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MG Midget and Sprite Technical - Testing a fuel guage

My Frogeye fuel guage isn't registering anything. It could well be knackered as the needle probably hasn't moved since the car was last on the road circa 1983. At the tank end I have a sender unit which I think is OK as before fitting it I tested with a metre to see the resistance change as I moved the float arm. The sender is properly earthed, not just through the tank but with its own earthing wire back to the main body shell.

I plan to remove the guage for checking tomorrow. What checks should I make and is there a way I could try rigging it up to test it off the car?

Perhaps I should add, the tank is pretty low on fuel as it hasn't yet been to a filling station. Its had maybe 1 & 1/2 gallons put in but has run for quite a while whilst setting the carbs and ignition up. That's why a test off the car would be good. I am not too bothered about accuracy at this stage, I just want to see that it responds reasonably well.
GuyW

I've got a Sprite Mk2 and it should be the same as Frogeye. On the schematic the gauge has a green wire (switched live feed) and a green/black wire which goes to the tank sensor which is a variable resistor to earth. If you earth the green/black wire on the gauge it should jump to full.
Rob
MG Moneypit

Thanks. So the first would be to check voltage on the green wire with ignition on?
Then if that is ok, I earth the green/black one.

Would removing it and similarly connecting it directly across battery terminals achieve the same thing?
It is a positive earth car.

I have a test lead that takes modern spade fuses that I would use. But I think 5amps is the lowest I have.
GuyW

Guy
If you have the April 2017 MASC there's an article on fuel gauge. In summary:
Basic resistance test: T terminal to earth is 100 ohms, T to B is 60 ohms. (Both give or take). If either are open circuit (high ohms) gauge is faulty. Wire to T (tank) terminal should measure a few ohms with low tank. Check that gauge body is earthed, if not it WILL NOT WORK.
Bill Bretherton

Guy
Don't wire your gauge directly accross a battery
Smoke will appear
Best to pull the wire off the sender
turn the key on and earth the wire and the gauge should go to full
Don't leave it at full for any time , just earth it long enough to check that it sweeps and then off

willy
William Revit

Thanks Bill. I had forgotten about that MASC article. Useful!
And I am pretty certain the sender body is properly earthed. It was one of the little upgrades I did when making the loom, to add a dedicated wire to earth the sender and tank to the car body.

I hear what you say Willy, about smoke, but why? Isn't 12v what it normally gets when connected and a full tank?
GuyW

The sender is almost down to zero ohms when tank is EMPTY (Frogeye, not later) so 12v to gauge with T terminal earthed should give ZERO reading. As the sender resistance increases with more fuel, the gauge gives an increased reading until FULL registers at about 80 ohms. That sounds counter-intuitive but it's how the gauge works. It's not a straight meter like later gauges but uses a "bridge" circuit to compensate for voltage variations (so stabilizer not needed). Another test: if you disconnect sender cable (T terminal) then, with ignition on, you should get FULL reading. If not re-check gauge is earthed then check gauge resistance readings with ignition off.

So I disagree with Willy (!). The early gauge is different to the later gauge.
Bill Bretherton

Thanks.
When my questions get down to how to check out a fuel gauge it's an indication that I am nearly finished with this restoration! Dangerous statement, that, as there will always be more that can be done!
GuyW

My bad Guy/Bill, had forgotten it wasn't stabilized
Sorry for the misguided guidance there
willy
William Revit

Some checks.
I disconnected the cables from the gauge, checking they had been correctly connected to T (tank) and B (battery)
Cleaned the terminals and brass washers.
Resistance B to T = 61.2
Resistance between the tank sender wire (at the gauge end, but disconnected from it) is pretty well zero. T to E = 1.3 ohms

I'm not sure how to interpret this. J E Davies' article says the internal resistance of the gauge is 100. Mine measures 61. Bill says it should be 60. And I presume that when reading T to E I am getting the resistance of the sender unit when on empty. That, or I have got my earth and signal wires from the sender crossed at the bullet connector in the boot.

With the B terminal on the gauge connected, ignition on, when I then earth T I just get a flicker of the needle, but it doesnt go to full.

The caption to the wiring schematic (as Bill quotes) seems to mis-match with the text in the second paragraph wrt the gauge internal resistance (60 or 100?) Plus if I read paragraph 3 right, earthing T should equate to an empty tank, not a full one.
GuyW

60 ohms B to T is correct. Just checked mine. Your sender sounds ok at low ohms for low fuel. Have you tried ignition on with T disconnected? That should indicate FULL with gauge properly earthed.
Bill Bretherton



With B connected and T disconnected you should get a full reading and then earthing T should give an empty reading which is what Bill says in his last post
If your gauge doesn't go to full with T disconnected then either the gauge itself is faulty or the gauge isn't earthed properly
William Revit

I think my brain cell is catching up now!
There are two resistors, one of 60 ohms between T and B and a 100 ohm between T and the gauge casing.

And for simulating full I need to earth the casing not T. I had missed the point about earthing the gauge and it has been hanging out if the front of the dash whilst checking, so no earth! Earthing it, with T disconnected and I get a full tank indication so I think all is well.

I see that there are two nuts on the back of the gauge with slotted fixings, suggesting a means of possible adjustment to get an accurate reading? I will leave well alone for now, but may come back to that later when I have a full tank.

Thanks for your help, Bill
GuyW

For future gauge calibration, lifted from Paul Hunt's site for MGBs but the principal is the same. -
"However it is important to do 'Empty' last as is the more important one to have accurate, and changing one adjuster does have a noticeable effect at the other end of the scale as well."
Nigel Atkins

Good that it works Guy! I think the deflection is the reaction between two coils. Clever design.
Ensure casing is well earthed when installed in dash.
Bill Bretherton

As with the sender, I added earth tails for each of the gauges when making the loom. Every unit has an earth wire to the point that I think the loom would work with a plastic car!
GuyW

The original loom has a specific earth wire to the fuel gauge. I had always wondered why. Makes sense now.
Bob Beaumont

There is a very nice gadget called Gauge Wizard that allows the gauge to be calibrated very accurately as you want it it, also with a programmable safety margin for E.

It is very fiddly to calibrate (ask Steve Gyles on the MGA BBS for his instructions (much better than what the supplier offers)) but then the gauge is accurate and doesn't swing around like mad either as fuel moves n the tank.

But you do need a working gauge as a starting point!
Dominic Clancy

Yeah
I wouldn't get into fiddling with the gauge adjustments- They would have been set up when new and worked for(60)years at that---good earthing is the main issue
One slot is for full sweep and the other i think for half, If your sender unit is good it should read ok----leave well alone, the gauge doesn't need to be dead acurate, it's only a guide the needles always have wandered around a fair bit going round corners hard and up and down hill
That's why the later cars went to stabilised systems which sort of flatten the peaks off the waves but no different really

It's a feature not a fault, enjoy it

willy
William Revit

Quite agree Willy. The plan now I know that it works is to leave well alone for now at least. Only if it is dramatically out would I want to start trying to adjust it.
GuyW

This thread was discussed between 01/12/2020 and 02/12/2020

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