ians Posted July 28, 2021 Report Share Posted July 28, 2021 OK this should be pretty simple to answer. The fuel gauge is not showing any fuel and I know its about half full. Trying to identify if the fault is in the Sender unit or the Gauge or the Wiring. There is 12v at the B terminal on the back of the gauge. Am I correct in thinking that if I earth out the sender input terminal on the back of the gauge it should read full? Quote Link to post Share on other sites
RobH Posted July 28, 2021 Report Share Posted July 28, 2021 No, if this a normal Tr3 gauge it works the other way round. If you disconnect the sender wire it should read full but earthing the terminal it should read empty. The sender resistance to earth should vary from zero Ohms at empty to 70 Ohms or so when full. Quote Link to post Share on other sites
ians Posted July 28, 2021 Author Report Share Posted July 28, 2021 3 hours ago, RobH said: No, if this a normal Tr3 gauge it works the other way round. If you disconnect the sender wire it should read full but earthing the terminal it should read empty. The sender resistance to earth should vary from zero Ohms at empty to 70 Ohms or so when full. Based on that comment I now realise that popping the gauge out of the dashboard into free air and not earthing the case of the gauge is not the brightest thing to do so tomorrow I will earth the case properly and see what happens! Quote Link to post Share on other sites
RobH Posted July 28, 2021 Report Share Posted July 28, 2021 For info, this link describes how the thing works (even though it is from the 'dark side') http://mgaguru.com/mgtech/electric/fg_01.htm Quote Link to post Share on other sites
ians Posted July 29, 2021 Author Report Share Posted July 29, 2021 Sorted: It was the needle stuck (glued?) to the Empty end stop. Goodness knows how it got stuck. Gave it a gentle nudge and all working fine. Quote Link to post Share on other sites
Ralph Whitaker Posted July 30, 2021 Report Share Posted July 30, 2021 22 hours ago, ians said: Sorted: It was the needle stuck (glued?) to the Empty end stop. Goodness knows how it got stuck. Gave it a gentle nudge and all working fine. They sometimes have a small rubber sleeve over the end stop which rots and becomes sticky. Have had this before myself. Ralph Quote Link to post Share on other sites
john minchin Posted May 15, 2022 Report Share Posted May 15, 2022 I need some help with my TR3 fuel gauge and I have tried some of the ideas in this thread. I have the original type TR3 gauge (which has worked for me for many years) has a power feed with battery voltage (B terminal). The other wire goes to the sender (T terminal). The symptoms are : -The gauge twitches off the bottom stop when I turn on the ignition but never indicates any more fuel even when I pull the float up with a piece of wire. Tests which I have done from the forum thread above are - disconnected the gauge wire at the tank - the meter should apparently show full - no effect at all. - grounded the gauge wire - the meter should apparently show empty - no effect at all. - I have had a digital and an analogue resistance meter across the sender when the supply wire is disconnected. I see no variation in the resistance when I lift the float the sender with a piece of wire. I was hoping for 70 ohms or similar when the float was lifted. -I have reversed the gauge to see what happens and it does not even twitch. Any ideas for diagnostics ?. Thanks, John Quote Link to post Share on other sites
RobH Posted May 15, 2022 Report Share Posted May 15, 2022 (edited) Sounds as though the gauge may have lost the earth connection (made via the gauge body and clamp to the dash). One of the coils inside passes current to earth all the time the ignition is on, and pulls the needle towards "full". The other coil passes current to earth via the tank sender and pulls the needle back towards "empty" . Since your gauge does not show full even when the sender wire is disconnected, it means the first coil isn't doing anything and that could be because it has no earth. Edited May 15, 2022 by RobH Quote Link to post Share on other sites
john minchin Posted May 15, 2022 Report Share Posted May 15, 2022 Rob, It was the earthing of the gauge, I've just added an temporary wire to see. Thank you for the prompt reply. Excellent, thank you. John Quote Link to post Share on other sites
Lebro Posted May 15, 2022 Report Share Posted May 15, 2022 It does not explain the lack of resistance variation of the sender, when float is raised ? Bob Quote Link to post Share on other sites
john minchin Posted May 16, 2022 Report Share Posted May 16, 2022 The lack of resistance variation is highly likely to be down to my lack of ability in electrical things. I can measure Volts and that is about it. Never mind it all works a treat now and that is what really matters. Full meter reading when I haul the float up with a piece of wire. My TR3 is now Salvador Blue, abit like yours Bob, I think that it looks stunning. Thanks again, John Quote Link to post Share on other sites
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