Fuel guage

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Colthebrummie

Well-Known Member
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Thatcham
for the last few months I thought my fuel guage had packed up. Today, I decided to investigate and when I looked at it properly, the needle actually rises about a millimetre even though there is more than half a tank of petrol. The earth to the sender is ok, first thing I checked but when I measure the voltage to the live wire to the sender, it was only about 7 volts. Is this normal or should it be 12 volts. I spose the sender varies the voltage depending on how much is in the tank but I would have though it should 12 volts at the wire going in. Any thoughts gratefully received.

Col
 
https://en.wikipedia.org/wiki/Fuel_...ic current is sent,i.e., inside the fuel tank.
If your wire from the sender is at 7 volts that implies it is actually working, any chance of you moving the float, i.e. by parking on a hill or taking the sender unit out? To move the float arm and test the voltage.
If you get 12 volts to the wire, disconnected from the sender does the gauge move?
Finally, can you get the gauge out to test it? Or at least find out which wires go to it and test them individually?
Sounds to me like a break in the connection/wire between the sender and the gauge.
 
Take the lead off the sender Voltage will be 12V on a series 2 10V on a series 2a/3 if you get that voltage earth the lead gauge should go to full but slowly. That proves your earth return and the gauge.
If your voltmeter is digital and system is 10V the digital meter will struggle to read the voltage as the regulator reduces the voltage by switching it on and off
 
Blackburn said:
Take the lead off the sender Voltage will be 12V on a series 2 10V on a series 2a/3 if you get that voltage earth the lead gauge should go to full but slowly. That proves your earth return and the gauge.
If your voltmeter is digital and system is 10V the digital meter will struggle to read the voltage as the regulator reduces the voltage by switching it on and off
Click to expand...
Thanks for that info. I have cleaned and reattached the earth but it made no difference. If the voltage should be 10v but is reading 7v I would have thought the guage should read about half or more. The guage has only ever gone up to 3/4 at the best of times. I'm considering applying a separate 12v supply to see what the guage does but it looks like the sender may be kaput. The ballast resistor must be ok because the temp guage works ok. The top of the sender is covered in rusty crud and I'm reluctant to disturb it unless I have to so I'll remove the dash first and take some measurements at the resistor and guage. The circuit is very simple.

Col
 
Have you removed wire to guage from sender and grounded it to chassis? If guage goes to full then it is working so suspect sender is faulty no need to remove dash to try...
Rob
 
@Colthebrummie It's worth noting that the wire to the sender isn't a 'supply', but is the signal going back to the gauge, indicating the fuel level. The sender is simply a lot of turns of wire, making up a variable resistor to ground, and the voltage you see there will depend on the fuel level. The gauge basically just shows that voltage, relative to the 10V 'supply' that doesn't get any further than behind the dash.

That might help explain what you're seeing.
 
I shorted the wire to earth and the guage did indeed go to full, so I guess it is the sender at fault. Do they just screw in? The top of mine looks pretty corroded, it'll take some Brummie persuasion to get it out. Looking on eBay, there seems to be at least two different ones for the 109. Mine has two tanks, I don't know if that accounts for the difference.

Col
 
Small screws that thread into brass collar on tanktop underside...do you have a sender on each tank?...once it is out re-connect it to guage and earth then move float arm and watch guage also check float is not fubar and got fuel in it...
 
Shippers said:
Small screws that thread into brass collar on tanktop underside...do you have a sender on each tank?...once it is out re-connect it to guage and earth then move float arm and watch guage also check float is not fubar and got fuel in it...
Click to expand...
Thanks. I've scraped off much of the rust and dirt form the top of the sender and can't see any screws that's why I asked if the sender body screwed in, I noticed the top of the sender had a couple of straight edges as if it has been drifted on with a mallet. need to look harder I guess. Both tanks have senders but the front tank has never registerd on the fuel guage, probably knackered as well. I don't use the front tank anyway but it does have two or three gallons in it. I consider it to be an emergency reserve.

Col
 
edit^^^^ yours might have the twist in type of sender...you might need to bend wire arm a bit or a previous owner may have fitted wrong resistor value sender...
 
You *might* have a problem matching the sender to the gauge - there was a thread here a while ago about someone who was struggling to get a new sender to give sensible readings on their original gauge.

I managed to get my original sender apart and gave it a good clean up and rebuild before refitting it. Might be worth hanging on to your old one in case you have any similar problems.
 
TheMegaMan said:
You *might* have a problem matching the sender to the gauge - there was a thread here a while ago about someone who was struggling to get a new sender to give sensible readings on their original gauge.

I managed to get my original sender apart and gave it a good clean up and rebuild before refitting it. Might be worth hanging on to your old one in case you have any similar problems.
Click to expand...

And checking out second one on other tank as well!!...
 
TheMegaMan said:
You *might* have a problem matching the sender to the gauge - there was a thread here a while ago about someone who was struggling to get a new sender to give sensible readings on their original gauge.

I managed to get my original sender apart and gave it a good clean up and rebuild before refitting it. Might be worth hanging on to your old one in case you have any similar problems.
Click to expand...
Nothing is ever straightforward is it. I will take a good look at the original sender, as you say, it might just need a good clean.

Col
 
Shippers said:
And checking out second one on other tank as well!!...
Click to expand...
I don't know if the senders are the same for the back and front tanks but if they are, and if I can get the old one working again I'll swap it for the dodgy front one. Having said that, Im not sure it is a sender issue with the front tank, I'll have to short the signal wire to see if the guage goes to full. The wiring to the change over switch is a bit complicated and I've always assumed that was where the problem was for the front tank.

Col
 
I have finally gotten around to replacing the fuel sender. After spraying the top of the old one with Plusgas it came out easier than I was expecting. The tank is about 3/4 full but the new sender only showed just under half. So I took the sender out and bent the float arm up a bit, put it back and it still reads the same. I then bent the arm a lot but it still only reads about half a tank. When I took the sender out for the third time, I connected it and pushed the float to its maximum height but still only read just over half a tank. Is there something I'm missing or is the new sender fault You? Removal and replacement of the sender was a doddle and whenever this happens it means I've done something wrong. The guage is definately working ok.

Col
 
Surely if the guage was reading low you need to bend the arm downward so the fuel pushes the float higher up.
What is the resistance of your new sender in low and high position?
These are the readings for a sender I recently fitted swb but same gauge in dash
Fuel Tank sender resistance range Full 24 ohms empty 260 ohms.
 
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