8 replies to this topic

[woodsy483]

Can anyone share their wisdom and help me out!

I'm going to be fitting four relays that I want to be switched on by the ignition switch. The four relays will supply the megajolt unit, facet fuel pump, electric fan (via temp switch), and spot lamp 12v supply.

My question is though, how do I supply the four relays from the one 12v ignition switched live, has anyone does this and how did you do it.
Do I need a terminal busbar or another fusebox??

The relays I've bought have built in 30A fuses.

Any help appreciated!

[Bungle]

the power for the relays will come from somewhere else (starter solenoid) and the ignition feed will just power the trigger to switch the relay

a lot of people power the fuel pump from a relay using a switch from the alternator (warning light) so if you crash and the engine stalls and the fuel pump stops

[cooperrodeo]

Bungle. So if the engine is stopped and so is the fuel pump, how do you start the car in the first place?

[Dan]

Not Bungle but, you can fit a cranking bypass as used on ballasted ignition so that the pump relay will activate under cranking or alternator run. But the carb should contain enough fuel to get the car started anyway, unless it's been standing for a very long time. If it has a carb that is! If it's injection I suppose you could use a pressure accumulator to get it started.

Or you can use an innertia switch instead which will cut power in a heavy impact.

[Ethel]

The relays will draw very little current themselves, connect all their number 85 terminals to the ignition switch (the easiest way is to "daisy chain" the spade connectors with 2 wires in each, 1 to bring the voltage in & 1 to pass it on to the next relay); connect all the 86's to earth - or, if you like, an extra switching device

http://www.eraturbo....l/technical.htm.

[woodsy483]

Great help, thanks, never thought of taking the supply off the solenoid, I've already planned to use an inertia switch also.

So if I connect to the solenoid on the starter for the supply for the relays what kind of gauge/ampage of wire will I need and also is there always a spare blade connector on the solenoid to connect onto.

Does anyone have an idea what ampage the individual relays should be rated to as the relays came with 30A fuses and this may be a bit high rating for the fan or lights, etc

they will be used for

Electric fan
Facet fuel pump
Megajolt supply
Spotlamps x 4

[Ethel]

You can calculate the lights using watts = volts x amps

I'd think, 15A for the fan & 10A fuel pump (both are likely to have installation info somewhere)

The MJ will depend on what you're running off it, don't include shift lights etc that earth through the user controls but do include the Edis & coil if they're sharing the same supply. Personally, I'd consider keeping the coil separate or look at using blocking diodes. Have a google, there's loads of info on people's MJ installations online.

Use wire that gives a decent margin over the fuse blow rating - supplier's site (autosparks, polevolt etc) will have info.

[MRA]

The downside to daisy chaining electrical items is if one fails they can all fail depending on which one fails of course

The correct way to do it is to get a fused busbar, which then has fused outlets to each relay, then if something goes wrong the fuse for that appliance will blow without taking out the rest of the electrical system..... this is the same approach that most modern car electrics are wired.

When I built my last harness for my self I used a fuse box from another vehicle which meant that each headlamp had its own fused supply, if one fuse failed it didn't take out all the lights

I used the rear fusebox from an earlier (1999) S type Jaguar, they are already fitted with bus bars, and can be taken apart and easily modified

[Wil_h]

a lot of people power the fuel pump from a relay using a switch from the alternator (warning light) so if you crash and the engine stalls and the fuel pump stops

I think that you are confusing the alternator light with the oil pressure light. the alternator light would be an odd thing to use because of the way it's wired.

I have an output from my ECU (megasquirt, SC Tornado) that controls the fuel pump. so if the engine stopps then so does the pump. It also does a little prime when you switch the ignition on.

Bungle. So if the engine is stopped and so is the fuel pump, how do you start the car in the first place?

If you are running carbs, you don't need the pump running to run the engine, it'll start and run until the fuel in the float bowl runs out.

If you run injection, you need the pump running, but you'll have an ECU which will control the pump as I describe above.