I have a 306 which wouldnt start, I ruled out the LP pump by powering it via a sealey power probe and a spare battery and the car starts and runs fine

After looking into common faults with this model and bearing in mind I probably know less about cars than the average 6 year old girl this might be a world class dumb question lol

So far I have figured out the following

The fuel on is pumped by the LP pump in a circle back to the tank and the HP pump takes only the fuel it needs

The LP pump primes for 10 seconds in the pre ignition phase to allow for a car thats run out of fuel but runs constantly once the engine has started

So rather than finding the fault thats stopping power from getting to the LP pump which icould be the pump relay, the connector, the wiring itself an ECU output or maybe even the emergency fuel cut off solenoid (its a diesel so not as risky as a petrol model) what, if any are the downsides of just powering this pump from a 15A supply that becomes live with the ignition?

The obvious downsides would be that if the ignition was in a pre ignition state then rather than just running for 10 seconds it would run constantly which if not paid attention to could flatten the batter obviously

But other than that are there any real downsides to just abandoning the ECU/Relay control of this pump and just running it straight from a hardwired 12v supply as this would "seem" to remove a hell of a lot of "common faults" from the system that seem to do little more than the 10 second priming feature before the car starts

Many thanks

Mike