This is a read-only archive of the old Madabout Kitcars forum. New posts and replies are no longer possible, but every thread is still here to browse and read.
I am fitting a push start button to the centre of the dash for no other reason than it looked good having a large swith button in the middle and two smaller ones either side for fogs etc..
The switch is one of the silk screen printed anodised alloy numbers from Europa. It is only rated at 6 amps so I will use the switch to fire a 30/40 amp relay ie starter load on pins 87/30 and ign live into 86 to ground on 85.
The question is has anyone who has done a similar thing found a way of only making the switch active when the engine is stopped.
The reason being I would prefer it do not engage the starter if the engine is running. Someone on the Cobra boards had used the charge warning output from the alternator onto pins 85/86 which in theory would work put I am concerned it may damage the diode pack in the alternator.
I will still be using the Sierra column switch and key so it will not be ppossible for the engine to continue runnng with the steering lock engaged, the only thing I will move to the push start is starter crank.
How about taking the alternator charge to an op-amp acting as a comparator, and take the op-amp output to drive the relay? I'm pretty sure that's possible, though it's based on a rather shady memory from electronics lessons at school many years ago.
1. You probably don't need a relay (unless see below). The starter switch doesn't operate the starter motor directly (which needs 400 amps) there is already a relay involved mounted on the started motor itself (used to call them a solenoid in the old days) This should take only a few amps.
2. Simple way to disable the starter switch is to run it through a relay operated by the oil pressure switch (Use normally open contacts on the relay) . Assuming that is, your engine starts before the oil pressure builds enough for the oil pressure switch to go off. Most modern engines start at the first flick so I think this would be OK.
I know in theory the starter takes the load, but take a look at the Europa switches, anyone with any electronics background will say RELAY.
I had thought of the lazy american route ie oil pressure switch but its not a clean way of doing things and mine has already got two adapters for the oil pressure and the oil warning light.
I can live without it, but I may have to visit the farnell website and dig back into my computer main frame engineer past (long past) and build something to do the job.
best to stick with the alternator signal I would have thought, the oil pressure should go out as soon as the engine is cranked but that may not mean that the engine has started whilst the charge light doesn't usually go out until the engine is running as the alternator needs x amount of rpm to switch the light out. I would run a relay off that signal in parallel to the charge light (check the current draw/resistance/impedance first thought)
Charge light is a much better idea but as Chris says you would need to check the current draw/resistance/impedance first.
Once my car is SVA'd I also intend to go the route of a separate start button so I will probably use this idea.
As far as how much current you need to supply to the starter circuit I suppose the best answer is just to measure it. I would be surprised if it as any more than 1 or 2 amps.