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How to test my fuel guage and where's the low fuel indicator ?


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  • Gold FFM

I've just whipped out the fuel guage (whilst changing light bulbs) as it only ever read 1/4 full (or 3/4 empty - depending upon personal persuasion).

Now it's out of the car ('85 MY TE) , is there any way I can test it (with a multimeter) to ensure it works, thereby leaving the sender unit as the most likely culprit.

Also, the wiring diagram in the WSM indicates the 4th light on the left hand side of the dash should be for the fuel warning light. Mine has no electrics, but the window has the "Hazard" symbol. Perhaps I'm reading the wrong diagram (LSL 248) . The fuel guage (Smiths)  has no indicator on it. 

TIA,

thebartman

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Hi, I tested mine by connecting a 500ohm variable resistor in series with the guage and a 12V supply. Sweeping the pot should see the guage go over its range.  I happened to have one in the drawer but easy to get off ebay.

 

My guage was fine, the sender was at fault. It needed stripping down and the crud gently scrubbing off the resistance wire

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  • Gold FFM

Cheers for the reply  @MikeJ,

I'm learning car electrics , so please forgive the simple questions ...

so why a 500 Ohm variable resistor? Does the fuel guage have limits? and if so, what are they? if I look on ebay - there are squillions of them - looks like outer space to me 😉 and they seem to have war-of-the-worlds legs, not auto spade terminals - does this matter? and if I get one with more than 500 Ohm. would this be a problem?

TIA 

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The gauge displays voltage returned from the sender. The fuel tank sender is a variable resistor operated by a float that allows more or less of the supplied voltage (10V from voltage stabilizer in the binnacle) to get back to the gauge. So here you are substituting the 500 ohm variable resistor for the sender in the tank to see if the gauge is operating. As you adjust the resistor, changing the voltage supplied, the gauge should change position if it's working.

Terminals on the resistor don't matter, just solder on whatever wires or connectors suit your purpose, or clip/twist/ crimp wires to them temporarily to do the test.

 

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On 27/09/2020 at 09:20, thebartman said:

As I don't posess a soldering iron etc., and I have the binnacle apart, could I use the rheostat that's in the binnacle as the variable resistor ?

Good thinking but I think the rheostats will be too low a resistance. 

This should do you: 500ohm linear pot. The wire that usually connects the sender to the guage - connect this to the center terminal of the pot. Connect either of the outer pot connections to ground. Put the pot in the middle position before turning on.

 

https://www.ebay.co.uk/itm/Potentiometer-Rotary-15mm-500R-2M-with-Colour-Coded-Knob-Sold-Individually-2/221810806316?hash=item33a4f44e2c:g:K3QAAOSwymxVQ05g

 

 

 

There's also another wire that goes to the sender which is the low fuel switch.

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Yes when I had a similar problem it turned out to be the float in the sender. The float had a pin hole in it so it filled up with fuel and would no longer float. If you rule out any electrical issues it will be worth taking the sender assembly out and checking it,

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  • 2 weeks later...
  • Gold FFM

So the sender is out, and the float doesnt. Also, the arm was corroded into place.

WD40 sorted out the lack of movement within hours. 

How to I empty then re-seal the float ? At this point I'm awaiting a new gasket before replacing, so have a few days to sort out the float.

Sender1.jpg

sender2.jpg

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Some plastics can be welded with a hot air tool.  I would (carefully) warm up the float and check the seams to see if there's a crack that the expanding fuel seeps out of and try to melt that back together, assuming you can't find a replacement.

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  • Gold FFM

Cheers @snowrx Dude, 

I'm happy with a little "DIY" but "Hot Air vs. fuel. vs. ignorance" is a stretch for me  - call it cowardice .... 😉

Can I just get a cork from a wine bottle? If so, what weight? and what "floatiness" characteristics shoud it have ?

I really don't want to spend out on a new sender, when I'm so close to fixing this one ... 

Thanks everyone ....  

 

 

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You mentioned earlier that you had a multimeter. Now's the time to set it to resistance and measure the resistance between spade and metalwork as you move the arm. I cant remember which spade is the low fuel switch and which is the guage. It doesn't matter, one should show a nice variable resistance as you move the arm, the other will suddenly switch to near zero resistance. If this doesn't work then it takes you down the route of either replacing the sender or taking it apart to clean it up (there's quite a lot of corrosion in that picture!).

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1 hour ago, thebartman said:

Cheers @snowrx Dude, 

I'm happy with a little "DIY" but "Hot Air vs. fuel. vs. ignorance" is a stretch for me  - call it cowardice .... 😉

Can I just get a cork from a wine bottle? If so, what weight? and what "floatiness" characteristics shoud it have ?

I really don't want to spend out on a new sender, when I'm so close to fixing this one ... 

Thanks everyone ....  

 

 

You can buy just the float from sj sports cars for £5.60, that's what I did and it worked a treat. They just twist in and out of that wire holder.

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  • Gold FFM

Thanks @snowrx and @MikeJ, have got the multimeter and tested as you suggested and got a range from around 26 to 191 ohms, so will assume the sender is functional.  :thumbsup:  

 

@robington Thanks - I had looked, but I missed it . 

However, this does look a different shape to the one I've removed , does this matter ?

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  • Gold FFM

Cheers, - I ordered it (along with other stuff)  - it WILL fit, even if I have to file it down 😉  🙂 

Can't wait to have it delivered, so I can put it all back together with working sender, new gasket/float and actually see how much fuel I am burning ...

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  • Gold FFM

My fuel tanks are intact 👍🔧🏎😀,

(given a post I saw of a bloke returning from Goodwood with rotten tanks .....  that you fixed)

 

if you remember the film Life of Brian ..   “my eyes are dim, I cannot see”

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On 11/10/2020 at 15:51, thebartman said:

So the sender is out, and the float doesnt. Also, the arm was corroded into place.

WD40 sorted out the lack of movement within hours. 

How to I empty then re-seal the float ? At this point I'm awaiting a new gasket before replacing, so have a few days to sort out the float.

Sender1.jpg

sender2.jpg

At £6 for a new item, doesn't bear thinking about any further......

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  • Gold FFM

Hussah !!!   New fuel sender float arrived, new gasket already here - no lost nuts/bolts, and renovated, free moving sender tested, installed and working  - Yipee - a personal milestone :thumbsup: Now I have to work out the evaporation rate of fuel - as I now have less than 1/2 a tank(s). 😉

My thanks to everyone whose advice and guidance helped me. 

 

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  • 7 months later...

My instruments are in hand with a technician for refurbishing and he asks what are the resistance values from top to bottom range of the fuel sender? Also, he seems to think the voltage is regulated to 5v via the stabiliser, is that correct?

Cheers

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Later cars with Smiths gauges regulated at 10 volts, I don't know for sure on a S2.

The regulator part number on my 1985 starts with A079***, which I think means they started using in 1979, so my guess is 10 volts rather than 5.

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  • Gold FFM

Definitley 10V DC regulated - as said above (for my S3/Turbo - 1985 MY)

A similar sender unit may come from a Jaguar XJ, and a new float (if required) from a Mini

Resistance values: are here - and I should note two things:- 1) Sender unit registers FULL when tanks are full and 2) Choke/low fuel registers on mine when aboout 1/4 full on bumpy ground around corners  - so looks like it's all OK  - happy days 🤗

On 13/10/2020 at 11:14, thebartman said:

Thanks @snowrx and @MikeJ, have got the multimeter and tested as you suggested and got a range from around 26 to 191 ohms, so will assume the sender is functional.  :thumbsup: 

ranges above were measured when sender was out of tank ....

Good luck ....

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