Home away from home
From Melbourne Australia (and likely under the car)
Some of the stuff I'm about to type might sound fairly 'elementary' to some people, but I'm doing it to cover all bases. Even if you were to name the most knowledgeable and skilled mechanic in the world - there was a point, at some time in their lives, where they had never held a spanner or a screwdriver, and knew absolutely zero about cars. That's where we all start, each and every one of us. So don't worry at all about any questions, we're all happy to help.
OK - probably first things first - as far as direct current (pretty much anything except for household electricity, to generalise to a fair extent) goes - you have a positive and a negative terminal. electrons flow out of the battery on one wire, and back in on another. So any on board device needs a positive and negative 'hookup' to function. Obviously this would take a lot of wire to do that. So in order to save on wire costs/weight/complexity, car makers did a nifty trick - they route the negative lead from the battery directly to the car body itself. So anywhere that is metal on the entire body. So to power a radio, or whatever - you'd simply have to run 1 wire from the battery positive, and then the negative wire from the radio, simply route it to any place on the body work. Radio makers and the like typically are 'on board' with the whole thing, and simply making sure the radio casing is in contact with the bodywork (screws attaching the radio to its cage/mount) and that's that (of course speaker wiring is extra, but I'm talking about actual power in and out of the radio itself).
To complicate matters - some older cars (particularly british ones) pre circa 1965 or so were 'positive earth' and they attached the positive cable of the battery to the bodywork. So a radio, or other gear of recent manufacture won't work 'as is' in a positive earth vehicle. But the beloved datto is negative earth, as is pretty much any post 60s car. I only bring it up out of trivial interest.
Due to engine mounts being rubber, there's no true 'metal to metal' contact to carry current from the car body to the engine block. So there's usually an additional 'earth strap' from bodywork to engine block somewhere. They also sometimes use one negative cable, with a fitting on one end to go to the block, and a terminal roughly in the middle of the cable length, going to the body.
There's still some pathways for 'some' current to find it's way there, so if an earth strap from body to engine was to come off whilst running, the motor would still run, but if it happened with the engine off, the next time you try and start it, well starting requires massive current, (which is why the big battery positive lead is thick, and routed to the starter motor like that) It'll flow through the positive side no probs, but there won't be enough cable/thickness/current carrying ability on the negative side. And believe it or not, it'll try and travel through any potential pathway. Believe it or not a couple of people have had this happen, and when they tried to start it, the current tried to flow through the accelerator cable and melted it almost instantly.
That's not likely what's happening here, but the reason I bring it up is to help explain 'hotwiring' the ignition. You run a wire from the battery positive to the coil + terminal. You don't need one to the coil negative, the coil negative goes to the distributor (to the 'points' ) - which are basically an on off switch when they are 'on' the power flows from the battery (through the ignition switch etc) to the coil positive, then through the coil to the distributor, to the points, and then to the engine block (earth strap etc) - a complete circuit. As the distributor turns, the points are open and shut, and when closed/on that first 'energises' the coil primary winding, then the points open, and as the power in the primary coil drops to zero, it creates a high voltage discharge out of the coil secondary wiring (through the dizzy cap, the spark plug lead, to the spark plug, and across the plug gap, then into the engine block/earth.
So that's the 'how' now if the ignition switch is stuffed, it might not be getting power to the coil in the first place. So to 'bypass' the ignition switch, run a wire direct from the battery + terminal to the coil + terminal. As DD has said - turn the motor over (have a friend help bridge the starter motor contacts ) with the coil lead removed from the distributor cap, and close to some metalwork (the engine block will do) and check for a spark. If it has spark then, it's a problem with power getting to the coil, and you can start checking why. But the thing to be careful of is this added wire. Since it's conected to the battery + if you accidentally drop the wire, and the end comes into contact with bodywork, it'll start sparking, probably melt the 'hot' wire rubber insulation, start an engine bay fire, you name it.
Another thing you can do is get a cheap on off switch from a local car/electronics parts store. Cut the 'hot' wire in the middle and add the switch in there. That way, if something goes wrong (or if it goes right!) and the motor starts, it'll be easy to switch it off without having to unscrew or rip the hot wire off out of there.
So these 'points' are an electrical switch. if we wanted the most powerful spark, we could (in theory) run 24 volts or more through them to give higher potential coil output. Problem is that this would quickly overheat the points, and they'd start to spot weld (then tear away) the surfaces where the points 'touch' and open and close to switch on and off. So we just can't put unlimited current in there. Even with 'ideal' current, they'll still eventually wear out. So in order to get satisfactory life out of the points, we need to limit the maximum voltage they will deal with day in day out. To do this, there is what is called a 'ballast resistor' which is basically a resistor (a resistor is something which reduces/lowers the amount of power travelling through a circuit in very basic terms). This drops the voltage the coil will see (and the points too of course) down to approximately 9 volts or so. And it all runs well (well enough) So when the ignition key is in the 'on' position, you'll get power going to the ballast resistor, on to the coil +, then from the coil - to the dizzy etc.
BUT - for starting, esp when cold, ideally we would want/need a stronger spark to kick it into life. Since it only takes a few seconds to start the motor (when all is well) - then _temporarily_ having full battery voltage (closer to 13.8 volts even though they are called a 12 volt battey) - well doing it for only those few seconds, isn't going to affect points longevity. So how is it done? Simple enough - with the key in the on position it goes through the ballast resistor, but when you turn the key to the 'start' position, not only does it power the starter solenoid wire, it also sends power out from the ignition switch through another wire, and this goes direct to the coil positive. SO, when you turn the key to start, it more or less bypasses the ballast resistor, and hey presto, full power to the coil/points.
Why did I bring all this up? Well simply because ballast resistors don't last forever, and often there's no external signs of their demise (i.e. you can't always just look at em, you'd have to test if they allowed current to flow at all or not). So if the starter was still able to be powered by the ignition switch, but the ballast resistor was broken, well when you turn the key to start, it'd power the starter, and turn the motor over, AND it'd bypass the ballast resistor, so the motor would fire into life. BUT when you let go of the key, and it springs bck to the 'on' position, so now there's no 'full' voltage going to the coil, AND the ballast resistor is broken, so as soon as you let go of the key, it dies. So it's kinda weird - starts and roars into life, then dies the second you let go of the key.
So normally that'd be a dead giveaway - but in your case, you've got (at least that we know of) trouble with the ignition switch, so when you are bridging the starter motor terminals to get the starter to turn the motor over, you (naturally) haven't go the key in the 'start' position, it'll be in the 'on' position. If that is the case then (if the ballast resistor is stuffed), then there'd be no power getting to the coil/dizzy etc and it'd turn over and over, but not start.
The ballast resistor itself is typically attached beside the coil - the ceramic insulated rectangular thing) You could run a wire across each side of it, so that current would bypass it (or just pull both the wires and jam a piece of wire into each 'plug'. Then with the key in the on position, it'll now get power (assuming the ballast resistor and not the ignition switch is to blame).
now it might still be the ignition switch, so to check that, run that hot-wire from the battery positive to the coil positive, then try starting, or just turn over to check for spark. If there's no spark then, maybe the points are so completely worn that they aren't working (or rarer, but possible, the coil itself has failed, which can happen if it gets full current, i,e, ignition left on for hours but motor not running so the points aren't switching the power on and off and allowing it to discharge thus burning it out eventually)). So check the points contacts for pitting/damage, (similarly the rubbing block itself can eventually wear down to the stage it no longer opens the points, or potentially the rubbing block can melt if over heated for various reasons) and repace the points. and replace the coil if needed. Then it should (with the hotwire attached to the coil + ) start,
So if the points or coil was stuffed replace and retry, and it should run. (or at least show spark if you'd rather look for spark at this stage than just go ahead and try starting it.
However, if it starts with the hotwire from battery + to coil +, with no new points or coil, then it (obvously) isn't the points or the coil, as it's now running. So then you start back tracking. remove the hotwire from the coil positive, and shift its attachment point to one side of the ballast resistor. If it starts, all good. If it doesn't then the wire from the ballast resistor to the coil + is damaged. But if it starts, then disconnect the 'hotwire' from batt + and reconnect it to the 'other' side of the ballast resistor, so the current has to flow through hte resistor, So put the hotwire there, and attempt to start (or look for spark). If it runs with the hotwire going to the other side of the ballast resistor, then it's the ignition switch (or wiring from ignition switch to coil) that is faulty. but if it doesn't run with the hotwire going to that other side of the resistor, then it's the ballast resistor that is stuffed.
Posted on: 2012/5/4 13:42