Twin fuel pump relay setup

[EURON8]

Thought I had this cracked but relays don't work like I thought.

Basically we have

Intank lift pump

External high flow feed pump

What we want to do is have each pump triggered by a relay obviously.

BUT

If the lift pump stops working for whatever reason we want the feed pump to cut out so it doesn't run dry.

I have the lift pump relay setup and working as per normal.

However I tried to be tricky and run the lift pumps earth wire to the second relay as the switching voltage for the feed pump so if it cuts out it'll stop the feed pump.

Anyways apparently that doesn't work and Google has info on every kind of setup except what I want.

I've been thinking about this too long now and I feel the answer is really simple but I just can't see it.

[My name is Russell]

Flow switch after the supply pump that goes to the lift pump with an earth or something something? Or are you thinking more simple than that? 

[EURON8]

Yeah we were thinking that.

We were just going to protect through electrical failure.

To protect against a mechanical fault in the pump we would need some from flow switch in the fuel system

[EURON8]

Ok. It's as we thought. The only proper way to do it is with fuel pressure and ECUs etc to have a true fail safe.

Not really a simple way to do it right.

[downtrail]

I can think of a few ways to do this. One simple way it to have a pressure switch straight after the lift pump. You can get adjustable ones. Should see around 5psi with line restrictions etc. So itll be closed at above 5psi. This is where you take the coil wire from the main pressure pump relay through so if it drops itll drop out the main pump.

Another way is triggering the main pressure pump relay off the power feed for the lift pump. But no good for mechanical fault.

Small flow switch would work same as idea 1.

Using ecu is best as youll want to kill motor as soon as pumps shut off to start with. If you have boot up it pump shuts off. Motor leans out untill it either stops or dies etc.

You could take initall pressure switch idea and depending on ecu you could have a digital input checking fuel pump relay state. Pressure switch trips main pump digital input sees this and kills ignition as well.

Many ways to skin that cat.

[Testament]

 pumps have to be set to shut off when the engine is not running if its going to be certed to teched for NZDRA or MSNZ - so no fuel = no engine running = no pump running ?

 

the amount of time the pump will run "dry" is going to be bugger all and it wont be 100% dry. you can also return the pressure pump to the surge tank which will help. provided the lift pump has more capacity than the fuel use most of time (it pretty much has to anyway) and you have a line returning to main tank from the surge tank fuel heating will not be an issue.

 

as you say pressure sensor is more belt and braces way to do it but even that is not infallible.

[Ned]

simple relay 1 must be on for relay 2 to operate

or use the fuel pressure to start relay 2

Or just run both pumps from 1 relay...?

I run fuel pump, ignition and injectors from 1 relay so they are all on at the same time, and if the relay fails, they all fail (really i just didnt wanna wire more than 1 relay)