additional fuse box?

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d95gas

Member - Posts: 10

Joined: May 18, 2008

Location: United Kingdom

Posted: July 13, 2008 at 5:48 AM / IP Logged

Hi Guys,

Well tried to search for my answer, but could'nt come up with what I wanted, so here I am begging for the answer :-)

Okay car is a 95 MR2 Turbo. In the engine bay is one of the main fuse/relay boxes where there is a handy 12v Source.

I am adding some accessories that require a 12v feed. Not wanting to put strain on the existing box, I thought of adding an additional 4 way blade fuse box. I assume rather than directly linking the new fuse box to the old box, it would be best to have a relay inline.....but what I need I am not too sure about, or what terminals to use on the relay.

For now the main item going on the new fuse box is a feed for water Injection system, which itself just needs a switched 12v source (this in the old box is) and has its own 30A relay.

Within the coming weeks there will be a couple of more accessories, but probably only looking at around 6 - 10amp.

Any advice and diags would be appreciated.

Thanks

Graeme

i am an idiot

Platinum - Posts: 13,671

This member consistently provides reliable information

Joined: September 21, 2006

Location: Louisiana, United States

Posted: July 13, 2008 at 8:17 AM / IP Logged

Above are pictures of relays with a simple visual explanation of how it works. The first pic is the view of the bottom side of the relay. The numbers of the terminals are the only reason for this picture. The middle pic shows the relay at rest. (No voltage applied to the coil) Notice that terminal 30 is connected to terminal 87A. There is nothing connected to 87. The picture on the right shows the relay in the energized state. (Voltage applied to the coil) Notice 30 is now connected to 87 and there is no connection to 87A.

For your application you will need to wire the relay as follows. Terminal 30 goes to the battery via a sufficiently sized wire with a fuse located as near the battery as you can get it. 87 will be the wire that feeds your new fuse box. 86 is the terminal that is going to turn the relay on, it needs to go to a switched power source. The gray wire of the radio would be a good place to connect. This will allow your relay to come on in accessory position of the key. If you only want it to come on with ignition, let me know. I will find you a suitable location. Terminal 85 of the relay needs to be grounded.

KPierson

Platinum - Posts: 3,527

Joined: April 14, 2005

Location: Ohio, United States

Posted: July 13, 2008 at 8:34 AM / IP Logged

If you're already under the hood why not just go right to the battery with the proper sized cable? Going through a relay seems like it is adding complexity to the system, unless you need the ability to turn it on/off - which you didn't mention.

Kevin Pierson

d95gas

Member - Posts: 10

Joined: May 18, 2008

Location: United Kingdom

Posted: July 13, 2008 at 8:59 AM / IP Logged

Thanks for the info, excellent explantation and made it all very clear.

One of the major pains when working on the MR2 is that the battery is in the front of the car, the engine is at the rear, where I want to work with the electrics. Getting a cable from the rear to the front for power is not the easiest of tasks.

Hence Toyota in their wisdom stuck a small fuse/relay box in the rear and within this is a large power cable that DOES actually run directly to the batter :-) I just wanted to add additional fuse facilities at the rear.

So what would be the best option....forget the Relay and just wire a fuse box directly to the Main power cable that comes to the rear ??? I really only want the fuse box live when the ignition is powered on.

Once again many thanks

Graeme

KPierson

Platinum - Posts: 3,527

Joined: April 14, 2005

Location: Ohio, United States

Posted: July 13, 2008 at 10:56 AM / IP Logged

If you only want it powered when the ignition is on then you will need a relay.

What are you planning on using to turn the relay on and off? You will need a low current igntion source to trigger the relay.

Kevin Pierson

d95gas

Member - Posts: 10

Joined: May 18, 2008

Location: United Kingdom

Posted: July 13, 2008 at 12:51 PM / IP Logged

In the main fuse box next to where I want to mount the Aux fuse box, is a major power cable that goes directly through to the battery, this is a constant supply.

I will have to find a switched source as well, presume then I just use the info about for the relay ??

Thanks

Graeme

i am an idiot

Platinum - Posts: 13,671

Joined: September 21, 2006

Location: Louisiana, United States

Posted: July 14, 2008 at 6:29 PM / IP Logged

Being that there are fuses and relays in that fuse box, you should be able to find a switched source to trigger the relay.

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additional fuse box?

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