relay for water pump power

I have a 5 pin Relay I want to use to setup power for a bosch water pump for an intercooler. I want it to trigger from the fuel pump, but that's all I know. I am confused which wire needs to connect to what. Anyone wanna help out a new guy?

Thanks

Ground terminal 85.  Fuel Pump positive wire to terminal 86.  Fused power to terminal 30.  87 will power your water pump.

85 grounds to battery?
86 gets 12 volts signal from fuel pump wire
30 get power from battery with in line fuse
87 goes to pump
what does 87a do? nothing just cap the wire?
And the water pump has a pigtail with positive and negative wires. Just ground the ground to the body then as well?

Just trying to fully understand

Thanks

85 to the ground terminal of the battery, or to the chassis of the vehicle.  87A is not used in this application.  Ground the water pump to the battery or to the chassis of the vehicle.  It does not matter.

87A is the other contact that is "normally on". (SPDT, or "change over" contacts.)

It will be +12V (connected to #30) when the relay is of (ie - not energised by the fuel pump).
Hence best to insulate it if there is risk of shorting to chassis/body/ground. But it could be used if the relay fails and doesn't switch over (very unlikely) - the move the 87 water pump wire to 87A - but DISCONNECT IT when you do not want the pump running.

[ Another way to guard against shorting is simply to reverse the fused +12V power and waterpump wires (ie, 87 to +12V, 30 to the pump). Hence 87A is connected to the pump (ie, no voltage) else not connected to anything when the pump (30) is switched to 87 (+12V). In that case, upon relay failure (assuming the 30 to 87A contacts & connection is ok), move +12V from 87 to 87A. ]


BTW - why not use the vehicle's water pump? Or is this a separate (coolant) supply?

Separate systems. The engine water/coolant is 180-200*. This is for an intercooler that sits below the supercharger to cool the incoming air charge. You want that as cool as possible and it only needs to run when the car is running. Thanks for the guidance guys. Really appreciate it.

If it only needs to be on with ignition then I wouldn't suggest running it off the fuel pump. You can simply run it off any ignition source (ie radio, cigarette lighter, etc). By running it off your fuel pump you are creating the possibility of being stranded if something shorts out and kills your fuel pump fuse, or the pump itself. Pick a circuit that is not crucial to the running of the vehicle so you can always get home.
Just my .02

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Kenny
Owner / Technician
KKD Garage LLC
Albany, NY 12205

Though IMO that is a consideration - and not incorrect - it should not be a practical concern given normal proper wiring practices and a relay.
A relay should be of negligible load, and its wiring secure. (Its coil could even be fused if it is such a concern.) But the chances of the relay coil causing the controlling circuit's (fuel-pump's) failure is remote.

But as offroadzj wrote... IF "it only needs to be on with ignition...", then use the ignition.


But if the (water) pump is only to be on when the engine is running, then the fuel-pump or similar circuit is desired.
In the old carby days, fuel pumps were controlled by the alternator as was some other equipment like chokes, fuel cutoffs, battery isolators etc. (IE - its charge-light circuit energised a relay that powered such loads.)
With EFI came ECU controlled fuel pumps (or in some cases, (ignition) pulse sensing relays etc).
In whatever case, the added relay should be powered from the fuel-pump's relay output UNLESS its controlling (coil driver's) capability is known.
And only if the engine can run on after the fuel pump relay de-energises (carby bowls, surge tanks) but the water pump is still required should perhaps the IGN be considered (or another other source chosen - eg, alternator charge light). [ But keep in mind, despite people worrying about engine coolant heating up after the engine stops, the heat critical engine parts (head gasket etc) are cooling down! If otherwise critical, electrical engine water pumps are used - not mere fan extensions.]

I agree that the chances of it happening are slim, but you never know when a wire may get pinched... even if you were extremely careful. Its entirely possible to miss something and have a wire short out... and on the slim case that did happen I would rather lose my radio for a bit than be stranded on the side of the road or worse have the car completely cut out in traffic.

However if you need it to work with the engine running (and not just with ignition) then stick with the fuel pump.

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Kenny
Owner / Technician
KKD Garage LLC
Albany, NY 12205

The only reason I asked about the fuel pump is that is what is normally used and suggested by other Intercooler users. What size fuse should I use for the 40/30A relay. The pump pulls 2.5 amps continuous. The wire routing will be carefully thought out and protected by convaluted tubing. Bought new Battery terminals to clean up the install.

The fuse (at the battery end) will be the smaller of:
- the relay rating (ie, max 30A)
- the cable rating
- connector rating

... which must all be larger than the load current.


At the 2.5A pump end (optional?), I'd probably use a 10A fuse or breaker; else maybe 5A. (The pump should have its own recommendation).