Mr Locust Posted December 12, 2011 Share Posted December 12, 2011 The generic circuit for the charge lamp is +12v ign switched to one side of the panel bulb, alternator small connector to the other side of the bulb. This provides initial field excitation to the alternator and charge indication. So before start, ign feed = +12v, alternator terminal = 0v. Bulb lights up. Once running (and charging) ign feed = +14v, alternator terminal = +14v. Bulb goes out. Link to comment Share on other sites More sharing options...
Sheds Moderator Posted December 12, 2011 Share Posted December 12, 2011 It's a bit counterintuitive this one but it does work. What you have on your bulb is one side to the battery +ve, via the ignition switch. The other side of the bulb is connected to the alternator output. When you switch on the ignition one side of the bulb gets 12V and because the alternator is not turning it's at 0V and so the bulb lights. It earths via the alternator. When you start the car the alternator output goes to 12V (well, 14) which is identical to the ign +ve. The light then goes out as there is no voltage difference across it, both sides are at +13.5V (nominal). So as you see it's counterintuitive but ingenious. Link to comment Share on other sites More sharing options...
Sheds Moderator Posted December 12, 2011 Share Posted December 12, 2011 Must type faster... Link to comment Share on other sites More sharing options...
Member Jonathan Kay Posted December 12, 2011 Member Share Posted December 12, 2011 That's the explanation of how ignition lights work... but are you sure your bulb is OK? Jonathan Link to comment Share on other sites More sharing options...
tiddy1 Posted December 12, 2011 Author Share Posted December 12, 2011 yes I understand the logic but i get +12V from the alternator to the bulb when its not running and zero when it is running (and charging) ??? but the zero is not earthed so the light does not even come on then.... and yes the bulb is fine what if the two wires coming out of the alternator were the wrong way round ? Link to comment Share on other sites More sharing options...
Member Jonathan Kay Posted December 12, 2011 Member Share Posted December 12, 2011 Can you connect your voltmeter to mimic the bulb exactly as BOSS describes? Forget about earth. What voltage do you get between the battery positive terminal (not just a wire attached to it) and the bulb feed from the alternator (ditto), firstly when the engine isn't running, then when it is running fast? Jonathan Link to comment Share on other sites More sharing options...
Mr Locust Posted December 12, 2011 Share Posted December 12, 2011 This may shed some light on the situation link to the dark side. It appears that the Denso boys do things in a somewhat more complicated way than the basic Lucas, Bosch, Iskra alternators which have large wire terminal for charge and small wire terminal for excitation. Do any of these types look familiar? Link to comment Share on other sites More sharing options...
tiddy1 Posted December 12, 2011 Author Share Posted December 12, 2011 Thanks but i have read all that , and thats how i wired mine. so with a volt meter replacing the bulb i get 0 volts when the engine is not turning, i.e 12 + 12 when the engine is running i get -12v, i.e 12 from dash and 0 from alternator, have just fitted a new pulley. and no i get a good 13.5 v at idle, but still no light. i can only assume that the regulator is shagged and will fork out 15 for a new one, hope I can fit it without taking the alternator out again, that was a real ba****d even with a air ratchet to get in the tight bolts I also tried swapping the two terminals on the plug but then it would not charge Simon Link to comment Share on other sites More sharing options...
Member Jonathan Kay Posted December 12, 2011 Member Share Posted December 12, 2011 You're way beyond with me the Denso alternator. But it sounds to me as if you're reading all voltages with reference to chassis earth, and that's why you're expressing them as paired values, and why you mention the dash. I suggest you do that test again with point to point measurements with no wires or earth involved. I don't understand why the bulb isn't lighting. Have you connected a battery across the wires leading to the bulb holder so that you can test those wires, the bulb-holder and the bulb all at once? Jonathan Link to comment Share on other sites More sharing options...
tiddy1 Posted December 12, 2011 Author Share Posted December 12, 2011 Jon when I said dash I only meant the +12v switched supply coming from the fuse box under the dash as opposed to the 12 v coming from the alternator, putting 12v across the bulb holder makes the bulb light I believe the reason teh bulb does not light is that the alternator wire drops to zero volts but is not connected to earth? Simon Link to comment Share on other sites More sharing options...
Member Jonathan Kay Posted December 12, 2011 Member Share Posted December 12, 2011 Here's the bit I don't understand... if you have 12V PD between battery positive and alternator terminal with the engine off why doesn't the bulb light up when connected in the same way? Some thoughts: You're not testing between exactly the same points in the circuit and there is a fault in the wiring that is different between the two tests. Your bulb and wiring are directional! Could it be an LED, or is there a diode somewhere. Could you try the bulb etc the other way round? Probably easiest to test this directly to a battery. Jonathan Link to comment Share on other sites More sharing options...
Paul Deslandes Posted December 12, 2011 Share Posted December 12, 2011 the circuit in the build manual shows a diode in series with the lamp Link to comment Share on other sites More sharing options...
Member Jonathan Kay Posted December 12, 2011 Member Share Posted December 12, 2011 Welcome back Paul. I was hoping for someone who knew more... Does that circuit usually/ often contain diodes? I can see why they would in order to stop the bulb lighting up when the alternator output voltage goes above the battery live... but I was getting there from first principles. Jonathan Link to comment Share on other sites More sharing options...
Paul Deslandes Posted December 12, 2011 Share Posted December 12, 2011 I've been following the thread but not quite understood what was going on. If the bulb has zero volts on one end and 12 on the other (at the same time) it should light up, unless its blown or has a diode in series. I don't know which configuration you have Simon but the Stack dash circuit shows the ign warning light with a diode in series. It looks from the manual as though the conventionally instrumented cars have the ign warning light in the tacho, is that correct? Mines a '95 car with a very different loom and a separate light so is not relevant to this problem. Link to comment Share on other sites More sharing options...
Sheds Moderator Posted December 12, 2011 Share Posted December 12, 2011 The alternator voltage will never rise above that of battery positive, as they are directly connected and as soon as the alternator spins up it raises battery +ve along with alternator +ve. The diode in the circuit is a new one on me though. Link to comment Share on other sites More sharing options...
Member Jonathan Kay Posted December 12, 2011 Member Share Posted December 12, 2011 Quoting batteredoldsupersport: The alternator voltage will never rise above that of battery positive, as they are directly connected and as soon as the alternator spins up it raises battery +ve along with alternator +ve. The diode in the circuit is a new one on me though. If the alternator is charging the battery there must be a PD or the electrons wouldn't know which way to go. But I'm sure you're right in that will be very small because of the low impedance of the connecting wires. Jonathan Link to comment Share on other sites More sharing options...
Paul Deslandes Posted December 12, 2011 Share Posted December 12, 2011 odd thing is, it looks as though its pointing in the wrong direction so it may be a red herring and best ignored for now. If you haven't already, download the build manual and refer to the circuit diagrams at the end. If nothing else, the spelling on the ECU connector legend is good for a laugh, e.g. Pin 58 Dignostics and Pin 61 Ingnition sensor Must have been done by an Inginere 😬 Link to comment Share on other sites More sharing options...
Sheds Moderator Posted December 12, 2011 Share Posted December 12, 2011 Quoting Jonathan Kay: Quoting batteredoldsupersport: The alternator voltage will never rise above that of battery positive, as they are directly connected and as soon as the alternator spins up it raises battery +ve along with alternator +ve. The diode in the circuit is a new one on me though. If the alternator is charging the battery there must be a PD or the electrons wouldn't know which way to go. But I'm sure you're right in that will be very small because of the low impedance of the connecting wires. Jonathan That's the point exactly, as you say. The battery is at 12V when you fire the starter, it drops off to (say) 9V under starting load, the engine fires, the starter disengages and the battery goes up to 12V again (off load) and the alternator is instantaneously at 13-14V. There is of course a massive rush of current to the battery but the cable between the two is about as thick as a pencil so the resistance/impedance is close to 0 and the battery +ve terminal goes to 13-14V in a fraction of a second. Don't forget also that if the battery is heavily discharged then the alternator won't make it all the way to 14V as the battery load will pull its voltage down. But take it from me (or test it with a multimeter!) the difference between alternator out and battery +ve will always be zero because of that bloody great thick length of copper cable between the two! It;s actually interesting to do this with a half-flattened battery and a multitester, you can watch the voltage climbing just after starting the car and hear the engine note changing with altrnator load. Especially so if you turn on the lights, you'll have noticed that most cars falter for a split second when you switch the lights on at idle, then the idle speed rises slightly when you switch them off. If you have a meter on the battery you can see its voltage fluctuating at the same time. Link to comment Share on other sites More sharing options...
tiddy1 Posted December 13, 2011 Author Share Posted December 13, 2011 when I say the alternator voltage, i mean the voltage coming from the small terminal on the rectifier in teh alternator, not the big red wire, of course the big red wire is at 12V as it is connected directly to the battery The only place there could be diode is in the alternator, the wire from the alternator (small terminal) goes straight to one side of the lamp, the other side of the lamp goes to a switched +ve from the fuse box No circuit diagrams as this was all wired from first principles Simomn Link to comment Share on other sites More sharing options...
Jason Plato Posted December 13, 2011 Share Posted December 13, 2011 glad to hear the charging rate was due to the alternator pulley size as I mentioned earlier re the bulb - do you have an earth strap from alternator to engine ? Link to comment Share on other sites More sharing options...
Jason Plato Posted December 13, 2011 Share Posted December 13, 2011 I wired alterantor in as - 12v feed from switched ignition - the second pin *grounds* the bulb when the alternator is not charging( the other pin on the actual bulb in the dash is ignition switched positive feed) I also have a earth strap from alternator bolts to engine block as my alternator is mounted using rubber bushes. Edited by - Dave Jackson on 13 Dec 2011 09:03:43 Link to comment Share on other sites More sharing options...
Member Jonathan Kay Posted December 13, 2011 Member Share Posted December 13, 2011 Quoting Paul Deslandes: I've been following the thread but not quite understood what was going on. If the bulb has zero volts on one end and 12 on the other (at the same time) it should light up, unless its blown or has a diode in series. Simon: Apart from the pulley and the earth strap and the regulator we really need to sort this issue about the bulb. There are two reasons for this. At the moment you're trying to debug a fault but can't be certain that the test equipment is OK. And once it seems to be working you won't know if it's charging properly or not until it's flat again. I'd still recommend you measure voltage from point to point, not through the fuse box or any wiring. Then I'd try the bulb assembly and wiring the other way round. Then I'd take that bulb assembly and wiring and try them (both ways round) in a test rig that has nothing to do with anything on your car. Jonathan Link to comment Share on other sites More sharing options...
tiddy1 Posted December 13, 2011 Author Share Posted December 13, 2011 Jon, I have tried the bulb and assembly every way round you can, ( its only a bulb) and if I connect it to a battery it lights up, if I leave it connected to the +ve 12 from the fuse box and earth the other side it lights up Now if i connect it to the wire coming from the alternator regulator and then earth the other side it does not light up, (engine running or not) if i plug in the multimeter from the wire from the regulator (as above) and earth the other lead of the multimeter then i get +12v when the engine is not running and 0 when it is running if check the continuity between this wire and earth it is never connected, engine running or not I do not have an earth wire on the alternator but the but the continuity between the battery earth and the alternator case is good so I should not need one. Link to comment Share on other sites More sharing options...
Member Jonathan Kay Posted December 13, 2011 Member Share Posted December 13, 2011 Thanks. For this test I suggest you forget about earth and continuity. What I don't understand is why you get 12V between alternator terminal (the one which we think should go to the indicator bulb) and battery live, but the bulb doesn't light up when connected to the same two points. But I don't think that's quite the test you have done. Jonathan Link to comment Share on other sites More sharing options...
Member Jonathan Kay Posted December 13, 2011 Member Share Posted December 13, 2011 Quoting tiddy1: I do not have an earth wire on the alternator but the but the continuity between the battery earth and the alternator case is good so I should not need one. Testing continuity like that does not guarantee that the connections can take a high current. This could still be part of the problem. I suggest fitting an earth strap and cleaning the connection surfaces. Jonathan Link to comment Share on other sites More sharing options...
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