Con-fused

Post by **Peetee** » Sat Jan 11, 2003 3:00 pm

I want to adapt the wiring of the car at the fusebox to introduce an additional fusebox for new accessories on the car (radio, heated rear window etc) I also want to separately fuse some existing units (eg: lights, heater) through this but the wiring diagram for the car is causing me a headache. It would appear that items are on both sides of the fuses. All the wiring colours check out according to the relevant page in the Workshop Manual but each consumer would appear to only have a feed – and no return. It is difficult for me to follow the wires beyond the fusebox because they are still in their original, and good webbing harness.
How do I add a new fuse in for the lights for example?

Post by **Willie** » Sat Jan 11, 2003 7:23 pm

Lo, the return for each item is via the earth(chassis) as is normal. The icon for the chassis connection is the same as that shown for the Batteryconnection which is connected to chassis. Later models are fitted withan in-line fuse which protects the SIDE LIGHTS only ( under the control box).The problem with fusing the HEADLIGHTS is that they must be individually fused, otherwise you could lose both headlights whilst doing50MPH down a dark country lane which could prove fatal!!
If you wish to add an in-line fuse for the side lights then find the RED
wire with GREEN stripe which comes from the lighting switch and insert
the fuse there. DO NOT fuse the BLUE wire from the lighting switch...
it supplies both headlights on main and dipped beam!
Willie.

Post by **Cam** » Sat Jan 11, 2003 7:33 pm

The easiest way of adding fuses in for new equipment is to use in-line fuses when installing the new equipment.

Obviously these can also be added at a later date.

It is possible to add a secodary fuse box taking it's feed from the bottom of the original fuse box, but I personally do not see the need.

You may find that with all the extra connections and wiring you will actually be making the system weaker than original.

To do a 'proper job' you will need to remove the loom, take it apart and re-design it with the correct connections and wires.

The returns that you mention are the earth connections which the body of the car is used for, there are earthing points all over the body for different devices.

If you want to fuse the lights, then you would have to isolate the lights feed at the regulator and either connect an in-line fuse or connect it to your second fuse box. It is no good putting an in-line fuse in at the back of the bulb as this would not protect the wire from the fuse box to the lamp (in case of chaffing on the body).

If you seriously want the protection that modern cars have, then you will have to re-design the system.

In short - Add new fuses for new equipment (in-line or 2nd fuse box) and don't bother with the original equipment.

Post by **Peetee** » Sat Jan 11, 2003 8:20 pm

My suggestion to fuse existing items separately is based on a suggestion by a moggie owner that I read somewhere. In retrospect if it has been good for the last 43 years I can't see it failing in the time I will have it.

As for the new items I'm reasonably confident about wiring them into a new fusebox but is there any preference for where to locate the fuses in the circuit ie positive side, earth side, between switch and consumer etc?
On my last car, for simplicity, I did them on the live feed side with a common heavy duty supply piggy-backed into each fuse - but that was a modern VW.

Cheers
Pete

Post by **Cam** » Sat Jan 11, 2003 8:53 pm

It is easier to fuse the live side, although electically it does not matter.

The fuses are best located as close to the supply (battery live or original fusebox input) as possible to protect as much of the system as possible (switches, wires, etc.).

You could try using a Mini fuse box (70's - 80's) as these have four fuses (same type as the moggy), I used two of these when I built my mini's audio system a good few years ago, and they were good for 1700 watts!

Post by **an_avenger** » Sat Jan 11, 2003 11:35 pm

Living with an electrican he has made it clear to me that using a in-line fuse board (comprising of  'blade fuses) is much simpler to understand when fitting and tracing problems!  I am installing additional lighting/gauges/radio/cigarette lighter etc.... obviously they require fusing..... anyway..... i bought a fuse board (from the autojumble, NEC) for about £4 - i thought it was a bargin!  Try Derby Wiring Products (bung those words in a search engine and you should find their website) I've used them before and they are very good!  But if you are after a good fuse board that's the one to go for!  Plus you can descretly  'appropriate' the fuses from those places called scrap-yards....ahem!!!!!!!! 

Post by **rayofleamington** » Sat Jan 18, 2003 6:54 pm

Hi,
Cam already made the most important comment, but I hope you don't mind if I explain the importance of it.
The fuse should be close to the battery end of the circuit on the live side.
That way if you get a short circuit, your wiring will not catch fire!!
It does happen - trust me.
I have to stress this to my engineers at work, as you can even cause a fire with an unearthed lead light. (one of them nearly burnt out a £250K prototype car when water got in his hot-wired bulb. He was filming a component with a webcam and laptop whilst driving and had just bunged the bulb on for extra light without thinking. Fortunately they were quick enough and only the front bumper was melted....)

Post by **Cam** » Sun Jan 19, 2003 1:12 am

Ray,

Not at all, mate.

You can't overstress the importance, as an electrical / electronics engineer myself, I am only too aware of it.

It can also be a problem when incorrectly rated fuses are used. I once had an MGB melt the entire loom together from the front to the back of the car because the fuse that was fitted did not blow.

These things are so easy to avoid and so very costly to put right.

Post by **rayofleamington** » Sun Jan 19, 2003 10:44 am

Exactly!
The general rule for fuse rating is that the fuse must be lower rated than the wire. If you fit 25 Amp rated wire, then a 20 Amp fuse is 'safe', but a 30 Amp fuse would risk a meltdown/fire. (which is why you should never replace a blown fuse with a big one!!) If the actual equipment fails short circuit then your wiring should be safe.

There is extra confusion based on fuse ratings (to me anyway). Some fuses are sold on their 'slow blow' value and others on their 'spike' rating. (ie what current is required to blow the fuse in a few seconds compared to what is needed to blow it in a few tens of milliseconds)
Being a mechanical engineer I ignore this and stick heavier wire on than neccessary, as this also reduces voltage drop.
Maybe one of our 'electrical' biassed friends will set me right..

Post by **Peetee** » Mon Feb 10, 2003 7:04 pm

Thanks for all the tips folks but it doesn't really answer the original Q. (Pay attention at the back!!

)
What I noticed was that each fuse in the standard fusebox has consumers on both sides. I would expect to see consumers on one side and a supply on the other.

Post by **Cam** » Mon Feb 10, 2003 7:29 pm

Right!! Let's try again shall we?

The thick Brown wire coming from the regulator box terminal 'A' is the unswitched constant live going into the fusebox.

When the ignition switch is thrown, it brings fuse 2 on AND supplies the coil and fuel pump from the unfused side.

The lights are fed from the A1 regulator terminal directly and are not fused (except the 'pilot lamps') on later models which are fused using the inline fuse found on the bulkhead in the vicinity of the main fusebox

and......that's about it.

So, we have unfused feeds coming from one side of the fusebox, and fused feeds coming from the other.

Post by **Peetee** » Tue Feb 11, 2003 10:51 am

Cheers Cam, That all makes perfect sense now.