Morris Minor Owners Club

Don't forget that there are only 2 fuses for the whole of the wiring system!

It would be best (IMO) to fit a relay system, so the main wiring takes a small amount of current, and then activates a big, thick heavy duty jobby (17A might be OK) to run the HRW.

Hope this helps. I might have a wiring diagram for fitting a HRW in an old book somewhere. Won't be for a Morris, but could be worth a go?

Sounds a bit like Tom Goode's "boys book of knowledge" doesn't it?

Have you asked the supplier of the kit what they recommend, dit the kit not have instructions ?

If your wiring gets hot then it is not of a suitable type for
the heated screen. A relay is the way to go. Another thought,
you are now in danger of exhausting the battery under'winter
traffic jam' conditions with a heated rear screen.....time to fit
an Alternator at the same time??

why would I want instructions ?!!!!!!!!!!!!!!!!!!!

Oh, how well do I know that one?

Usual practice re the 'headlining wires' tie string to wire
pull through,attach new wire to string and pull it through!!
RELAY....identify which relay operates on say a FORD model
when the heated screen is turned on(Ford because they are
plentiful) and then obtain one from a breakers yard.

A local motor factor/accessory shop should have a suitable relay, I would have thought that one similar to the type used for fog/spot/driving lights will be sufficient. (with instructions! )

tie string to wire
pull through,attach new wire to string and pull it through!!

Or just neatly solder the bigger wire to the old one and pull it through without the intermediate stage of the string try and mae the joint neat and use tape to remove shar padges on the join as it may snag.
If using string the knots can be tricky as they catch on stuff.

STOP! These screens are great, I've fitted them to five travellers over the years, no porblems at all. There are two type of kit normally available: Stick on or replacement glass. One of the old stick on ones was wired in parallel, the other in series. All the current replacement glass kits MUST be wired in SERIES. If you have them in parallel they will draw too much current, overheat the wiring, and burn out the elements; no matter how many relays you fit.

Back to basics: Use wire rated to at least 17.5 amps. If you want to disable heater with ignition switched off then use a relay on the supply to the switch, operated from the ignition switch. If you don't do this and leave the heater on by accident the battery will be flat in a very short time!

Use a heavy duty (20amp) switch or make the switch operate another relay to control power to the heater. Do not pick up power from any existing wiring. Fit an inline fuse in the supply to the switch / relay.

From the switch or relay, take the cable into the element on one door. From the other terminal on the same door, take a cable to the element on the other door. The other terminal on the second door is taken to earth.

If you split the one cable to both doors, and take both doors to earth, then this is parallel, and will casue the problems that you describe, and could even cause a fire in the wiring.

General purpose relays / fuseholders are available from any motor accessory shop or mail order. Total price typically £5, not worth a scrp yard!

Hope that all makes sense. Good luck.

A relay is a good idea, but bear in mind that:

If the wire is getting hot, a relay won't help only bigger wire. If it's blowing a 35A fuse, your drawing well over 35A, and not many HRWs take this much, so the Series / Paralell suggestion is probably your problem.

If you are using 17A cable, you should be fusing at less than that (say 15A). That way the fuse blows, not the wire gets hot (sorry about the Grammar).

If you are using a relay, you'll want a battery feed. Remember to fuse this close to the battery. The fuse is to protect the wire, and if you get a short on the unfused bit, you get a fire! I see so many installations with the fuse at the 'appliance' (radio or whatever) you wouldn't believe.

you should be fusing at less than that

You must ALWAYS fuse lower than the wire.
(unless you don't mind if you car catches fire..)

You must ALWAYS fuse lower than the wire

I thought thats what I'd said?

There is a little more to it, though. The longer the wire, the more you must lower the fuse rating compared to the wires nominal rating. The current capacity of a wire is really a combination of it's size (in square mm) and it's length.

As a general rule, always use the thickest wire you can. As well as the fuse issue, the wire can affect the performance, especially of things like lights and screen heaters.

I thought that the thicker the wire, the greater the resistance, and therefore less power it will ultimately deliver... so don't go too thick with the wires - "thick enough" should be plenty.

The fuse is to protect the wire, and if you get a short on the unfused bit, you get a fire! I see so many installations with the fuse at the 'appliance' (radio or whatever) you wouldn't believe.

This was a bit lost on me... what did you mean by "fuse at the 'appliance'"? Was it that the fuse was closer to the appliance than the battery? If I was to put the fuse on the positive side, to avoid a short from the negative when the system was used, are you saying that I should fuse my earths??

I'm not the world's greatest electrician, or even the best in my town/road... er house, so I apologise if this post has been a bit dim, but could someone please put me straight? :S

No, the thicker the wire the less resistance, also the longer the wire the more the resistance.

Was was meant was that if you put the fuse as close to the battery as possible then if the wire after the fuse (to the appliance) shorts out then the fuse will pop. If you put the fuse next to the applicance and the wire shorts out then you will have no fuse in-between the short and the battery and the wire will melt / catch fire.

Aha!

Thanks, Cam. That all makes sense now... Also explains why the fusebox is right next to the battery! ;)

Crikey, I'm glad I don't have to teach Physics in Primary School... there again, maybe I'd better find out how all this stuff works...

Cam is right. That's exactly what the fuse is doing, and why it's near the battery.

The reason modern cars have so many fuses is the wires have to be kept thin because there isare so many. So every time a wire gets split to feed a number of items, each wire is fused again to protect it from the split point (which is usually in the fusebox).

Compare it to your house, and the fusing for the sockets. There is a big fuse to protect the wires from the meter to the fusebox. This is usually in the meter cupboard with the meter and varies in size depending on the house but is around 100Amps. On a ring main (the wiring to the sockets), the wires will handle around 35 Amps, so the fuse to protect the cable in the walls is 30 amps (it varies depending on if its a fuse or a trip). If you had thin wire to a lamp from a plug in a socket, and shorted this out, it might not blow the 30A fuse, so we put a smaller one in the plug, of a size to protect the wire to the lamp. If you had some electronics, there would usually be a fuse in the plug to protect the wire to the unit, and a small one inside the unit to protect any electronics.

Cars are pretty much the same, but older ones tend to use thicker wiring for everything, and can get away with just a couple of fuses. On Mogs, there are some circuits, though, that are still at risk. The output from the Dynamo to the battery is one, and the feed from the battery to the fuses is another. This is why you have to be careful when extending the wires through the bulkhead if you want to fit an ammeter as you are cutting into an unfused section and making the wire longer, therefore more risk of it getting shorted.

This doesn't just apply to OLD older cars either. A few years ago I had a Montego (C reg) that the wire from the battery to alternator shorted out and caused a bit of a mishap. I wasn't overimpressed because I was in the middle of France on my Honeymoon when it happened.

you are cutting into an unfused section and making the wire longer, therefore more risk of it getting shorted.

Yeah... that rather explains my melting ignition system, after the DPO's "secret security switch" shorted...

Cheers, guys!

I thought that the thicker the wire, the greater the resistance

I guess you may have passed GCSE science, but as Cam already mentioned... your logic has got crossed.

The longer the wire, the more you must lower the fuse rating compared to the wires nominal rating.

??
Maybe if you inted to rap the wire in bundles, then the self heating effect will increase. However the wire will have a resistance per unit length creating a volt drop per unit length (power consumption). The wire can cope with a certain amount of power (heat dissipation) as heat is reasonably uniformly lost in ration to surface area, -so in relation to unit length.
As you mentioned there are other factors at work, which really have to be considered (eg if wiring a 9kWatt shower using 50m of cable), but at the normal automotive wire lengths I've never heard of an issue to downrate fuses.
Maybe it comes from the power loss issue - eg if wiring a heated rear screen, a high proportion of power could be lost due to the wiring itself, so bigger wires are prefferable, but the fuse rating stays the same. (ie - same size fuse but bigger wires, for long runs that use high current)

Bigger wire, or smaller fuse. Achieves the same thing.

As you said, there is a resistance per length. So where the resistance of half a metre of cable may be low enough to make sure the fuse blows quickly when you have a short, with a long run of wire the resistance of the cable is higher. Under short circuit conditions where the current should be very high the fuse blows quickly, but if the resistance of the cable is too high, it limits the current and slows down the fuse blow. If the fuse takes longer to blow, the wire heats up and the resistance goes up still further, reducing the current even more. So you see how a short wire might blow a fuse, but a long wire of the same size might not blow the same fuse.
The issue is most important when you are providing supply to things that take really small currents. You don't want to make the wire thicker (cost and weight) when you don't need to, but you need to be sure the fuse will open if the end of the wire gets shorted to ground.