Extra fuse box

[mattyguk1]

Hi all, having now replaced my dynamo with an alternator I’m ready to wire in my previously fitted rear heated screen. I’ve decided to add another fuse box, am I correct in thinking I can take a feed from the original fuse box to the new fuse box as long as I fit a relay between two ?
If that’s correct do I use a 4 or 5 pin relay?

[geoberni]

Hi all, having now replaced my dynamo with an alternator I’m ready to wire in my previously fitted rear heated screen. I’ve decided to add another fuse box, am I correct in thinking I can take a feed from the original fuse box to the new fuse box as long as I fit a relay between two ?
If that’s correct do I use a 4 or 5 pin relay?

Why fit a Relay between fuse boxes
What would you be switching with it? A Relay is only a remote switch.
Decide what you're going to do about splitting the electrics up and fuse it accordingly.
If you're just fitting the fuse for the Rear Screen, it will depend how the alternator has been connected but essentially, just take it from the Alternator output, through a fuse to the Rear Screen Heater.
Are you perhaps mixing up the idea of a relay between the 2 Fuse Boxes, as you've stated, with the idea of having a relay to control the supply to the heater?

[oliver90owner]

The couple of high current ‘extras’ I have fitted to cars, over the years, have been wired directly from the battery live terminal through an in-line fuse. It avoids any risk of overloading the original wiring harness.

[mattyguk1]

Thanks, so if I were to wire direct to battery + terminal as well as the inline fuse I’d need to run through a relay and also the switch ? Is that correct ?

[geoberni]

To save long replies here, take a look at this page I found which indicates the different systems used to power the rear heater on various MGBs over the years.
Starting at the top...
1- An inline fuse from the supply side of the fuse box direct to heater
2- From the fuse box direct
3- Using a Relay. In this image, the terminals W1/W2 are the connections to operate the relay, C1/C2 are the Contacts being switched.

http://www.mgb-stuff.org.uk/hrw.htm
4th and 5th diagrams are complexities of designs actually used in later MGBs, so ignore them.


If you need more explanation, please ask.

[simmitc]

One thing to consider is whether you want the additional high power accessories to be active only when the ignition is on, or to be active at all times - be aware of draining the battery. If using the ignition switch to control the power, then you will need a relay that is activated by the switched circuits already on the car. The relay will switch a supply from the battery circuit to the new fuse box. The original ignition switch and wiring will not take additional high power loads.

A 4-pole relay will have a supply terminal and an earth terminal for energising the coil (electromagnet) plus two terminals for the high capacity circuit that is switched. A 5-pole relay is almost the same, but it acts as a changeover, so the high power goes from a common terminal to one other terminal when the relay is not activated and to another terminal when the relay is activated. Either could be used as a simple switch, but the 4-pole is ideal for your application. Ther terminals should be marked so that you know which is which.

[mattyguk1]

Thanks simmitc , as you say it may be better to only have it live when the ignition is on. Where would I take the power from, the the original fuse box ?

[geoberni]

Hi Matty
Let's establish the Base Line here, regarding:
A - How was the Alternator installation carried out, was it just the kit where the RB106 Control Unit is used as a Junction Box?
B - Is the original wiring at the fuse box still undisturbed, i.e. White Wires (switched live) and Brown Wire (un-switched live)?
C - What wiring and switches came with the rear window heater, was there an integral or separate warning lamp included?
D - Your skill level at installing wiring, i.e. have you done this sort of thing before?

It was a couple of years ago you were asking about wiring for the Rear Window and I'm not sure what other 'bits' you had.

[simmitc]

Matty, (a) what year is your car? (b) Please post a picture showing your starter solenoid, existing fuse box and any control box or terminal strip that is in the area. This will answer some of Berni's questions and allow us to comment further. Thanks.

[mattyguk1]

Thanks for the replies chaps, it’s a1970 4 door saloon. The rear screen was fitted a couple of years back as I needed a new one and it seemed a good idea to fit a heated one in readiness for when i needed to do the alternator conversion.
The alternator was a kit from ESM, so the original control box is being used as a junction box only. The fuse box is untouched with the white wires and brown wire as they should be, solenoid and wiring are all original apart from the new connections for the alternator.
The screen didn’t come with switches etc,I have grounded it and brought the other wire to the dash area I’ll get pics tomorrow

[mattyguk1]

Hope these are ok

[simmitc]

Lovely. There are a couple of ways to do it, buit here's what I would do. Try to use the correct colour wire to make future work easy

Take a white wire from the spare terminal on the white side of the fuse box (bottom left in your middle photo) and use it to feed the relay coil. Fit an inline fuse, this will protect things in case of relay failure (unlikely but possible) or human error, but will not blow the fuse protecting other systems, which is what would happen if you took the feed from the green side. The other fuse is not switched by the ignition. The other side if the relay coil needs a black wire to earth. The relay will now energise when the ignition is switched on.

Take a brown wire from
The battery side of the solenoid (ring terminal)
OR
the large spade terminal on the side of the solenoid with the other brown wire (join wires in one connector or use a piggy-back).
The brown wire will go to one switched terminal on the relay and then another brown wire from the other switched terminal to the supply side of the new fuse box.

Lastly, take the relevant colour wires from the fused side of the new box to feed the new circuits such as the screen heater.

Use the correct gauge (thickness) of wire and correct fuses to match the loads on the circuits.

[mattyguk1]

Excellent simmitc thanks, what size fuse is recommended for the inline fuse on the white wire coming from the original box? Would it be the same rating as the original fuses?

[geoberni]

I admit to getting a bit confused here, pun not really intentional.
Why are we fitting a relay between fuse boxes?
Why do we want to switch the new fuse box on and off?
Any additional circuits fed from the new fuse box will hopefully have their own on/off switch.
I'm not sure if the subject is fully understood by Matt or just not coming across well in the posts.
I think Simon and Matt may be at slight crossed purposes.

Are we installing a new Fuse box for future proofing or are we just installing a Rear Screen heater?

Quotes just to give them a notification....

[simmitc]

My thinking is that the heater needs its own fuse, and there are other potential additional circuits too. The additional circuits need to go off when the ignition is off - leave the heater on when leaving the car and the battery will go flat, therefore the supply to the new fuse box has to be switched; but the ignition switch and wiring are not rated to switch additional high currents, hence the need for the relay to control power to the new fusebox. Of course, I could have misunderstood what is needed. but it all seemed fairly straightforward...

[geoberni]

My thinking is that the heater needs its own fuse, and there are other potential additional circuits too. The additional circuits need to go off when the ignition is off - leave the heater on when leaving the car and the battery will go flat, therefore the supply to the new fuse box has to be switched; but the ignition switch and wiring are not rated to switch additional high currents, hence the need for the relay to control power to the new fusebox. Of course, I could have misunderstood what is needed. but it all seemed fairly straightforward...

I thought that was where you were coming from and I agree with your concerns about the Ign Sw and wiring, I'm just not sure Matt is on totally the same page as there's been no mention of the on/off switch and it's rating for example. His screen is going to draw around 7.5 - 9 Amps.
So perhaps best to have a relay on the control side?

I'm going to make the assumption that Matt had brought the cable from the rear screen up to the dash area, rather than into the engine bay.
I'd run a suitable cable from the Brown (Battery) side of the existing fuse box to a small additional box or an inline fuse holder, with a 10 Fuse.
Then have a switch* pick up from the Green ignition switched circuits behind the dash to control a relay that will switch the feed from that new fuse to the rear screen. The additional load of the relay coil on the existing wiring, about 0.15A, will be negligable.

* either an illuminated switch or have a separate warning lamp.

There's often more than one solution to a problem, and I do see how you are looking at it.

[simmitc]

There's often more than one solution to a problem

Agreed, as I wrote:

There are a couple of ways to do it

Both our ideas are perfectly acceptable, up to Matt which he uses.

[mattyguk1]

Thanks guys, I’ll report back when jobs done

[geoberni]

Thanks guys, I’ll report back when jobs done

The important thing to remember is that components need to be a suitable rating; much as the discussion a couple of years ago was about the cable and fuse rating.
Whichever way you install it, when switching the actual supply to the screen you either need to use a relay or a switch that is capable of handling the current.
Relays will generally always be OK (I don't think I've seen a car relay that is less than 20A rating, they're typically 30A or more) but many 'classic' switches advertised don't give the current rating and are often 'lightweight' in that regard (perhaps only 3-5 amp), hence my suggestion about using a relay.
But FYI, I've noticed that there are some reasonably priced higher rating examples out there, such as these:
https://www.ebay.co.uk/itm/263253023633
https://www.ebay.co.uk/itm/293586476512

Note: Switches are described by their Poles and Throws.
A Pole is the live connection inside and the Throw is what happens to circuit, is it simply an On/Off or is it a 'change-over'.
S1 is a Single Pole Single Throw, it's just an ON/Off
S2 is a Single Pole Double Throw, it's a change-over, the type of switch used in your home to control the light on the stairs from either the top and the bottom.
S3 and S4 are a single switch used to switch 2 circuits at the same time...
When applied to equipments, including vehicles, the Double Throw can be available as a basic 'Change-over', or have a Mid 'Off' position, but there's no additional letters, it would just be called 'SPDT Centre-Off'...

Switches.JPG (23.92 KiB) Viewed 1021 times

[mattyguk1]

Well I finally got around to fitting the extra fuse box and wiring in the heated rear screen today. I decided to go with simmitcs way and am pleased to confirm all Is working well thanks again