Fuel pump amperage

[moggles]

I'm building a remote immobiliser system (long story!), with 4 separate independent feeds to standard SU fuel pump, coil, solenoid and everything else.

I need to know the amperage of both the fuel pump and coil circuits to decide whether I need to use high amp slave relays to switch them.

Can anyone advise the pull-in amperage of the fuel pump? Cos it's essentially an instantaneous pulse, a voltmeter won't react fast enough - I really need an oscilloscope to see the amp draw.

Anyone help with either or both fuel and coil amps?
Ta

[Alec]

Hello Moggles,

running I think about 1 to 2 amps for pump and coil.

As for instantaneous current, do a resistance check and use ohms law, that will give the start current which drops off as the coils energise.

However, I don't think that it is a real worry, after all the ignition switch copes easily enough, and I assume you will close the disabling switches before switching on the ignition anyway so they will only need to be rated for running current.

Alec

[moggles]

Thanks Alec,

My circuit should theoretically handle up to about 2 amps per output. I think that should be ok for the coil, but I suspect is at best marginal for the fuel pump; and I will have to drive a slave relay for the starter solenoid. I will see if I can get a static resistance reading on the fuel pump as you suggest, but I still reckon the instantaneous spike may be a problem.

Cheers,

[bmcecosse]

As Alec says - the in-rush current is entirely dependent on the initial resistance you measure on the coil. As the coil saturates - the current decreases.