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MG MGB Technical - Smiths Battery Condition Gauge

Does anybody know how to connect the above gauge to a 1970 BGT ?
Bruce Mansell

I'm pretty cetain these arte just a voltmeter with a scale marked in chat rather than volts. I would put it to the switched side of the ign.
What does it say on the dial?
Stan Best

I believe they're nothing more than a voltmeter. Thus they don't require the heavy gauge wires that an ammeter need. Should be able to hook it up between any reliable 12V (nominal) source and a good earth. I've got one lying around somewhere but have never got around to installing it.

TTFN

Derek Nicholson

Beat me by that much!

I do agree about hooking it to a "switched" source, though. Good point, forgotten.

TTFN

Derek Nicholson

Aye, just a voltmeter calibrated in good, bad and indifferent. It's also something of an antique (OK, period) in itself. As well as switched you might also like to consider fused rather than unfused i.e. the green circuit rather than the white. An unswitched circuit e.g. the brown (unfused) or the purple (fused) would tend to discharge the battery albeit very slowly. The only problem with the white, and even more the green circuits is that in use these exhibit less voltage that is at the battery and hence can make you paranoid about charging. If you are going to fit one you really need to use them as a 'change' indicator and not as an absolute value indicator. i.e. if it normally indicates 'OK' and suddenly drops to 'no charge' then you do ne4ed to investigate things. But as John Twist has said "Except in the rarest of circumstances the ignition warning light will tell you all you need to know, buy yourself a nice pair of driving gloves instead".
Paul Hunt 2

Hi all.

Just for completeness..

The instrument has two connections for the measurement circuit. One of these is marked with a blob of red paint on mine and I assume that this is intended to go to +12V (but read on).
The actual movement is a heater-and-bimetallic-strip type and appears to be insulated from the case, so is not polarity concious, does anyone know why one tag is marked red ?.

Regarding illumination, the lamp needs to be tapped into a red/white wire and the case of the instrument needs to be connected to earth unless the lamp holder has a black earth wire connected, as some do.

As Paul says, I would certainly connect it to an ignition switched feed rathere than a permanently on feed (it takes about 0.1A).
Ideally it would have it's own fuse from a white feed as this will avoid the small volt drop that will develop across the 'green wire' fuse and wiring as the 'green wire' circuits draw current, though in practical terms I doubt this is significant.
It needs a fuse somewhere as there is a small risk of an internal short to earth if the internal very thin wire insulation broke down.

BTW, with any fuse the best place to put it in the circuit is usually as close to the voltage source as practical, so that there is protection should the wire going to the load short to earth due to insulation chafing, melting or otherwise breaking down.

Don
Don

Does it have internal illumination? If so maybe the return path of that is internally connected to the terminal that *isn't* marked with a red blob, which would go to earth/ground.

Whilst in theory there should be no volt-drop, in practice, and especially on cars our age, you can get a successively higher and noticeable volt-drop through the brown, white and green circuits, especially in a GT with the heated rear window and the brakes on. Personally I think it is this that makes people keen to fit higher rated alternators, but it is the voltage at the *battery* which is important and can be a volt or two higher than that on the green, especially one or two 4-way bullet connectors from the fusebox.
Paul Hunt 2

This thread was discussed between 02/11/2006 and 06/11/2006

MG MGB Technical index

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