Wiring diagram for installing a rear windscreen demister

[dennissutera]

Can anyone provide or advise me of the above? I've got a 1957 4 door morris minor 1000 saloon & I've got an old but unused 12v Air Flow Major demister, with on/off switch, 2 rubber suckers 1 at each end, that I'm thinking about installing. It contains a wire filament & when I temporarily connected it to a spare battery & switched it on it seems to work & radiate gentle warm air. My car is fitted with an alternator & negative earth battery. As there's not any extras fitted I'd hoped that I could fit a Duritc push/pull illuminated switch on my dash-board & wire it up using a 35A (glass) in-line fuse & 17A single wire from Halfords, marked suitable for rear screen heater. Thank you in advance.

[The vast minority]

Hi
Couple of points, there is no such thing as a negative earth battery and you cannot fit a 35 amp fuse in-line in a 17 amp cable (well you can, but you most certainly shouldn't) because the wire is then the fuse and the fuse is not a fuse. This is how fires start in DIY autoelectrics

Id recommend some general reading on DC electrics as a starting point and it will be simple to work out from that point.

[BrianHawley]

Can anyone provide or advise me of the above? I've got a 1957 4 door morris minor 1000 saloon & I've got an old but unused 12v Air Flow Major demister, with on/off switch, 2 rubber suckers 1 at each end, that I'm thinking about installing. It contains a wire filament & when I temporarily connected it to a spare battery & switched it on it seems to work & radiate gentle warm air. My car is fitted with an alternator & negative earth battery. As there's not any extras fitted I'd hoped that I could fit a Duritc push/pull illuminated switch on my dash-board & wire it up using a 35A (glass) in-line fuse & 17A single wire from Halfords, marked suitable for rear screen heater. Thank you in advance.

Does it have an information plate showing how much current it draws, or the wattage? (Divide wattage by 12 to get the current.)

Wire and fuse need to be appropriate to the current.

Also, DC motors run backwards if connected the wrong way round so it needs to match your negative ground car if the case is connected to one of the supply wires or is used as a current return. Does it have two wires, or one and the case is the other connection?

Is it the very old Bakelite case Air Flow Major, or the metal cased one?

[dennissutera]

Thank you for the 2 replies so far. In reply to the first one, I should have stated negative earth, I apologise for this confusion. Regarding the point about 17A wire & considering using a 35A in-line fuse & possible adverse consequences. According to the note on the spool, the wire is rated up to a max'm of 200A, so I'm unsure about whether or not the wire would be the weakest point. I confess to not being electrically minded, hence seeking helpful advice from more experienced members, before starting any work. Regarding the second & possibly more helpful response from Brian Hawley, the demister does not indicate an amperage/wattage rating anywhere. I'm also confused as to the reference to DC motors as there's no motor within the demister. From one end of the case only, protrudes 2 wires, 1 red the other black, both diameters are physically smaller than the intended 17A wire I anticipate using. It's the metal cased Air Flow Major.

[BrianHawley]

I had assumed a motor to blow the air over the element, but I guess I’m wrong. It must just work by convection. If there’s no motor to hear, you might like to consider adding a warning light so you don’t leave it on by mistake.

With no information plate we are flying in the dark here to some degree. But we can make some guesses. Let’s say it’s about 300 Watts. Divide by 12 volts, it should draw around 25 Amps. Probably it’s a lot less, but let’s take that as a middle figure.

If your cable is rated to 200 Amps, it’s well in the clear, even if we are way out on the power estimate. Can you check that? Where does the 17A come from? Any other info printed on the cable or drum?

Regarding the fuse, that’s there to protect the cable, not the load. The fuse should be rated significantly less than the cable, but more than the load. A figure of 1.25 times the load is typically quoted as a minimum. So we need a fuse of more than 31.25 A and significantly less than 200 - so the 35A fuse is theoretically fine.

So much for theory. But we are guessing a lot of things here. And caution is needed any time a fuse bigger than 10A is used.

What would I do because of all the unknown factors? I would mock the whole thing up on the garage floor using a battery, fuse, realistic length of wire and the heater. Run it for a good long time and see if the fuse blows or the wire gets warm AT ALL. If it does then you need thicker wire. And I would start with a 5A fuse and work upwards if it blows. My guess is the heater is more likely at the lower end of the power range, although we don’t know that for sure.

There’s a slight chance that the whole thing could go up in flames of course, but at least it’s not the car.

Remember of course to take the power off an ignition switched circuit on the car, so the heater can’t be accidentally left on. If it’s a circuit with its own fuse and your new wire is the same or thicker than the existing wire, then you are protected and may not need a separate fuse. But you may blow the existing fuse if you exceed the original designed load. There are ways round that, but let’s cross that bridge if we come to it.

[BrianHawley]

Oops missed a point in your second post that makes much of the above redundant. If the wires coming out of the heater are thinner than the wire you plan to use, the wire is fine.

I’d just hook it up to an ignition switched fused circuit and observe carefully.

[The vast minority]

Sorry if my response wasn't helpful.
I sought to illustrate that its necessary to have a good understanding of DC electrics if you want to DIY them.

Let me know where you get that 200 amp 17 amp cable, sounds like pretty good stuff.

[GavinL]

With respect to the warning light issue, for those that are interested, you can get a 20A 10 minute delay timer,
https://www.autoelectricsupplies.co.uk/ ... tegory/210 which seems like a good idea, you can wire a warning light across the output as well, but 10 minutes should be plenty to demist the window. if you want further flexibility, they also do an adjustable relay for between 0.5s and 6 hours.

[BrianHawley]

With respect to the warning light issue, for those that are interested, you can get a 20A 10 minute delay timer,
https://www.autoelectricsupplies.co.uk/ ... tegory/210 which seems like a good idea, you can wire a warning light across the output as well, but 10 minutes should be plenty to demist the window. if you want further flexibility, they also do an adjustable relay for between 0.5s and 6 hours.

Handy tip!

Now if they’ve got one that will push the choke in for me...

[oliver90owner]

Seems like this thing (if it draws a high current) should be wired via a suitable relay, not directly through the ignition switch. I doubt the wiring loom would cope with an extra load of 25A or more...

Question: Do you have an alternator fitted? Using this for any length of time, particularly in the winter with lights and other ancillaries, could easily exceed the output from a standard dynamo.

[amgrave]

Read the post properly, he states he has an alternator fitted. Good idea re the relay though if it's a heavy draw.

[dennissutera]

Gentlemen, thank you all for your advice. I've a 1981 booklet titled Autodata Car Repair Manual Minor 1000 1956-71. Towards the end there's a section on installing a rear screen heater, with 2 examples, the first & more simpler, shows a diagram of the heater circuit without a relay, the second diagram is with a relay. Both diagrams indicate via ignition circuit, using a 35A in-line fuse, the heater shown is from Lucas. My intention is to make a temporary mock-up in my garage. My metal Air Flow Major demister has an on/off metal flick type switch, no motor, just a single filament & the heat given off is gentle convection. I intend using an illuminated push-pull switch as my primary means of turning the demister on/off, as well as a visual reminder. As previously stated there's no rating plate indicating amperage/wattage on the demister & so far searches on the web haven't provided any information on rating. The demister has 2 wires , red & black, approx. the same diameter as an 8-10A spool from Halfords. I was thinking of fitting the in-line fuse, initially 5-8A, near to the demister, rather than the ignition side & see how the mock-up performs. Meanwhile I will look into the option of a delay timer.

[BrianHawley]

Is this it?

s-l1600.jpg (363.63 KiB) Viewed 3982 times

Infuriatingly it says "low current" but not how low.

I would guess this is a fairly low powered unit and should not need a relay - but no guarantees. Could try it without and see if it blows a main fuse.

I stand to be corrected, but my guess is it is under 5A.

Your test rig fuse can be anywhere if you are just checking to see how small you can go before it blows. Your final working fuse should be at the battery end of the cable as the fuse is to protect the cable, not the heater.

[dennissutera]

Brian, yes that's the one, although mine didn't come in a box, it seems from the description that the body is plastic (bakelite?), apologies as I thought it's a black metal casing. Does the box in your picture refer to a date? Regarding your suggestion of the "final working fuse should be at the battery end of the cable", my intention is to start with a fairly low rated fuse say 5-8A, especially as your "guess is under 5A". As the demister will be mounted at the bottom centre of my rear windscreen & there's a small shelf below & the thinnest wires in the system are the 2 on the demister, I thought it might be safer & easier to locate the in-line fuse at the demister end. There's little flammable mat'l present & it's visible/accessible, just in case it over-heats & there's a good local earthing point. Route the new feed wire previously 17A, (but maybe lower) down the rear seat squab, under the sill carpet driver's side, up to a new illuminated push-pull switch mounted below the right-hand glove box, then through the opening (via grommet) adjacent to the driver's scuttle into the engine bay & connect to the ignition side of the fuse box. I have a multi-meter & thought about trying to check the amperage/wattage drawn by the demister in a mock-up mode. Any comments will be greatly appreciated.

[philthehill]

In my experience these demisters have a poor performance record as regards demisting a window.
At best it will be a talking point.
Phil

[BrianHawley]

Brian, yes that's the one, although mine didn't come in a box, it seems from the description that the body is plastic (bakelite?), apologies as I thought it's a black metal casing. Does the box in your picture refer to a date? Regarding your suggestion of the "final working fuse should be at the battery end of the cable", my intention is to start with a fairly low rated fuse say 5-8A, especially as your "guess is under 5A". As the demister will be mounted at the bottom centre of my rear windscreen & there's a small shelf below & the thinnest wires in the system are the 2 on the demister, I thought it might be safer & easier to locate the in-line fuse at the demister end. There's little flammable mat'l present & it's visible/accessible, just in case it over-heats & there's a good local earthing point. Route the new feed wire previously 17A, (but maybe lower) down the rear seat squab, under the sill carpet driver's side, up to a new illuminated push-pull switch mounted below the right-hand glove box, then through the opening (via grommet) adjacent to the driver's scuttle into the engine bay & connect to the ignition side of the fuse box. I have a multi-meter & thought about trying to check the amperage/wattage drawn by the demister in a mock-up mode. Any comments will be greatly appreciated.

No harm in having an extra fuse at the demister end, but the wire still needs to be fuse-protected at the battery end. As my old electronics lecturer used to say, well scream at the top of his lungs actually, the fuse is to protect the cable. But depending on where you take the feed, it’s quite probably fused already.

Yes, you can measure current with a multimeter. But you need to be more careful than when measuring voltage. The entire current flows through the meter (assuming it’s not a clamp type) and you can blow its internal fuse if you are on the wrong range or have it the wrong way round. How do the meter leads compare with the leads on the heater? If they are thinner I wouldn’t try it.

Easy to over think these things. Myself, I’d just try it with a fuse as a mock for 20 mins or so, then go for it.

[BrianHawley]

In my experience these demisters have a poor performance record as regards demisting a window.
At best it will be a talking point.
Phil

Cool to have it but cool in temperature? Boom boom

[dennissutera]

Brian, thank you for your advice, the wires on my multi-meter are about the same diameter & it's fitted with an internal fuse, however, my friend has an industrial type, that has a higher fuse rating, so I will ask him to check the amperage on my mock up. I'll install a second in-line fuse at the battery end, as it's better to be safe than sorry.
Phil, I realise that this type of demister doesn't give off a lot of heat, however, as I've got it I might as well try it out, rather than leave it on a shelf.
Finally, I like to thank everyone for their contributions.

[BrianHawley]

Good luck and let us know how it goes.

[dennissutera]

Hi Brian, I'm at a branch mtg tonight & hopefully the guy who's got a better multi-meter, which I think has a 20A internal fuse, mines only a 5A, will be there & I will ask him if I can borrow it. Meanwhile, I've inserted a 10A fuse (I've got a range of other lower/higher ones ready) near the spare battery end on the mock-up in my shed, but the weather's been dreadful. I'll keep you apprised of progress.