Lotus Elan

Voltage choke ... I think. Need confirmation

PostPost by: mn1 » Wed Jun 12, 2019 7:07 am

Can any one identify this? It has 2 terminals one of which is marked "IGN" and "SB" enclosed by a circle is engraved into the backplate. It sits close to the fuse box and has Green/B wire in and out. It wasn't earthed. I think it may be an inline choke fitted to later +2's (to avoid fluctuations of fuel and temp guages, according to the manual). Mine is a rebuilt +2 with new loom circa 1993 and the wiring to this box, I think, is incorrect, which leads me on to why I'm digging around. The fuel guage has always read low and as part of my recent radio replacement, tacho fix, fuel guage testing and fixing a few other minor niggles I removed this box and just direct connected the 2 wires (Green/black) on the basis they were inline. Lo and behold fuel guage appears to be accurate (or at least more so - I have tested and number matched the compatibility of sender with the guage) but now the temp guage reads high when previously it was perfect. Thought I'd put the box back in line to positively id its removal as the cause of the temp guage issue and in trying to determine which wire went to "IGN" I voltage tested both wires with the ignition on. They both read a bit under 12v...so something is weird. Anyway I'm taking a step back and starting with positive ID of what this box is .... before pulling the dash apart for some serious wire tracing.....again!
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PostPost by: nmauduit » Wed Jun 12, 2019 10:00 am

looks like a original Smiths voltage regulator (for the dashbord instruments requiring 10V)
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PostPost by: Andy8421 » Wed Jun 12, 2019 11:07 am

The fuel gauge and water temp gauge need to be run from a 'stabilised' 10V supply. Smiths fitted a dreadful bimetallic thing behind the instrument panel to take the battery voltage and drop it to an average 10V.

I suspect what you have found isn't a voltage regulator - or if it is, it is in the wrong place.

Firstly, it needs to be earthed to work, secondly the standard Smiths unit has different markings to yours, and thirdly Green / Black connections usually run from the Fuel gauge to the sender directly. The voltage stabiliser should have Light Green connections between it and the instruments.

Given someone has messed with the wiring, who knows what is going on. Have a look at this - it shows the correct connections for the stabiliser and may help.

http://www.lotuselan.net/forums/viewtopic.php?f=38&t=32907&start=
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PostPost by: nmauduit » Wed Jun 12, 2019 12:17 pm

on Smiths 10V bimetal voltage regulators the ground is connected by the casing (if rivets/spot welds are bad that can cause an intermittent malfunction), then there is an input (battery side) and output (10V instrument side), but since it is a thermal effect one should be able to swap terminals and see no difference in operation (or lack thereof).

see for instance:

Image

viewtopic.php?f=38&t=41413
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PostPost by: billwill » Wed Jun 12, 2019 12:23 pm

nmauduit wrote:then there is an input (battery side) and output (10V instrument side), but since it is a thermal effect one should be able to swap terminals and see no difference in operation (or lack thereof).




I don't think that is true, the heater for the bi-metalic strip is presumably connected to the output terminal, else it would not regulate.
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PostPost by: nmauduit » Wed Jun 12, 2019 12:40 pm

billwill wrote:
nmauduit wrote:then there is an input (battery side) and output (10V instrument side), but since it is a thermal effect one should be able to swap terminals and see no difference in operation (or lack thereof).




I don't think that is true, the heater for the bi-metalic strip is presumably connected to the output terminal, else it would not regulate.


you are right, I had never tried actually (replaced mine with solid state inside...): with terminal connections swapped the heating would never stop then it would be only very briefly on (I was conjecturing that from the chassis pos/neg tolerance of these components without thinking very far - maybe I just had it reversed when I tried mine... oh well )

had a glance at that page for internal schematics
smithsvoltage-02.gif and
Smiths voltage regulator schematic

https://www.minimania.com/Smiths_Voltage_Stabilizers
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PostPost by: mn1 » Wed Jun 12, 2019 11:06 pm

I have used the term "choke" deliberately. as it refers to a Lotus workshop manual item description (section M page 82 in my manual), but in order to help clarify the query. I have a voltage stabilizer correctly wired sitting behind the speedo, earthed and wired with LG and G /b wire in and out. It is exactly the same as the item in the photo from nmauduit - 2 terminals, I - instruments, B - battery. The item I am querying is down stream from the stabilizer.
I agree it looks like it should be earthed but even installed as it was it had an affect on the fuel guage and temp guage as I originally described.

Maybe if I change the question. Does anyone know what a CHOKE looks like. "Commencing chassis #50/1661 - Part number 50 M 6017"
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PostPost by: Bud English » Wed Jun 12, 2019 11:26 pm

A choke is an inductor that blocks high frequency AC while passing DC and low frequency AC. Maybe added along with the alternator upgrade to block that high frequency noise?

They can take several forms. The parts manual shows this one in Section MB -- Electrical -- Fuses & Relays, item 20, but it doesn't look anything like your picture. I have an earlier car so I'm no help with an actual picture. Hopefully someone that has a later car will chime in.

Edit: The choke I referenced appears to be used between the fuse block and the radio for noise suppression, so probably not what you're hunting down.
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PostPost by: mn1 » Thu Jun 13, 2019 5:34 am

Tks Bud, The word choke may be technically inaccurate in the broader sense but I am using it as Lotus have in the manual...we'll get there one way or the other.
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PostPost by: alan.barker » Thu Jun 13, 2019 6:23 am

"I" is for Instruments
"B" is Battery supply.
The body must be Earthed.
Voltage Stabiliser is it's name.
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PostPost by: mn1 » Thu Jun 13, 2019 6:43 am

Almost 100% sure it's not, actually. Just filter MN1 posts and ignore the rest.
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PostPost by: john.p.clegg » Thu Jun 13, 2019 7:57 am

Pretty sure it's not the 10V Stabiliser/Regulator with the SB and IGN marking,five minutes with a sharp knife would open it up for all to see , easy ro reinstate..

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PostPost by: billwill » Thu Jun 13, 2019 1:21 pm

Trying to imaging what SB stands for ??? IGN is probably the coil, but could be almost anywhere else on the ignition circuit.

Perhaps what you have there is a ballast resistor.
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PostPost by: john.p.clegg » Thu Jun 13, 2019 2:13 pm

Bill
Am I mistaken or isn't SB used on old coil connections before + and - ?

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PostPost by: Craven » Thu Jun 13, 2019 2:18 pm

I think that was CB contact breaker.
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