Engine upgrade

[chrisyork]

Hi Stina

Nice basic bit of O level physics (Power = V I) gives you a current draw of around 7 amps. But car electrics actually run a bit above 12V - usually nearer 14V - and electric motors usually draw a heavier current when they start than when they are running. So lets allow it 10A running and 15A when starting. That'll be why they gave it a 25A fuse.

So now we're working to supply a 15A load. You are going to supply the relays from the general area of the fuse box (the ammeter shunt?)? So there's around 10 feet of wire to run by the time you've included all the twists and turns? Back to ohms law. The resistance of the wire is proportional to its length. A big fat wire has lower resistance for a given length than a thin one (nore copper in it so more space for the 'tricity to rush about in!). So now you're looking for two results. You don't want to knock too much off the volts before they get to do something useful at the fan - that's ohms law V = I R . And you don't want the wire to get too hot - another version of the first one - Power = Heat = V I. This is where the science goes a bit pear shaped. If you really did want to minimise problems with all these, then you'd finish up with a wire as big as the car, so a compromise is needed. Science really doesn't do compromise, so we need an engineer instead. Engineers understand the science and then ignore it and stick an educated finger in the air and say - that'll be about right!

So go to your accessory shop or other chosen supplier and buy yourself the relays. (you are going to do the headlamps while you are at it aren't you?) Ask for a 15A relay for the fan and I'll let you work out how big the relays for the headlamps ought to be. Don't forget that one day you may want to upgrade the headlamps for ones with a higher wattage than they are wearing currently. It doesn't matter, in fact it's a good idea, if you finish up with 25A relays for everything! Hint - if you start running out of amps, simply add another relay.

Now we come to the engineering compromise bit. How big are the terminals for the "power in" on the relays? So how big is the biggest cable you can get onto them? Ah, well that'll be the size to go for then!

Simples!

Chris

[ghce]

In terms of cost, go for the cheapest relay (probably 25 amp Hella relay) the 15 amp relay may well cost you double the 25 amp one, in the high current world bigger is always better hence the cheapness of 25 amp relays. As for wires get wire of 1.25 mm2 or larger, me I would opt for 2.0 mm2 wire for the main fan circuit, drive to the relay coils can be 0.5mm2, for the lights 2.00 mm2 or even slightly bigger.....dont forget its the copper that costs.

Graeme

[chrisyork]

Note to Stina

Remember, you only need ONE power cable to all the relays! That's why it makes sense to do the headlights at the same time.

Chris

[unstable load]

Here is a good reference chart for wire diameter/length/current draw.
http://www.wattco.net/infopdf/wireguagechart.pdf
and another
http://www.powerstream.com/Wire_Size.htm

[stina]

O.k guys thanks for the info , Chris my ears have only just stopped bleeding Thanks for the lesson though , I was probably bunking off again the day they did that lesson
I went out and had a look at what i've got , the fan motor is 12amps , the controller relay is 30amp , and their is a 25 amp fuse in the fan live .

I had a look at the blade terminals i plan to use and 17amp wire fits into them . Or should i look for wire closer to 25amp I'm planning to get my mechanic friend to find the correct terminal on the ammeter shunt to take the feed from .

Yes i plan to do the lights at some time in the future , But not worried or wanting to cloud the issue with that at the moment . Don't use it much at night anyhoo .

I do want to bypass the the ignition ballast while the dash is out , i have identified the white and pink ballast wire on a scrap shell so i know where to find it on mine . Question is , you got it ! What size wire do i need to replace it with ?

[chrisyork]

No No! Doing the lights at some point in the future doesn't cloud the issue now! When you run the wire to supply the power to the relay for the fan, you wll then have a "strong" power supply at the very front of the car. If in the future you want to change the lights, or even just put relays in for the existing ones, then that same power supply can serve those relays too. It's only the control cables to the various relays that are specific to the functions they serve - the power is common across all of them. I'd lay bets that you will place the fan relay at almost exactly the same point that you will want to install the light relays, so make sure the power wire is big enough to do both! Then when you do get round to doing the lights, all you have to do is loop the power wire on to the extra relays. Otherwise you'll finish up running a second power cable from the same place to the same place!

Actually, if you are having a mate round to help with the wiring, do the relays for the headlamps now. That will save you worrying about a melting (or even burning!) fuse box and will probably make the existing lights quite a lot brighter too. If you want to change lights in the future then you can do it without worrying about the wiring.

So the upshot of that is use the biggest wire you can get in the terminals - at least 25A.

Chris

[stina]

Hi Chris . Yeah i understood about the one big live feed to the front of the car and will do , was just saying the light relays will come at a later date . 25 amps it is then , so it is written so it shall be done

[stina]

Hi all . Time for an update . Spent the afternoon in the garage yesterday with my mechanic friend sorting the wiring for the fan and the ignition feed ( i had like most been running off the washer bottle feed for over a year and was never really happy with that )
I stripped the dash out Saturday so we could get a clear start on it Sunday . The fan is switched with a revotec thermostatically controlled switch , and i fitted a manual over ride switch in the car for those times when your stuck in heavy traffic on a hot day ( not much chance of that down here at the minute ) For the manual switch i used a spare heated rear window switch and mounted it next to the original so it looks standard to the untrained eye .
Someone on here ( Testrider i think ) wanted to know which was the correct wire out of the ammeter shunt to splice into for the main feed to the fan that will show a draw on the meter when the fan cuts in . It's the brown with white tracer that runs from the shunt up to the back of the ammeter Ran a 27 amp wire from here through an inline 25 amp fuse to the front of the car to feed the fan . I can also use it at a later date to feed the head lights through relays ( your advice Chris ) Also spliced in a non resisted wire from the white ignition wire off the fuse box to the coil , bypassing the ballast resisted wire , left that in the loom though in case of ever wanting to run points again , can't imaging why though
Couple of pics Cheers Stina

[testrider]

It was me, thanks Stina. What temperature have you set the fan to come on at?

[rockdemon]

looking good... you've kept the engine driven fan in for now?

Rich

[Dave3066]

Nice work there stina

The fan is switched with a revotec thermostatically controlled switch , and I fitted a manual over ride switch in the car for those times when your stuck in heavy traffic on a hot day

Belt and braces then. However, if the revotec controller is set and works correctly then you'll never need to use the manual switch. If you're stuck in heavy traffic on a hot day and the revotec has switched on the fan then the manual over ride becomes redundant. If you do need to use it to switch on the fan then that would indicate that you need to adjust the controller or that the controller has failed somehow. Hopefully that will never happen though

Dave

[testrider]

I think the idea is to switch it on early. The worst case scenario is barrelling down the motorway then coming to a stop in a queue, the engine's hot but the fan will take a 30seconds or so to catch up so you manually put the fan on as you come to a halt.

It's all speculation of course as I don't have a manual override switch.

[stina]

Hi guys , well spotted Rich , yep engine fan still fitted at the moment , time i got the dash back in and tidied up i never had time to mess with the controller , so i just turned it up for now so it doesn't cut in . Waiting for a nice warm day so i can remove the engine fan and find the best setting under different conditions .
Dave , yes it is belt and braces , but as Paul said my thinking was that if i see a situation arising where it's likely i'll be stuck in traffic i can cool it down before it gets hot , plus if i suffer any fuel vaperisation it will help blow the heat away . That's my thinking any way
Now for a bit of advice from you electrical experts out there . When i switch the fan on the ammeter swings way to the left towards -60amps for a fraction of a second before it settles at around -5 to -10amps . lifting the revs doesn't bring the meter up to 0amps . does that sound about right ? 80 watt motor . I have 25 amp fuse protecting the fan and wiring .

[rockdemon]

its drawing a heavy current on startup. not sure how you soften the blow though.... Graeme usually seems the most practical on electrics?

Rich

[ghce]

Hi Stina if it's an 80 Watt rated motor at it's rated output it will only consume about 6 Amps. Starting the fan is effectivly starting it from a stall current and depending on the type of motor they have used it may have a higher start current, that coupled with possibly a bit of meter needle over swing may give you a slightly higher than expected reading, 25 Amp fuse should be more than adequate for the task however I wouldn't be putting a lighter current fuse in, you dont need nuisance blowing.

Graeme