408 Fuel gauge not working diagnosis - opinions, pls?

[dwomby]

My gauge has sat unmoving, pointing at E the last few months (prior to that it tended to bounce between E and F). Today I disconnected the two wires on the fuel sender and attached a variable resistor between them. With the ignition on but engine not running, I could move the gauge pointer position quite precisely by varying the resistance (as the fuel sender should be doing!).

Am I right in thinking this means the gauge and the wiring are OK and it is the sender needs attention? Thanks.

David

[sv8predator]

Well. as you've removed the sender from the circuit and varied the resistance externally, it would appear that you are correct, the sender is faulty. Should be easy enough to remove it and discover that it is horribly clogged up and corroded.

But also check if the fuel gauge circuit uses a voltage stabiliser is the circuit. It is there to stop the fuel needle swinging about as the fuel in the tank swings about. They can be faulty and give spurious readings.

It will be a small alloy-bodied relay behind the dash.

[dwomby]

Thanks but I found a problem now I have removed the sender. The float is fine - empty. The body of the sender in the tank is coated in aluminium/aluminum oxide but otherwise everything seems clean. The problem seems to be the bar holding the coil of wire had been dislodged to one side and the sliding pointer was no longer in contact. I nudged it back and the pointer makes contact again. It reads from 0 Ohms empty to ~60 Ohms full. Is that right for our cars?

The bar does not seem a tight fit and I am concerned it may get dislodged again so I plan to pack something insulating between the side of it opposite the moving pointer and the casing. Hopefully that will hold it in the correct place. If not, it's time for a new one.

David

[Roger Morrall]

0 to 60 ohms sounds about right, but I’m sure you’ll check it actually drives the gauge correctly before you put it back. I say this as I’m slightly surprised that your gauge was showing empty with a duff sender - my experience was that the gauge showed well over full when the sender was duff and open circuit.

There is a relatively simple explanation of how these gauges work at Fuel Gauge - Working or not?

I know this describes a pre war Austin seven but the principle didn’t change over the years and it’s quite a good description.

Note that the body of the gauge itself has to be earthed, also that because this type of gauge relies on two opposing electromagnets it’s accuracy is relatively unaffected by battery voltage and it doesn’t need a voltage stabiliser - so don’t go looking for one as there you’ll never find it, there ain’t one. This type of gauge gives you an instant indication of the level in the tank when you switch the ignition on unlike the bimetallic gauge which was used on many other cars of the period which takes a little time to show what’s what and does need a voltage regulator. On the other hand the electromagnetic gauge does rather wobble about as the fuel sloshes in the tank which the bimetallic gauge doesn’t.

You won’t find it at all easy to find an original sender if you do need one but these guys

https://www.smiths-instruments.co.uk/


will be able to supply a modern substitute - you will need to quote all the letters and numbers on the original - if you go to their web pages Products then Senders you’ll understand why - the variety available is totally bewildering

Good luck

[dwomby]

Originally Posted by Roger Morrall

0 to 60 ohms sounds about right, but I’m sure you’ll check it actually drives the gauge correctly before you put it back.
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Good luck

Roger, thank you, as always, for your helpful response.

Indeed, this morning I did test the sender with the gauge via the car wiring. It worked! However, despite all my efforts, I cannot get the sender to go down to zero resistance and the sender at empty produces a gauge as per the attached photo. The lowest I can get it down to is ~3 Ohms and that resistance seems to be in the stud (the ungrounded connection) itself that the coil wire is attached to. I'll try cleaning it up but I am not hopeful. I'll probably live with things as they are since I will have a functioning gauge and there's always the reserve to rely on but I am considering buying a replacement. In any case, the sender will be out for a few days as I have ordered new gaskets for it.

Oddly, the sender is marked FT.5300/79 and not the FT.5301/17 listed in the manual.

With the sender out I got a good look inside that end of the tank. Shiny steel!!! I was pleasantly surprised by that.

Thanks for the links to the sender/gauge operation and Caerbont. I had found them. I agree, the array of senders is surprising. I wonder how many are electrically the same but just have different float arms to fit the tanks of different cars.

As an aside, two new developments - the fuel pump I ordered is delayed with no new delivery date so I may have to pick something else; and, somehow my playing around with the sender and pump electrics has triggered the brake fluid warning light which is now on whenever the ignition is. It's not because of low fluid because my aftermarket master cylinder doesn't have a level sender anyway but the light was not on before the recent work. Ah well, something else to look into in due course.

David

[Roger Morrall]

I’d counsel trying to clean up the stud and even if that doesn’t work sticking with what you’ve got, in my experience the Caerbont alternative is pretty fiddly to set up and you may well lose a lot of hair before you get a better result!

By the way the resistor wire in the sender does take solder (something I discovered when I took my original sender apart and mended it after it had already been replaced. In my case the wire had parted half way down the track)

The brake fluid warning light was only connected to the ignition feed on one side and a crude contact to earth that activated if you were lucky when a float dropped in the master cylinder header. I’d suspect that the loose end that was left when the master cylinder was changed finally decided to touch the bodywork and that it’s pure chance that this happened whilst you were fiddling about with the fuel gauge and sender. It’s astonishing that an apparently inanimate object can choose the right moment to confuse and annoy one quite so accurately, but things like this happened to me all the time.

[dwomby]

I am studiously ignoring the brake fluid lamp.

I epoxied the resistance coil bar in its correct position in the fuel sender and added some spacers to stop it moving too far away from the slider if the coil ever does get dislodged again.

I have now re-installed it with new gaskets and it performs in the car as it should. When there were about 2 US gallons in the tank, it read about 1/16 full as it did when I tested it. That's as low as it will go but I can live with that. After adding about 8 US gallons, it read above half full. During driving it did swing about (as it should) in tight bends and over bumps but always returned to above half full.

David