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Starter solenoid current draw


neilnaz

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Snappy title eh?

I was wondering whether anyone knew the amount of current drawn by the starter solenoid. I am after getting a button to churn the car over after sitting for a few days. The car starts more or less instantly at the moment unlike in the colder weather so I want to give the starter a good churn before switching the ignition on.

I don't want to get a push-button that is rated too low. I can't imagine it will draw much more than 1 amp but you guys might know.

I am popping off to Maplins so thought I'd get a button from there. I don't want to spend silly amounts on some Max Power rallytastic starter button when a £1.79 button will do. It will be hidden(ish) and subtle - no holes in the dash (etc).

Cheers, Neil

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No idea what it draws (not much I'd think) but 10A buttons aren't much more expensive than 2A ones anyway. I'd start by looking at Maplins stock and see what you like the look of. Then check its rating, you'll probably find it's quite high. Even some of the small switched in Maplin are rated for 5A mains.

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Bit too Max Power although it is subtle compared to some. I have seen a red button with chrome surround that looks more period IMO and may go with that. It is also smaller and £4 rather than £30.......

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Neil - early starter solenoids (Spit Mk1/2/3/early 4 - basically anything pre-ballast resistor coil) are 2.7 ohms, later ones are 2.2 ohms.

Therefore assuming a nominal 12V, early cars draw 4.44 amps and later ones 5.45 amps. A 6A or 10A rated button should be fine.

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Just one point though guys, a switch that is rated at 10 amps mains will be unlikely to switch much more than an amp at 12 volts DC.
The reason is that mains are AC whilst a cars 12 volt system is DC.
Sorry if this is a bit simplistic but here is why!! The wear that happens in all switches is when the current is switched off, as the contacts are moved apart an arc is formed which burns away the contacts very rapidly.

In an AC (Alternating Current) circuit, the directions that the current flows changes 100 times a second (for a mains 50 hertz supply) which means that 100 times a second the voltage and therefore current draw become zero. so going back to the switch, as an arc is created it will only take a maximum of 100th of a second before it is extinguished.
Now in a DC (Direct Current) circuit like a starter solenoid, the current always flows in one direction so when your switch opens the contacts have to bear the arc until the gap becomes large enough to decay due to the resistance of the air. That is why mains rated switches will have a very low DC switching capability. DC switches have much larger contacts that move further apart at a faster rate to try and suppress the life of the arc.

Sorry to go on, but i hope this explains the problem.

Cheers
Tim.

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Thanks for the explanation chaps. I have bought a switch and will see how I get on. I just need to connect it across terminals 1 and 2 on the back of the ignition switch. I just want to be able to turn the engine over and build up oil pressure before starting. It is quite cool that I will also be able use it to start once the ignition is switched on.

Cheers, Neil

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Yes James I found loads of options of starter buttons rummaging around Maplins. I am locating the button on a black switch panel close to the ignition switch adjacent to the oil pressure gauge in the picture attached. The aim is to be able to crank with ignition off for 10secs or so as I normally do then ignition on and vroom. The current (or recent) warm weather has meant the car just starts straight away and I'd rather avoid that. How ironic!  ::)

Anyway ordered the switch panel from AES switch is waiting, wires (etc). So job will cost all of £6 and that's just cos I wanted a black crinkle switch panel.

I presume you are going for a sprung toggle switch?

Cheers, Neil

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Nah - just an on-off switch...

A momentary could be easier than a push button - as lever is nicer to press for a long period...

Of course these are all fine points - really its just a switch you need to press/push to do the job :)

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OK James. I thought that would be rather awkward for starting as opposed to cranking but I don't suppose it makes much odds!  ;D

Charlie yes it is a twist for intermittent. The further you twist the knob after it clicks on the longer the delay between wipes. It's a Hella switch as sold for VW camper vans. Dead simple to use.  8)

Cheers, Neil

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I'm suprised no one has thought of it before, but how about a circuit with a relay that automatically cranks until the oil pressure light goes out and then stops cranking.

Note the use of the ignition switched 12V, so that if the oil pressure never gets high enough to trigger the switch, the cranking can be switched off.

Another option I'm thinking about is another relay that lets you start the car as normal using the key, but wont allow the spark until the pressure is high enough.

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Kevin I have heard, I think, of people doing exactly that (i.e. inhibiting ignition until oil pressure light extinguishes). I, being a simple bod, am quite keen on simple is best although I like your idea. I can add the switch and 2 wires piggy-backed on what is already there. The ignition will work as usual should I choose and I also have the option of cold cranking. It is all under my control. I also like the starter button but that is merely the child in me.

I take it that the wires on the back of the ignition switch are lucar connectors?

neil

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Nice idea and all that but why don't you just crank the engine without choke when cold 'til the light goes out, then pull out the choke?
If the answer is "Cos it starts anyway" then the mixture is way too rich - sorry if I'm being a pain, but that's how I've always done it, it just seemed so obvious I had to pipe up.        :P

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Dale - that was one of the first things I learnt from the PO of my first Spitfire - "it doesn't fire until you pull out the choke - but that means the mixture is correct anyway"

In fact I do it one movement... pull the choke whilst still cranking...

Choke still worries the wife... "it's broken..." such a pessimist with cars... we've only ever broken down once... twice if you count 100 metres from home as a breakdown...

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Dale that is what I have always done but, probably with the warmer weather and possibly slightly rich mixture (I'll check), the car has been catching and starting without choke or throttle. I am going to check and adjust mixture and timing this weekend. I recently blocked off the vacuum advance feed from the carbs (as you have mentioned to me) as I have suffered from mid-throttle pinking so the timing is probably retarded too which won't help matters.

Even if things go back to as they were 2 wires and a switch costing £6 with mounting panel included and a bit of fun means I've got something else to play with.  ;D

I do like the extra control I will have over starting/cranking and a nice starter button to boot. Yes I am that childish.  ::) :D

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Well I have been playing today. Timing adjusted as it was 5-6 degrees retarded after getting rid of the vacuum advance. Mixture needed a slight tweek as it was running slightly rich. I will have to check over the weekend with a seesion of spark plug readings. The car is going very well.

I fitted the proper catch tank rather than my Heath Robinson affair. The catch tank sits where the washer bottle sat and the washer bottle has moved towards the master cylinders.

A couple of paint chips have been sorted on the door where I had a slight mishap when reifitting the drivers door. Doors adjusted.

And finally the starter and cranking button has been wired in but I am waiting for the switch panel to mount it close to the ignition. It works exactly as desired. The panel should arrive tomorrow so I can mount it to the drivers glove box rail. I cranked the engine until the oil light went out switched the ignition on applied a smidge of choke and slight throttle and Vroooooommmmmmmmmmm  ;D :D 8) :P :)

Childish moi!!!!!!!!

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This ( http://cgi.ebay.co.uk/Spitfire-Engine-start-button-NOS_W0QQitemZ150247442448QQihZ005QQcategoryZ2983QQssPageNameZWDVWQQrdZ1QQcmdZViewItem) has to be the only cool Spitfire starter button.
No bling, no flash, just cool.
JOhn

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