SP250 - Temp & Fuel gauge voltage stabiliser

[Magnor]

I have been informed that Smiths temperature and Fuel gauges require a voltage stabiliser unit on supply side to keep the instrument readings stable. I cannot see one fitted to my car and cannot see one on the wiring diagram? Does anyone know if there is one fitted and if so where?
Thanks
Clive

[Ian Hastings]

As far as I know the SP never had one fitted - but I may be corrected.
There definitely isn't one on my car and the gauges are quite steady. Unsteady needles are usually due to bad electrical connections and/or senders.

[Fossil]

My 1961 B spec did not have one either. I fitted one during the recent re-trim (because the temp gauge was inaccurate when lighting and other loads were switched on) when rebuilding the dash. It is relatively simple to do, for example by unscrewing the centre panel and pulling it out to give access to the gauge and switch wiring, rather than trying to do it from below.

Regards

Geoff

[Vortex O'Plinth]

The SP fuel gauge is the moving coil type which relies on the relative value of coil currents rather than absolute values and is thus stable over a wide range of supply voltage. No voltage stabiliser was fitted as standard to the SP.

Despite this insensitivity to voltage variation it is very difficult to achieve an acceptable degree of linearity by adjusting the gauge. Once it moves off 'full' there's not much reliable information until it suddenly reads almost empty.

[Magnor]

Thanks for all the replies. As I have been given a stabilser unit I may fit it and see if it improves the fuel and temperature gauges stabilty. The guy who told me a unit should be fitted actually worked calibrating and repairing Smiths gauges so I guess he should know. It is strange that the SP did not have one fitted but MG's did?
Clive

[Vortex O'Plinth]

Whether a cars instrumentation (Temperature, fuel level or voltmeter) will benefit from a voltage stabiliser depends on the type of gauge fitted. Post-war cars until the early '60's were usually either moving coil or moving iron instruments and relatively insensitive to voltage variation. They are recognisable in that the gauge gives a reading immediately the ignition is switched on. Fuel gauges of this type would fluctuate as the fuel sloshed around in the tank and were sensitive to jolting from bad road surface, so in the mid-sixties these were gradually being replaced by the bi-metallic gauge which was more robust and was insensitive to temporary changes in fuel level. However, this type is sensitive to voltage variation and required a voltage stabiliser. It can be differentiated from the moving coil/moving iron type in that the gauge reading is not immediate but climbs gradually when the ignition is switched on. Smiths Instruments made both types of gauge.

The contemporary MGA did not have a voltage stabiliser and I believe neither did the early MGB, although it did appear on the later cars. I took this to indicate a change in the type of instrument fitted during the production run.

[Magnor]

Hi Nick,
Thanks for the explanation. Fountain of knowledge as usual and a good reason to join the club and post. Maybe I won't bother fitting stabiliser when I rebuild the dash! Is there any way of telling which type instrument is fitted from looking at them (without taking gauge apart). The temperature gauge I have looks a bit better than the rest (cleaner face) so may be a newer unit.
Clive

[Vortex O'Plinth]

If your car has the original instruments then the fuel gauge will be of the moving coil type. Bi-metal gauges were never fitted as standard to the SP.
If any gauge has been replaced at some time in the cars life, then I suppose it could conceivably be of either type, but it's very unlikely that the dial face of a bi-metal gauge would match the original. Below are the original gauges from a later SP.

If a bi-metallic gauge has been fitted there will almost certainly be a voltage stabiliser in the circuit, since without it the gauge would be unreliable and probably irreparably damaged by the over-voltage.

[Ian Slade]

The moving coil gauge has two nuts on the back, loosing them allows you to calibrate the instrument by moving the coils slightly, can take a long time but with a bit of patience you can usually get empty with power on and full ( short circuit and open circuit), then hopefully the mid point but that does take a bit of juggling.

[Vortex O'Plinth]

If you do want to try to re-calibrate your moving coil fuel gauge (and give yourself many hours of harmless fun) then you may find this article useful. It relates to an MGA gauge but the principle is exactly the same for the SP. However I do think Ian has rather understated the amount of patience needed! The fundamental non-linearity of the system can easily be seen from the relative positions of the 'F', the '½' and the 'E' markings on the gauge. I think you are doing well if you can get a reasonably accurate reading for both full and empty!
Before having a go do take note of the paragraph headed NOTICE and CAUTION in the article. It can be very easy if you're not careful to break the very fine wire connections to the coil - and from personal experience, re-soldering those can be a life shortening experience!