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MG MGB Technical - Ignition lamp help?

Hi all,
Sorry, I'm from the Dark side...I'm a midget chap looking for a little help.
First off.
Planning to replace the RVI Tacho in my Mk III Midget with an RVC Tacho from a rubber bumper BGT.
The gauge I have does not have a hole for the ignition lamp, so I guess I shall have to fit a separate warning lamp as on an MGB.
I THINK the part ( from the Moss catalogue ) is BCA4780Z
My question is, as I don't have a separate warning light on my Midget, how do you actually fit this part? In the Moss catalogue , it LOOKS as if it just pops into a suitable hole I will drill into the dash, is this right?
Second off. As I said , I'm a MG Midget man, and I don't have an MGB wiring diagram, so does anyone have an MGB GT rubber bumper circuit diagram they could kindly scan up for me please, or could they point me to one , I have checked on the web but cannot find one.
Seasons greetings etc,
Thanks all
Colin
colin frowen

Here you are http://www.advanceautowire.com/schematics.htm Just click on the MGB link and you will download a PDF with all the diagrams for all MGBs, both UK and US models. My 1969 MGB had the ignition light in the tacho as you describe. When I installed electronic instruments I had to fit separate ignition and high beam lights.

The ignition light is really a charge light. One side is connected to the dynamo or alternator and the other to the battery. When the dynamo or alternator is charging, the voltages on either side cancel out and so the bulb goes out Paul Hunt tells more here http://mgb-stuff.org.uk/electricstext1.htm#theory
Mike Howlett

Google advanceautowire for MGB wiring diagrams according to car model,year and country.
Sorry can't help you with installation of ignition lamp. Good luck.
C Harvey

Colin can I ask why you want to replace your RVI tacho? Is it because it’s not working? If this is the case are you running an electronic ignition system? If it’s yes to the first question and no to the second it may be easier to fix it rather than replace it. As long as it’s not a mechanical problem, in that the movement isn’t damaged and most are electrical problems, then I can probably repair it for a reasonable cost. This might be an easier fix than replacing it with an RVC tacho. Bob
R.A Davis

FAO Mike Howlett
Hi Mike, many thanks for that link help
----------------------
FAO Bob Davies.
Hi Bob,
The damned thing works then stops then again and again etc .
So, I have an RVC tach from an MGBGT which I intend to fit as a " fit & forget" then get the original RVI gauge made into a RVC gauge…. least, that's the plan!
Could you possibly give me an idea of how much that would cost please?
PM to colin.frowen@nhs.net
------------------------------
Cheers all for your help
colin
colin frowen

Turning an RVI into an RVC wouldn’t be that big of a deal electrically, you can do this a couple of ways. You could fit the electronics from an RVC to the RVI face and movement or there’s a company on ebay that supply RVI to RVC conversion boards. The cases of the RVI and RVC units are different to accommodate the electrical connections, so it would be preferable to use the case that goes with the electronics. The most common problem with these units is failed capacitors or potentiometer. The electronics in the RVI and RVC units is quite similar and neither should be unreliable once the above parts are replaced. You will need to run a wire to the coil to drive the RVC unit.

Bob
R.A Davis

Did Midgets always have the smaller 80mm main instruments then?

For the ignition warning light you need a bulb holder with two connections, not the type used for gauge illumination that only had one wire and the other connection comes from being pushed into an earthed metal panel. Can't speak for the Midget but rubber bumper MGBs had both the ignition warning light and the main beam warning light of this type as they push into a plastic panel, but you need the ignition one of course as that comes with a red lens, BCA4780.

Changing from an RVI tach to an RVC (or after putting RVC electronics into an RVI) is a bit more involved as the ignition supply to the coil goes via the RVI tach, but direct to the coil on an RVC. Then there is a trigger wire coming back from the coil -ve to the tach input. There should be two white wires at the back of the RVI tach going to the pickup. These should be joined together then a new wire run back from the coil -ve to the tach. You could run a new ignition feed to the coil +ve from the fusebox white, say, then move the existing coil +ve white to the coil -ve, then use that wire at the tach for the trigger. The remaining white at the tach carries unfused 12v ignition so should be insulated carefully so it can't touch any metal parts.
paulh4

Incidentally I note that some claw-type holders do have two wires, but one of them is connected to the body of the holder that pushes into a panel hole. These would NOT be suitable for use in an earthed metal panel for an ignition warning lamp.
paulh4

Only US market Midgets had 80mm clocks.

UK cars had 100mm throughout production.
Dave O'Neill 2

In that case an 80mm RB tach is going to be smaller than the speedo. Whilst RVI tachs might be rare I'd have thought CB RVC tachs would have been more common, and they have provision for the ignition warning light.
paulh4

Colin,
if you have a bulb holder that has two wires and one is an earth wire with a connector you can simply cut off that connector and put a bit of (yellow or whatever colour other than black) shrinkwrap or insulating tape over the black wire and then connect the wire in whatever way suits you or the situation you have.

You need a live bulb in-line to have the alternator charging (I might have wrong terminology but I think you'll know what I mean).

But I'd see how much Bob charges to recondition your original rev counter and prefer to go with that.

Nigel Atkins

As mentioned above a bulb holder with two wires is only suitable if BOTH of them are insulated from the 'claw' fitting if it is to be fitted in an earthed metal panel.

As Nigel says the warning light is part of the priming circuit to start an alternator charging at about 900rpm or so, but in practice they will usually start when revved to about 2k or so, and then charge normally down to about 600rpm or so. However without a warning light or a voltmeter you won't easily know if it is charging or not.
paulh4

This thread was discussed between 23/12/2019 and 27/12/2019

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