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Brian Vogel
Grand Master
Username: guyslp

Post Number: 1470
Registered: 6-2009
Posted on Wednesday, 08 July, 2015 - 02:32 pm:   Edit PostDelete PostView Post/Check IP

I know very well I've posted a picture of this item when I was cleaning things up and replacing relays on SRH33576. I even recall having talked about cleaning up the terminals on it, but I can't recall for the life of me what it is (and, so can't search, either).

Someone put me out of my misery!!

Mystery Item

Brian
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Jeff Young
Prolific User
Username: jeyjey

Post Number: 217
Registered: 10-2010
Posted on Wednesday, 08 July, 2015 - 05:04 pm:   Edit PostDelete PostView Post/Check IP

Thermal delay switch to hold the choke closed when very cold.
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Brian Vogel
Grand Master
Username: guyslp

Post Number: 1474
Registered: 6-2009
Posted on Thursday, 09 July, 2015 - 01:42 am:   Edit PostDelete PostView Post/Check IP

Jeff,

Thanks. I thought I'd remembered this thing being called the scintilla switch as well, and have confirmed that, but still can't find the photos I recall having taken and posted.

This thing, and its role, are also not at all well documented in the workshop manual, or else it's buried somewhere that I have yet to find. It is referred to once.

Brian
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Jeff Young
Prolific User
Username: jeyjey

Post Number: 218
Registered: 10-2010
Posted on Friday, 10 July, 2015 - 02:50 am:   Edit PostDelete PostView Post/Check IP

Hi Brian,

IIRC, the internal circuitry is at least shown on the wiring diagrams. (Either that, or I found it somewhere else. I'm old too, you see. ;)

I still have one to rebuild. Paul Yorke sent me a second-hand working one for reasonable-enough money that the non-working one just went on a shelf....

Cheers,
Jeff.
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Brian Vogel
Grand Master
Username: guyslp

Post Number: 1480
Registered: 6-2009
Posted on Friday, 10 July, 2015 - 03:01 am:   Edit PostDelete PostView Post/Check IP

Jeff,

It definitely is shown on the wiring diagrams. See the thread entitled, What is this electrical component?, on rollsroyceforums.com. The original person asking the question posted the pertinent bit from the wiring diagrams.

Based on what's been discussed here it appears to be two small resistors that generate enough heat to get a temperature-sensitive switch to close. I imagine it has to work with the heat causing the switch to close and then, as it cools, it eventually opens. I think this is the "flip side" of the behavior of the switch at the top of the stove pipe, but I could be completely mistaken about that. I always look up when a given switch is open versus closed any time I contemplate working on one (and, of course, how that opening and closing is accomplished).

If/when you ever take the thing apart be sure to document what its internals actually are.

Brian
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Geoff Wootton
Grand Master
Username: dounraey

Post Number: 821
Registered: 5-2012
Posted on Friday, 10 July, 2015 - 03:08 am:   Edit PostDelete PostView Post/Check IP

Jeff

Re: I still have one to rebuild.

If you ever get the chance to break into your spare unit, could you put some pics up. I'm interested in the internal workings. My assumption is they consist of a bi-metal strip with a heating coil. The time delay is generated by the time it takes for the bi-metal strip to heat sufficiently to throw the switch. I may of course be totally wrong about this. Since the device is not documented, it would be interesting to see if my assumption is correct.

Geoff
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Jeff Young
Prolific User
Username: jeyjey

Post Number: 219
Registered: 10-2010
Posted on Friday, 10 July, 2015 - 09:44 am:   Edit PostDelete PostView Post/Check IP

I did look inside before shelving it, and you are both correct: a bi-metal spring contact switch and a coil (or maybe two) to heat it. There's also a setscrew to change the set-point (by pre-flexing the bimetal leaf IIRC).

Anyway, I'm on Brian's side of the pond right now but I'll put some pictures up when I return to Ireland.

Cheers,
Jeff.
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Brian Vogel
Grand Master
Username: guyslp

Post Number: 1481
Registered: 6-2009
Posted on Friday, 10 July, 2015 - 10:41 am:   Edit PostDelete PostView Post/Check IP

Jeff,

If you happen to be passing through the Shenandoah Valley send me a PM.

Brian
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Bob Reynolds
Prolific User
Username: bobreynolds

Post Number: 283
Registered: 8-2012
Posted on Friday, 10 July, 2015 - 09:15 pm:   Edit PostDelete PostView Post/Check IP

This is another of those items gathering dust on my garage floor!

Scintilla Switch

The 3 wires go to the 3 brass rivets at the top. The brass circle at the bottom is not an electrical contact but an adjustment screw that presses on one of the bi-metallic contact springs.

The (intended) purpose of this unit is to control the choke solenoid and switch it off almost immediately after the engine has fired (when the Ignition light goes out) except in sub-zero temperatures where it holds it on for a few seconds longer.

On a Shadow 1 it is in the Relay Box at the back of the engine compartment and very difficult to get to.
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Brian Vogel
Grand Master
Username: guyslp

Post Number: 1494
Registered: 6-2009
Posted on Saturday, 11 July, 2015 - 08:41 am:   Edit PostDelete PostView Post/Check IP

Bob,

Thanks very, very much for posting this photograph.

Now, for the second question just to make things clear for me: Is this switch normally open (which is what I'd expect) or normally closed?

It would seem to me that the heating on the two "arms" is what would cause the switch to close and natural cooling (and rapid natural cooling) would cause it to open again. I'm trying to figure out if only one "arm" is heated when ambient temperatures are above some level while both are heated when it's really cold so that it stays closed longer.

The concept of this thing is intriguing. How it might actually implement its intended function is even more so. (Yes, I am letting my inner geek run wild!!)

Brian
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Geoff Wootton
Grand Master
Username: dounraey

Post Number: 824
Registered: 5-2012
Posted on Saturday, 11 July, 2015 - 01:52 pm:   Edit PostDelete PostView Post/Check IP

Bob

Many thanks for the photo from me also. I quite like to understand the inner workings of these devices.

My guess (since the scintilla switch is not documented) is the switch is normally closed when the engine is switched off and cold. This makes the circuit to ground for the choke solenoid. When the engine is started, the choke solenoid is energized and remains so until the bi-metal strips in the scintilla switch heat up enough for them to turn the switch off and thus break the choke solenoid circuit. This means of course, that the whole time the engine is running, the coils are heated to keep the switch open and prevent the choke solenoid re-energizing. The power for the coils comes from the control box, which means it is only available when the alternator is turning - i.e. engine is running. This is my interpretation of how the system works. I could well be wrong. If anyone could clarify the working of the choke solenoid circuit, I would be grateful.

I have read that this entire system only comes into operation at less than 4C. However, I have not been able to locate an otter switch in the wiring diagrams (SY1 and SY2) that would effect this.

Bob - You mention the scintilla switch is in your junk pile on the garage floor. Is this because you have found the choke solenoid/scintilla switch is not necessary, or is it just a spare?

A couple of years ago I repaired the choke solenoid on my car; one of the wires had broken off. As an interim measure, I made a plate to fit over the mounting aperture for the choke solenoid. I have not yet re-fitted the repaired choke solenoid. I have never had any trouble starting the engine, first turn (other than when I experienced unrelated ignition faults). This is because the car has always been used in hot climates (Nevada and latterly, Florida). I guess when Rolls export their cars to the US, they have to cater for extremes in temperature. In the Northeast, temperatures are regularly well below zero. -10C is quite common. This explains why they incorporate such devices as choke solenoids and air blend valves on the intake system. So if you live in a place with a hot climate then there is no necessity for such things. You can take them off and throw them away (figuratively speaking, that is).

I think I am inadvertently following in Bob's footsteps and building a lightweight Silver Shadow, by removing all the unnecessary junk in the engine bay.

Geoff
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Geoff Wootton
Grand Master
Username: dounraey

Post Number: 825
Registered: 5-2012
Posted on Saturday, 11 July, 2015 - 02:28 pm:   Edit PostDelete PostView Post/Check IP

I think I've figured it out. There is no external switch to disable the system when the ambient temperature is below 4C. This function actually takes place within the scintilla switch. The contacts within the scintilla switch are only closed when the temperature is below 4C (depending on the fine adjustment inside the switch). Above this temperature, the switch is automatically open, because of the positioning of the bi-metallic strips. So, if the engine is started at above 4C the coils are still heated, but this just causes the bi-metal strips to open further. i.e. the switch goes from open to more open. If the engine is started at below 4C the switch goes from closed to open. If my conjecture is correct, it still means the coil is always being heated whilst the engine is running, which seems a bit wasteful.

Geoff
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John Kilkenny
Prolific User
Username: john_kilkenny

Post Number: 205
Registered: 6-2005
Posted on Saturday, 11 July, 2015 - 05:43 pm:   Edit PostDelete PostView Post/Check IP

Geoff and Brian,
Refer to Tee One Topics Issue 45 Page 670.
This covers the Shadow 1 but is mostly applicable to later cars.

John
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Bob Reynolds
Prolific User
Username: bobreynolds

Post Number: 284
Registered: 8-2012
Posted on Saturday, 11 July, 2015 - 10:58 pm:   Edit PostDelete PostView Post/Check IP

Geoff has got the gist of it. I have actually posted an article about this system a few years ago, and that is basically what I said. Yes, it's a poorly designed circuit, the Scintilla switch is heated up for the entire journey, and doesn't actually do anything unless you're at the North Pole - and then only for a few seconds after starting.

My switch wasn't working when I got the car. It took me 2 years to find it and get access to it. When I did, I found that the adjustment screw had been backed off completely and the contacts were never making.

Rather than just disconnect it, I made up a nice little electronic control box to replace it and mounted it next to the Choke Solenoid, for easy access. There is an adjustment knob to adjust the time delay, and an LED to indicate when the Choke Solenoid is energised.

Electronic Choke Controller

However, I soon found out that anything but the slightest delay made the engine 'hunt' immediately after starting, due to the mixture being far too rich. I had initially guessed that the choke would need to be held on for about 30-60 seconds (as on a conventional manual choke SU carb) but No.

In normal temperatures the choke doesn't need holding on at all after starting. The Choke Solenoid only needs to hold the choke fully closed during cranking, after which it can be released straight away. Remember that when the Choke Solenoid releases, the choke is still being controlled by the bi-metallic spring in the black cylinder next to the choke flap. It doesn't suddenly spring wide open!

So I did the obvious thing and rewired the Choke Solenoid relay (called the Choke On Start Relay on the diagram) directly to the Starter Relay, so it just operates during cranking. The car starts first time every time throughout the year, here in the UK. I wouldn't drive the car in sub-zero temperatures anyway.

So although my control box was a waste of effort at the time, it is now destined to provide a Variable Wiper Delay facility.
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Bob Reynolds
Prolific User
Username: bobreynolds

Post Number: 285
Registered: 8-2012
Posted on Saturday, 11 July, 2015 - 11:54 pm:   Edit PostDelete PostView Post/Check IP

As a footnote to this: I have been considering a complete re-design of the Shadow automatic choke and fast idle system on my car. I am thinking of removing the choke control spring and its housing and replacing them with a servo motor, which just happens to be a similar size and shape to fit! The motor would be driven by a circuit which monitors the engine temperature (and possibly the ambient temperature) and would provide very accurate control of the choke flap and the fast idle cam at all temperatures.

This would also mean that the 2 stove pipes could be removed, along with the Choke Solenoid and other associated electrical equipment and devices!

There's nothing fundamentally wrong with the original system, but I find that the fast idle can be a bit hit-and-miss in certain conditions. Sometimes it seems as though it is going to stall when first starting off, even though the fast idle speed is set correctly. Other times, when the weather is warm, the fast idle seems to race until the choke has opened up sufficiently.

The fundamental problem is that the choke control spring is a linear device, but a linear response is not the best one for controlling the choke flap and fast idle. I would actually prefer to have a manual choke and fast idle control to the original one fitted on the car.
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Brian Vogel
Grand Master
Username: guyslp

Post Number: 1497
Registered: 6-2009
Posted on Sunday, 12 July, 2015 - 12:46 am:   Edit PostDelete PostView Post/Check IP

Thanks to all for the additional detailed descriptions and pointers to resources. I'm actually amazed that the scintilla switch continues to function over the decades with those (admittedly tiny) heating elements being "perpetually on". It would not be good were they to fail when the ambient temperature were below 4C, since from what I've read that would mean the choke solenoid would stay in perpetually energized mode were there nothing to heat up the contacts causing them to flex away from each other.

I do drive my car when it's below freezing so I suppose this thing has kicked in on those occasions, but I really don't know. I've never had trouble getting the car to fire up (except before I understood that you must wait to hear that the fuel pump has stopped ticking before starting to crank the car in my early days of ownership).

Brian
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Geoff Wootton
Grand Master
Username: dounraey

Post Number: 826
Registered: 5-2012
Posted on Sunday, 12 July, 2015 - 12:59 pm:   Edit PostDelete PostView Post/Check IP

Bob - Many thanks in forwarding all this information. I am now clear about how the scintilla switch/choke solenoid works. I'm also clear that as long as I am living in a warm climate I need not worry about refitting the choke solenoid.

The idea about using a servo in place of the choke spring has also occurred to me, but mainly due to fear of it becoming unavailable. However, I have just checked and see choke springs are still listed on FS website at a very reasonable $60. I understand your point about the more accurate control of the choke flap and it would certainly be an interesting project.

John - My first port of call when looking for information is Tee-one topics. I was surprised when you mentioned Issue 45 page 670 as I had searched the index twice with the keywords "solenoid" and "choke" and only managed to pick up issue 5 page 38. This gave a lot of great information, but not the whole story (and was not intended to). I rechecked my search and the reason I did not pick up the relevant page was the index has the misspelling "chocke". My "choke" search obviously jumped right over it. Interestingly, I had read the article, but a couple of years ago, which is why I had this notion of the otter switch "disabling the entire system", as I put it. In fact, re-reading the (excellent) article shows the early cars did use an otter switch to disconnect the scintilla switch, after the engine was running. Strange that the engineers discarded this early on and just allowed the scintilla coils to run continuously.

Brian - interesting point about the heating coil failing during normal running in cold weather. I guess the engine compartment temperature would prevent the choke solenoid from re-energizing. However, re-starting the engine from cold would cause some problems. This is why all this information is so useful - another possible cause if the engine refuses to start/run properly.

Geoff
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Bob Reynolds
Prolific User
Username: bobreynolds

Post Number: 287
Registered: 8-2012
Posted on Sunday, 12 July, 2015 - 07:37 pm:   Edit PostDelete PostView Post/Check IP

"I'm also clear that as long as I am living in a warm climate I need not worry about refitting the choke solenoid. "

I would refit the choke solenoid, because it does hold the choke flap closed during cranking. The bi-metallic spring is not strong enough to keep the flap closed during the sudden inrush of air when the car is started. You can see this by watching the link rod bouncing up and down when you operate the starter!

Just wire it directly to the Starter Relay like I did.