Ignition Light Switchbox - Rolls-Royce and Bentley Forums
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post #1 of 11 (permalink) Old 10-11-2019, 03:06 PM Thread Starter
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Ignition Light Switchbox

Hi I have been having an ignition light problem on my 1976 Silver Shadow and have noticed that there are two resistors attached to the generator light , I wonder if anyone could answer the following question for me, What are the resistors for on the Ignition Light Switchbox
many thanks
Martyn
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post #2 of 11 (permalink) Old 10-12-2019, 02:25 AM
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Martyn,


I researched these from your original topic when you called them diodes and I said they looked like resistors.


They are not shown on any drawings YET that I can find and my initial thought was if there were any light dimming going on in any circuits but that was not the case.


I do not have them on my car as earlier cars don't seem to have them either fitted when I looked at various RR/B switchboxes that were up for sale.


I certainly intend to fit some to my car as an update as it is quite interesting what they are for. Simple but a real brain teaser.



At the moment I don't see they are anything to do with your ignition light problem but without seeing how RR have actually wired them in on later cars I am on the fence at the moment if they could.


I have asked my friendly RR breaker to send me a later switchbox so I can see excactly how RR wired these up. I know what they are for but not yet how they were actually wired into the circuit.



At least they are not a mystery to me anymore.


Hope the wiring diagram I emailed you helps. I know you said you struggled with colours but at least if someone is with you it is good to have on hand.



All the best, and keep us updated as I am still concerned about your red brake warning lights so make sure they have not been disconnected.





Steve
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post #3 of 11 (permalink) Old 10-12-2019, 07:49 AM
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My guess would be to keep or increase the initial field current.
Mike

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post #4 of 11 (permalink) Old 10-12-2019, 08:39 AM
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Hi Mike,


You are absolutely 'spot on'.


With an alternator the magnetic field is produced by the internal field coils. Although there maybe always some residual magnetism in the metal it is not like a permanent magnet so maybe not enough to create the initial alternator output because this could be very minimal or after a period of non use actually none existent.


The field coil circuit is actually initially supplied through the ignition light filament, yes the actual lamp element itself. That's the brain teaser......



So if the lamp actually fails you could get no magnetic field created in the alternator so no 12v supply output and the car would just be running on battery power until eventually that runs out and the car would stop.


To overcome this probably very rare problem that anyone would actually experience the resistors were put in parallel with the lamp filament. Should be of a similar resistance to the lamp. That way if ever the lamp actually failed the alternator field feed circuit would still be provided via the resistor.


I researched up and it is actually a modification on several forums recommended on old classic cars that have an alternator.


I don't know why Martyn's car has 2 as I see some only have one and I don't know how they have been actually physically wired into the wires. Theory is IIRC my schoolboy electronics resistors in parallel half their nominal value proportionately ie a 10ohm and a 6ohm in parallel has net effect of a single 8ohm so maybe just something in relation.


As I said nothing on schematics so I want a switchbox to poke around and test the R value figure of the resistors and how they are actually wired.


I also read that if this lamp failure did happen you can always flash a supply temporarily to the alternator field as once it is fed 12v it becomes self exciting so to to speak. It is only required for the initial creation of the alternator output.


Obviously cars fitted with an ammeter you should see it straight away too but not all classic cars were fitted with them.


Hence I can't see these resistors even if faulty would have the effect of the ignition light staying ON which is Martyn's main problem.


All the best


Steve
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post #5 of 11 (permalink) Old 10-12-2019, 10:30 AM
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Hi Steve


Nicely researched.


There are 2 resisters on my car (SRX18501). They appear to be 60 ohms, from the markings. The pics below are the best I have since I was just taking a panoramic view with the top roll removed i.e. I was not specifically photographing the switch box. Hence the pics do not show how the resisters are wired in.
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1974 Silver Shadow
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post #6 of 11 (permalink) Old 10-12-2019, 11:55 AM
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Hi Geoff,


No sooner had I posted but the neighbour knocks on the door and says this parcel was delivered to him as I was out.


It must have been delivered Wed/Thurs when I was fixing my friends cars as I have been in all today. Good job I was not after it urgently.


Anyway...... agree the resistors are 60ohm 5watt and are wired in parallel to each other reducing resistance down to 30ohm then wired in parallel across the 12v and 0v/12v terminal(another story as it is not a ground) of the ignition warning lamp wires. The lamp itself measures 10ohm across the filament and says its 3watt. I am not sure of the science behind the resistor sizes or why 2 and not just a single 30ohm. That is a mystery that remains 6ft under in some old RR designers head...........maybe I will think about it one day...



This confirms though no way a fault with these resistors would cause the ignition light to stay on but will work the alternator field feed if lamp blows.


Now I have to make a key for it and send it back........that was the deal with the breaker........but one more mystery solved.


All the best


Steve
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post #7 of 11 (permalink) Old 10-12-2019, 12:04 PM
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Maybe two 30 ohm resisters in parallel in case one fails and goes open circuit. A belts and braces approach.

1974 Silver Shadow
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post #8 of 11 (permalink) Old 10-12-2019, 12:17 PM
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Geoff,


I will settle for that explanation........belt, braces and an adjustable waistband......


Steve
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post #9 of 11 (permalink) Old 10-12-2019, 12:46 PM Thread Starter
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Hi Steve,
just been working on my wiring, the resistors are wired up with one end to the "live" terminal on the bulb holder and the other end to the "earth" terminal so creating a link between both bulb terminals
hope this helps
Martyn
post #10 of 11 (permalink) Old 10-12-2019, 12:58 PM
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Martyn,


Technically it is NOT an earth at the ignition light. Just a 12v supply and a 0v from the alternator. When the alternator produces its 12v then 12v is supplied to either side of the ignition light which results in no potential difference across the warning lamp so no current flows to cause the lamp to light up.


Just saying


Steve
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