-tank pump.

So for safety with a petrol engine especially they are essential, I do not question that. I do question why they are fitted to vehicles without any in tank pumping arrangement though.

I suspect its parts standardisation or suchlike as on mine I do have an unused power feed (with a reasonably heavy wire!) present at the sender plug.