best place to get power for electric pump

[digger]

i'm installing an electric vacuum pump under the hood on the LHS. i will be using a 15A fuse and relay. where is the most convenient place to pull 12V, the pump should only run when the ignition is on so the source needs to be powered when the ignition is on

[AndrewBird]

There is an auxiliary fuse box on the side of the main fuse box just for this sort of thing.

Looks either like this:


or this:

[Ether-D]

Yep, one is switched, one is constant.

[digger]

mine doesnt have that

[whodwho]

Those have a big power demand, I would pull the 12v from the closest battery cable post like at starter or jumper post at the firewall. Run that to a relay and trigger the relay from the ign switch or off the 12v side of the coil.

[digger]

I’m wiring in the pump that is switched on by the ignition switch and pressure switch via a relay. My wiring diagram is as follows.



The relay is standard kind of relay with pin ID 30, 87, 87a, 85, 86 etc.

There will be an inline 15A fuse on the 12V side of the pump which is stated to draw less than 15A during operation. The pump runs through pins 30 and 87.

Do I need to fuse the ignition side that powers the relay through pins 85 and 86? If so how much current would be drawn through the ignition side of the relay.

The pressure switch is p/n 20623 here

http://www.designflexswitches.com/switches/psf109s.php

Which states a resistive load of 15A at 125VDC ? does this mean it’s a 8 to 16 Ohm resistive load, and thus when the ignition is on and pressure switch closed I can expect approx. 1.5A to 2 A to be drawn with 12V?

presumably the relay as some intenal resistance so the current draw will actually be much smaller?

[whodwho]

That looks good. I wouldn't run a fuse there but wouldn't hurt anything to run a 1 amp if concerned, the std Bosch relay draws ~200ma if that

[digger]

That looks good. I wouldn't run a fuse there but wouldn't hurt anything to run a 1 amp if concerned, the std Bosch relay draws ~200ma if that

yeah, the resistance of the relay is such that this sounds right

[AndrewBird]

You don't need a fuse as the switched 12v source is already fused. Fuses are generally in place to protect from shorts, not overloads caused by normal operation as things are designed to work without blowing fuses or burning wires to begin with.

[digger]

wiring install went fine, there was already a 12V ignition switched and fused wire for the wideband sensor, just spliced into it