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Fuel tank sender unit resitance

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Magnor
Posts: 56
Joined: Sun Feb 28, 2016 7:31 pm
Location: Bampton, Devon

Fuel tank sender unit resitance

Post by Magnor »

Having trouble with my fuel gauge. I think it is the sender unit. Ignition on and the needle jumps to Empty mark but wont move further. If I remove wire from terminal at sender unit gauge goes full scale? Not sure if this should happen?
I need to check sender unit. Does anyone know what the resistance should be at empty and full?
Thanks
Clive

User avatar
Jez
Posts: 89
Joined: Mon Feb 15, 2016 8:00 am

Re: Fuel tank sender unit resitance

Post by Jez »

The SP has a moving coil type of fuel gauge, which measure the difference between current flowing through a coil to earth and current flowing through a second coil via the fuel tank sensor to earth. Disconnecting the wire to the fuel tank sensor should cause a full reading so it seems the gauge is fine as is the first coils connection to earth.
If the gauge will not otherwise register a reading above empty it suggests that more current is flowing down this wire than should be, indicating either a short circuit on the wire (probably as a result of worn insulation etc) or a failed sensor (though I suspect they would normally fail open circuit rather than shorting).
Check at the sensor end if the wire from the gauge could be shorting against anything. If all ok, disconnect the wire. If the gauge goes to full, then the wiring is ok. If it remains at empty you have a short along the wire.
Sensor resistance should I think be approx 70 ohms with a full tank and decrease as the tank empties.

Cheers - Jez
Jez Stow
1960 Daimler Dart
and a few other toys

CHRIS J
Posts: 53
Joined: Sun Aug 14, 2016 7:01 pm

Re: Fuel tank sender unit resitance

Post by CHRIS J »

On the v8 250
E TO T 250 ohms empty
E TO T About 0 to 15 ohms full
E TO W is used for fuel warning lamp empty 0 to 5 ohms
this may not be the same as the SP

E = EARTH

May be of some help

Magnor
Posts: 56
Joined: Sun Feb 28, 2016 7:31 pm
Location: Bampton, Devon

Re: Fuel tank sender unit resitance

Post by Magnor »

Thanks Jez and Chris for your replies. I will check wire to gauge from sender tomorrow. If all OK will have to take the tank out to get to the sender unit to check what it is putting out at full and empty.
Clive

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heh101353
Posts: 157
Joined: Sat Sep 30, 2017 11:34 am
Location: Basel, Switzerland
Contact:

Re: Fuel tank sender unit resitance

Post by heh101353 »

I ran into a similar problem some time ago. I decided to build a small device (kudos to MGA Guru) which allows to get pretty close to the figures (ohms) needed for the (main) readings of the fuel gauge

From the pictures below you may get an idea what resistance the sender is supposed to deliver at the various stages.
My findings match with those 70 ohms (full) mentioned by Jez.

Image
Close to empty
Image
30 ohms nearly dead on
Image
52 ohms a bit too much; I guess 48 would be dead on
Image
57 ohms might be accurate
Image
74 ohms already to much

Hope this helps
cheers Hans
:o owner since 2015 :mrgreen:
https://www.wyhe.ch/

Ian Slade
Wise Man
Wise Man
Posts: 791
Joined: Sat Feb 13, 2016 9:54 am
Location: Sevilla Spain

Re: Fuel tank sender unit resitance

Post by Ian Slade »

You can also adjust the deflection to read at the full and empty marks by loosening the two nuts on the rear of the gauge and minutely moving them which will control the needle deflection, they interact so it's not as simple as moving the sender to its extremities and adjusting each one. When originally sold they were in pairs so over the years with either the gauge changed or sender they will be inaccurate.
Owner since the 70's, Genghis is slightly to my left.

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