Chicken Switch Problem

[PetenAli]

Afternoon All,

We are off to the Lakes in a fortnight and planning a trip that will take in all the big passes over three weeks. So I thought it would be a good idea to protect my 025 Motorsport 2.5 WBX as much as possible and fit a chicken switch for the rad fan. The temp guage does have a tendency to creep up higher than I like before the fan kicks in. Also - although it is wired for 2 speed only one speed works when I do the paper clip test on the thermo switch.

The wiring for the chicken switch goes like this:
- power direct from battery +ve to 40 amp relay via a 30 amp fuse (using 39 amp 4mm sq cable)
- then from relay direct to fan using 39 amp cable.
- signal wire to relay from side light terminal on lighting switch via on / off switch
- also warning light from on / off switch as well (obviously that goes to earth as well).

Using a 30 amp fuse because that's what the standard VW one is (No. 1 on my fuse box)

When I first did this it seemed to work well however first time we went out I tried it once and it was fine, then tried it again and it blew the fuse. Since then I've checked all the contacts, tried it again and it blew the fuse. This has happened several times. Sometimes it runs fine, I switch it off, then on again and it blows the fuse.

Does anyone have any ideas please? I think that the wiring set up replicates the original (i.e. position of fuse in relation to relay, capacity of wire etc). I've checked all the connections and they seem sound. Is the relay up to it or do I need a 70 amp one?

Any advice gratefully received...

Thanks,

Pete

[itchyfeet]

Why not just wire a switch in parallel with 1st speed fan thermo switch?
I assumed thats what people did

Why run a new feed your fan already has one

[PetenAli]

Thanks itchyfeet. I put the relay in as I thought the current draw would burn out the switch. But I suppose there's no relay for the first speed anyway. So it's OK just to wire a switch in either side of the thermo switch? And then its protected by the original 30 amp fuse?

Pete

[Oldiebut goodie]

Are you putting the 30A fuse in the 50A side? Might explain why the fuse is blowing. Check the 50A relay/strip fuse by the earth crowns.

[PetenAli]

Are you putting the 30A fuse in the 50A side? Might explain why the fuse is blowing. Check the 50A relay/strip fuse by the earth crowns.

Ahh - so are you saying the 30 amp in line fuse should go between the relay and the fan? Rather than where I've put it - between the power source (cranking battery) and the relay?

As you will have guessed, my knowledge is quite limited so this is a great help. Never too old to learn...

[itchyfeet]

Thanks itchyfeet. I put the relay in as I thought the current draw would burn out the switch. But I suppose there's no relay for the first speed anyway. So it's OK just to wire a switch in either side of the thermo switch? And then its protected by the original 30 amp fuse?

Pete

yes, short red/white and red/black by a seperate switch or by the thermoswitch or both together, nothing extra to go wrong

you can pull the rad switch connector and join the two wires to prove it to yourself

connector for red white behind fusebox but red black needs a wire out front by switch
just looking at this myself as I have a french trip coming up, new rad switch in and will get pics up when I do.

Stage 1 is 200W so you need bigger than a 16A switch, not too hard to find 25A switches.

[PetenAli]

Thanks itchyfeet. I put the relay in as I thought the current draw would burn out the switch. But I suppose there's no relay for the first speed anyway. So it's OK just to wire a switch in either side of the thermo switch? And then its protected by the original 30 amp fuse?

Pete

yes, short red/white and red/black by a seperate switch or by the thermoswitch or both together, nothing extra to go wrong

you can pull the rad switch connector and join the two wires to prove it to yourself

There are connectors in the dash that would be easier to pick up from, just looking at this myself as I have a french trip coming up, new rad switch in and will get pics up of where to wire to.

Stage 1 is 200W so you need bigger than a 16A switch, not too hard to find 25A switches.

That's great - thanks again. Will pick up a 25A switch tomorrow.

[itchyfeet]

just edited my post above, red black wire is only accessible outside.

[syncroandy]

In case its helpful I scanned the wiring diagram page for the radiator fan 86>, there are three different setups, a Syncro DJ probably has the 450/500 watt one, with a separate relay on the A-pillar.

The simplest way to provide 'max cooling' under manual control, would seem to be to add a switch between the output of fuse S1 (red/white wire) and terminal 86 of the relay J101 (red/blue wire). The switch would only see the relay trigger current so can be quite small. No other components needed.

If you wanted to switch the 'low speed', the switch should be rated the same as S1, so 30A@12V or 2A@240V.

http://syncrosport.com/info/wiring/T3_Radiator_Fan.jpg

[itchyfeet]

Good idea to use stage 2 ^^^ as you say relay triggered and so you can pick up both wires in the cab behind fusebox.

Think I will do that, noisy though

[itchyfeet]

30A@12V or 2A@240V.

Don't agree, current rating of switch must be 30A regardless of voltage.

[syncroandy]

It's the power rating that matters surely ? It's normal to derate the current rating of a switch with increasing voltage, so the converse applies IMO.

E D I T. Done some googling and not so sure now.. there seem to be various consideration when rating switches... I have a nice 240v / 15A toggle switch here that I feel would be fine for 30A @ 12V but how I'd demonstrate that by calculation I've no idea !

Can you point me to the proper calculation method ?

[itchyfeet]

It's the power rating that matters surely ? It's normal to derate the current rating of a switch with increasing voltage, so the converse applies IMO.

V=IxR

so a switch with a given resistance will generate a voltage across the contats of V with a given current

P=VI so the power(heat) generated by the resistance of the switch is directly related to the current that passes through it.

heat generated is the limiting factor of a switch rating.

Voltage ratings of swotches are all to do with arcing ovef and breakdown of insulation.



Same applies to cables

[syncroandy]

Well obviously I know ohms law.. but Google suggests there are other factors, AC vs DC, arcing with increasing voltage, leading to choice of contact materials depending on voltage.. its more complex than just Ohms law.

Hmm. Just looking on Farnell the vast majority of switches are rated at 240v, and there are very few rated 30A. I think some sort of derating must be possible as otherwise the sensible choice of switches is going to be very limited.

[itchyfeet]

yes alot more to it.

Next question, does the fan give the full power when only stage 2 is activated?
I never tried just triggering stage 2, I wonder if it has two seperate coils inside and needs both feeds for full power?
if so just doing stage 2 will be less power than stage 1