The Triumph Dolomite Club - Discussion Forum

GrahamFountain wrote: ↑Thu Jan 21, 2021 3:34 pm Well if I believed a 500W heater bought for £14 off ebay was, even at 13.8v, 500W and would drew anything like 36A (where does 48A come from?), I might be concerned. If it draws as much as 10.4A (500W/24V*12V/24V) off a nominal 12V supply I'll be very surprised - the only thing that cheap on ebay is hyperbolocks.

And I've got high capacity supply cabling in for a sub amp, i.e. for the low impedance, I was gonna fit on the back ledge but I never did. That should do nicely (the amp would probably have de-misted the back screen too).

And I do have an uprated alternator, though I forget what it is.

And all the aftermarket, stick-on demisters I've ever seen were crap. So I didn't look. They may have got better in the last 3 or 4 decades though - I hadn't thought of that. But £47.64 inc. drat & P&P seems a lot just to find out. Or is that being a bit Yorkshire.

And are you allowed to drive off, i.e. need lights and wipers, before the screens are clear?

Graham

I kind of (wrongly it seems ) assumed 500w at 12v (500/12 = 41.6A). To give 500W at 12V would give an element resistance (when hot) of 0.29 Ohms (12/41.6) Up the volts to 13.8 then the same 500W element will have the same resistance ( or very close to) so 13.8 / 0.29 = 47.6A ! So that is where the 48A came from. You cannot take a 500W element and assume that the current is the watts divided by the applied voltage, that will only be correct at the rated or specified voltage, which I assumed was 12V.

Anyway you seem to have it all worked out so I'll leave you to it

Roger

Carledo wrote: ↑Thu Jan 21, 2021 3:16 pm My wiring diagram (>VA15000) shows the HRW having it's own dedicated terminal on the fusebox, 1 of 5 green leads so connected. My memory tells me that of those 5 greens, 2 are siamesed together on a single tag and the other 3 are singletons, however I can't say for sure WHICH 2 are siamesed, I THINK its the VS feed and rev counter/voltmeter feed but could be wrong. However it works though, the HRW is DEFINITELY fed from the fused ign live on ALL models. If you want to fit a relay (and I have no quarrel with THAT, i've done it myself) i'd keep the switching feed ign live and power the relay from a permanent source.

Dunno about the hot air blower rear demist, it's an interesting concept and may well clear the window more efficiently than the original heater element. But it will involve some unsightly plumbing and the matter of current draw, big wiring and possible bigger alternator requirement make, IMO, a good used window a more practical and cost effective proposition. A toughened HRW is a lot easier to fit (and less potentially disastrous if you get it wrong) than a laminated front screen! I recently fitted both rear AND front screens to a Sprint in less than 1/2 an hour (empty shell after colour change respray and new vinyl/headliner) and had them sealed in and mouldings in place in not much more than another 30 mins. On my own!

Steve

It was that the connection was moved from the switched to the unswitched side of the fusebox at some time - could have been me even. But there's only two green wires on that one Lucar connector. I think the other one is the reverse light switch supply. So had I parked it in reverse and did the reverse light actually work, I could have drained the battery that way.

I've got to look again at the vent holes in the back shelf. If memory serves, the two outboard rectangularish holes under the cover have the magnets of the subs hanging through them (I don't care that they share an infinite baffle - they're only subs), but the inboard sort of half trapezoidal, wider at the back ones are free and, I think, nicely placed for a pair of vents. So if the tubing's the right size to fit, I might get the old Herald vents mostly in those holes, and they'll hardly show - might need a bit of MDF as an escutcheon round them, covered in that sicky backed leather effect vinyl. So then the wiring, heater box, and pipework will all be in the top front of the boot, out of sight and reasonably out the way of the shopping. If the current draw really is a problem, I'll have to look at a switched mode DC-DC converter for the heater supply to limit the draw - leaving the fans on the full 12. But they're only a few quid and 3 weeks from China. But I'm not expecting it to be an issue, if I guess what they mean by 500 Watts on ebay - I'm betting on under 150 actual Watts at 12-14V if it's 500W at 24-28V (on a downhill slope with a following wind). But it does depend on the thermal co-efficient of resistivity of the element, and its two temperatures (by proportion in K, not C) (fan's going twice as fast too), so it's only a guess pro temps .

The back window was recently out to replace the rubber (and for an insurance job respray of the front end), which was in a bad way at the bottom nearside corner. I guess that's when I should've done it. But I didn't. I might still think about it if I can re-use the newish rear screen rubber and I can get a cheap screen delivered to the soggy bottom (left-hand corner of the Kingdom of Jorvik) that is the Fylde. But since I've got the heater box on order, and the wiring mostly in place, I think I might have a go at the venting and such - assuming there's a dry-spell in West Lancs.

Graham

Assuming the resistance is constant, which it won't be, then halving the voltage will give you 1/4 of the power.

Power = Volts Squared / Resistance.

marshman wrote: ↑Thu Jan 21, 2021 6:14 pm Assuming the resistance is constant, which it won't be, then halving the voltage will give you 1/4 of the power.

Power = Volts Squared / Resistance.

Well, it's not impossible the resistance is constant, because if it were and thus running half the current it would be cooled by about half the airflow. So it could be about the same temperature and so be about the same resistance. That's a bit of a crude analysis even by my sloppy standards. But the point is that it won't be as far off as it would be if the airflow were constant.

Graham

Cheap electrical heater off of eBay driven via a cheap DC-DC highish current converter/controller from a similar source, running in an enclosed space at the back of the car ....... having seen the quality of some of this stuff I would be very careful about how it is all mounted and assume that it will get hot and possibly emit flames. Cheap stuff manufacturers are not known for their concern over using the correct materials - i.e. decent "high temperature" flame proof material supporting the heating elements, or bothering about proper design practices on PCB's or fail safe designs. Assume the worst will happen and fabricate the enclosure accordingly, and make sure you have a decent fire extinguisher!

What heating element does the heater have? Ceramic or some sort of wire? Ceramic is inherently safer.

I'll be having a look at the infernals of the heater, but the box it's in looks like a metal one from the pictures. I used to chair a BSI committee, so I have confidence if nowt else.
The possible problem is if the fan/fans fail and the heater keeps going. It's a bit unclear about it having a thermal cut-off or if its just guaranteed against winds of high and terrifying velocity and evil spirits. The statements about its microprocessor control and running off the owners fuel supply are a bit disconcerting. But 10 Amp thermal switches in a range of values and from the UK are under £2 inc. p&p on ebay. And I've got a point and shoot thermometer as well (if the kids ain't done for the battery).

Graham

Well, I found the problem with the reversing lights. Fortunately not the switch on the box or wiring to it. But the bulb holders: Both seem to have been gRimmer ones, though one looked older and more dirty than the other - maybe from an early owner. Slapped some solder on the junction between the copper strip that contacts the bulb body and the plated spring bit that contacts the sides of the hole on them both, which seems to be a real weak point in the design/constructions, and they work now. But it took some work to get the dirty one to tin.

Graham

Can’t quite remember where I read about the intermittent issue with those holders, but wherever and whoever it was, it certainly resonated with me, and I thank you.
So when on my rebuild and as most of us have multi rear lights as they seem to be plentyful.
Check those cars being scrapped as the body is a teabag and those lights still survive.
I made sure everyone was soldered up on the copper tab before installation.

The heater arrived and I opened it up to look in side:



It at least looks reasonably well built and laid out. I'll be looking at sealing the box a bit better with some duct tape or sim. I've yet to check the current draw at 12-14V. If it is over 40A, like I don't expect, some of the wiring will need to be replaced I think.
I also wonder it I might want more powerful fans than 0.3A ones.

Graham

It draws more than 10A, which is no surprise. But I need a 0.01Ohm resistor to measure higher currents than that and it'll be a few days.

I'm right about wanting higher power fans.

Graham

what is it going into? sprint......dont see many sprint road in this weather so whats the point in over powered rear demister? hope you bench test this first it look very chinese to me

tamtrucks wrote: what is it going into? sprint......dont see many sprint road in this weather so whats the point in over powered rear demister? hope you bench test this first it look very chinese to me

If you lived round here, you'd see my Sprint in all weathers, cos it's my daily drive.

And this fan heater really is Chinese, but it I can't see what's dangerous about it's build. It may be that the element will burn out, but there's nothing I can see that's combustible in or close to the element. Or the fans may stop and then the element will burn-out. But even so, it's in a metal box that will be mounted 1 cm off a metal panel under the parcel shelf. Also, if I do get concerned about the wiring, I can replace that. And I already plan on replacing the fans cos they are a bit weak - possibly a fortnight. I have been looking at thermal fuses as well. But I would want a resetting bi-metal one not a melting link one. So still looking.

Graham

wow a sprint thats works in all weathers....thats must be a first for this forum,i always thought that they were for shows and polishing in the garage

tamtrucks wrote: ↑Mon Jan 25, 2021 8:15 pm wow a sprint thats works in all weathers....thats must be a first for this forum,i always thought that they were for shows and polishing in the garage

My Dolomega Sprint will be my daily too once it's finished, and Galileo also dailies his, i'm sure there are more! No point in having a fast car if you don't drive it!

If this gadget works on Graham's car, I might even incorporate one on the Dolomega, it's booked to go on the RBRR in October and, as I mentioned above, the back seat area in a Sprint (or any other Dolomite, come to that) doesn't get exactly hot via the standard heater!

I've got a boot mounted battery so it'd be even less trouble for me to plumb one in and switch via a relay.

Steve