Two relays off fuel pump driver?

[ejs262]

Can the ms3 handle 2 relays(in series or parallel) off of the fuel pump relay driver? I want my intercooler pump to run when My fuel pump is running and that seemed like the most streamlined way to do it.

Thanks

-Eric.

[DaveEFI]

The fuel pump driver circuit is good for up to 1 amp.

[ejs262]

The fuel pump driver circuit is good for up to 1 amp.

so I should be fine then, the current draw for 2 relays in parallel should only be something like .1a right?

[Marek]

That depends on the relay Eric.
As Morrisey never said, "Some relays are bigger than others".

A relay whose winding usually pulls 50mA in parallel with another identical relay, will have 100mA flowing where the wires are joined.

kind regards
Marek

[ejs262]

fair, I am running bosch style 5 pin relays, resistance is about 65-85 ohms on the coils. I checked resistance of the 2 coils in parallel, and came up with about 42 ohms of measured resistance. did some quick math, assuming 18 volts, and low end of coil resistance of 65 ohms per relay (32.5 total resistance in parallel) for a huge safety factor:

i=V/R

I=18V/32.5 ohms

I=0..55A

With a more realistic 14 volts, that number is 0.43A, well within the 1 amp max.

is there a fault in this logic?

Thanks-

Eric

[R100RT]

What if you run both pumps off the power feed on the single contacts of one relay - seeing as how it sounds like both will be run full time - no feed back issues.

[ejs262]

What if you run both pumps off the power feed on the single contacts of one relay - seeing as how it sounds like both will be run full time - no feed back issues.

I want them fused separately so that a fault in the ic pump will not effect the fuel pump.

[DaveEFI]

What if you run both pumps off the power feed on the single contacts of one relay - seeing as how it sounds like both will be run full time - no feed back issues.

Always worth fitting a series diode to each relay coil if you're worried about interaction. Or you can buy them with one fitted internally. Of course they then become polarity sensitive.

[R100RT]

Diodes and Relays - with enough of those you could rule the world
And didn't note your concern over fuses. Measure the cooler pump draw under start up and running and place a slightly higher rated fuse post relay feed. Provide a relay common feed fuse that is above both pump draws enough for durability but still protects wiring. Or, run the second relay off the fuel pump feed, the coil draw will surely not be a concern for the fuel pump operation.