Smiths Fuel gauge Coil resistances

[Pete4]

I can only get my 1958 smiths fuel gauge to read from empty to ¼ full ......even with adjusting the coil positions. Sender reads correctly from 2-80 ohms. Earth is good.

I suspect one of the coils inside is shot - Does anyone know the resistance (ohms) of the EM coils and shunt inside 1958 smiths fuel gauge.

[Calum]

If one of the coils was dead I would expect it to either read full all the time or empty all the time. What does the needle do with the sender wire shorted to earth? With the wire disconnected? Is the arm free to move when the sender is installed in the tank? (I once had the float foul on the side of the tank)

[Pete4]

Thanks Calum.  It moves to empty when earthed.  pretty sure it moves to "full" when disconnected but i'll check.  The sender is new and reads 2-80 ohms and when connected up with good earth the needle responds to it appropriately but only to max of ¼ full.  Ive adjusted the little coils but cant get better than this....moving the "empty" coil has good affect, moving the "full" coil doesnt.  Ive opened the casing and the coil is  working on meter testing but I wonder if its gone bad.  On the previous one the shunt had burnt out and must have shorted at some stage. 

[Wittsend]

One assumes that you have the correct "early type" tank sender that goes with your fuel gauge

.... and even though your sender may be new - it could be faulty (cheap pattern parts copies).

 

[Pete4]

Yes it's the correct sender and has the correct resistance readings at full and empty. I can't tell how good it is in between  yet. 
I'm guessing it's a coil or shunt problem but  haven't got a baseline resistance to compare to. I did find this somewhere on the internet.... Can't find it since. Did anyone know if these values are correct?

[NoBeardNoTopKnot]

If you truly want an accurate gauge get yourself a Gauge Wizard. Use it to calibrate ANY sender to ANY gauge. All such that you can have your gauge set to precisely show empty the split second it is. FULL when it is FULL. 1/4 full when it is 1/4 full. No messing. Not nearly, not just about, bang on.

In my case, I've not even got a resistive sender, my sender is 1.3v empty; 4.2V Full,  sends volts not a resistive value, yet this weird and wonderful sender displays to a standard Smiths / Jaeger cluster: 20Ω full 220Ω empty (BF type gauge).

ie if you can do that with the thing, what you want to do is a walk-over.

Put any old sender in there, and be done. Don't prance about worrying if it'll match, it will, and this gadget will have it better than the 'correct' item. If you want an actual fuel-gauge that gives PRECISE indication it'd be the only way.

AND... it gets better. If you've got an existing gauge and it reads all over the place, this doodad zeroes it in.

See it at: https://spiyda.com/fuel-gauge-wizard-mk3.html

Doubt this, here's a CB08 VW Jetta sender sitting on my engine-bay, wired to a BF. (Suspect you have an FG?) It works well, now if that works, anything will! Certainly any Smiths FG.

[Pete4]

Genius!  I'll take a look. Thx for advice 😁

[Serious Series]

The gentlemans guide to smiths guages has this inforrmation for moving iron fuel guages in it.

[NoBeardNoTopKnot]

Yes,  nice enough only you actually will not need that stuff.

In short - Tear-up the rule book. ... provided both sender and gauge DO work, it's irrelevant if they match. The Gauge Wizard allow you to calibrate in 5 x points ...

EMPTY of sender to 'who cares where' EMPTY is on the gauge
1/4 of sender can be a mile from 1/4 on gauge
1/2 of sender can be a mile from 1/2 on gauge
3/4 of sender to somewhere 3/4 on gauge
FULL of sender to whatever is FULL on gauge

And it matters not. Each will be a mile away from the other at those 5 x points. No matter how good you eventually get the stock sender and gauge to work, you can never match in 5 x points. That's impossible on standard resistive gauges because the graph can only cross at one point, NOT x 5.

Hence we can head this  off at the pass now. The "I've been doing it this way for forty years; time-served design; I've never had any problems" line may be all very well, and work OKish, only you'll never see it work spot-on, because it can't. Gauges of this era never were that precise.  It's not possible to calibrate to 5 x points.

This doodad translates all 5 x points, and brokers the deal, such levels; sender and gauge, each require. All regardless. This is why my extreme example, a VW Jetta sender to a LR gauge works. Thus any  that tell you to look in the LR parts book have not understood what is happening here.

[Pete4]

Valid points & looks to be a good solution.
My latest ebay fuel gauge didn't work at all - once opened (quite fiddly) the wires had broken on both coils. Unwound a bit more, stripped the enamel coating and resoldered - worked beautifully. Full was full after adjusting coil position. Empty only ever got to ¼. So manipulated the needle and readjusted coils and all good. Its "original" but if misbehaves will definitely look at the wizard mentioned above. Thx for the help all.

[NoBeardNoTopKnot]

Be aware with the BT gauge you're wanting a 10V regulator. Thyey're 20p each. However, the classic-car set are happy to pay £15-20 for them. You'd think this was something special. It's not...

You want a LM7810, for £15-20 you should be able to get 150-200 of the things, I would hope more....

Postage is10 x  higher than the real-world cost of these...