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Sold
by simon8201 - 09/09/26 01:46 PM
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Wales
by Joske Vermeule, September 1
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Forums39
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Jun 14th, 2026
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Joined: Sep 2011
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OP
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I have been having some issues with my new fuel and temperature gauges.
I wanted to check the calibration and ran the tank right down to zero and into the 5 litre reserve. At least I now know that when it starts bouncing on zero it is reasonably correct and I have about 5 litres left.
In part of my checking, pouring in 5 litres at a time and then checking the resistance to the sender in the fuel tank, I discovered that for the last 12 litres or so to a full tank (55 litres)there was no change in the reading.
This explains why the tank never reads full. In effect There is no change in the resistance from a fraction over 3/4 or about 7/8 to a full tank.
I don't know if this is because I have a faulty sensor or not but since it seems to be a common problem it may simply be that the sensor that Morgan uses is maxed out at 7/8s of a tank.
Peter ![[Linked Image]](http://bradley.it/public_images/drive.jpg)
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Joined: Sep 2010
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Talk Morgan Expert
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Talk Morgan Expert
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I think they all do that Sir!
1969 4/4
1995 plus 8
1996 Plus 4 4seater
2002 Aero S1
2013 M3W
2014 Plus 8
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Joined: Sep 2011
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Very strange things happen in Morgan circuitry. One example is the cabin ventilation fan. The switch and two speed control is on the mass (earth) side so that the fan is constantly powered when the ignition is on. Get it going at high speed with some of the warning lights (say seatbelt) going and then turn the key off. The fan then turns into a generator and the lights dim down over about 1 second. If the fan is off the warning lights turn off instantly  PS. I don't have any of that circuitry now: it is all hard wired and through relays.
Peter
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Joined: Jan 2009
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what sort of sensor is in the tank. Some older style ones are a float on a lever arm working a rheostat so to get it to show full when the tank is full you just bend the arm a bit.
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Joined: Jan 2009
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Charter Member
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Charter Member
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what sort of sensor is in the tank. Some older style ones are a float on a lever arm working a rheostat so to get it to show full when the tank is full you just bend the arm a bit.
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Joined: Mar 2012
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Learner Plates Off!
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Learner Plates Off!
Joined: Mar 2012
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Yes, despite the tank being brimmed, in the last few weeks my gauge has stuck on both just over half and three quarters. Even when I had used most of the tank it stuck at those points. Now it seems to be OK but takes an age to fill with the pump clicking off constantly whilst filling.
Very odd and a tad annoying.
Stuart
Rich Maroon and Royal Ivory (ish) +4
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what sort of sensor is in the tank. Some older style ones are a float on a lever arm working a rheostat so to get it to show full when the tank is full you just bend the arm a bit. If it is anything like the sensors used with modern fuel injected motorcycles it is in a unit with the pump and senses the pressure of the depth of the fuel. The problem is that it works on a range of resistance from 9 ohms when full to 60 ohms when empty. It works on such low current that any little problem with connections can cause a variation in the resistance "seen" by the gauge. Caerbont told me that for the London Taxi cabs they finished up recommending an earth lead directly back to the battery to resolve issues. I think that in the case of the Morgan this signal is even passed through to th MFU and perhaps even the ECU (as is the coolant temperature) this gives scope for even more points where things can go wrong. For the fuel gauge a tiny resistance of say 10 ohms somewhere along the way would ensure that the guage never reads beyond a little over 3/4 full.
Peter
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Roadster Guru Member of the Inner Circle
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Roadster Guru Member of the Inner Circle
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My Plus 4 reads full when its full, but the needle drops almost immediately and in 30 miles is at 3/4, from which point it slows down.
DaveW
'05 Red Roadster S1
'16 Yellow (Not the only) Narrow AR GDI Plus 4
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Tricky Dicky Member of the Inner Circle
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Tricky Dicky Member of the Inner Circle
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Mine is pretty accurate it is maybe the bent arm and float proven technology being 19 years old and not all this smoke and mirrors modern stuff.
2009 4/4 Sport Henrietta
1999 Indigo Blue +8
2009 4/4 Sport Green prev
1993 Con Green +8 prev
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Joined: Apr 2012
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Charter Member
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Charter Member
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what sort of sensor is in the tank. Some older style ones are a float on a lever arm working a rheostat so to get it to show full when the tank is full you just bend the arm a bit. If it is anything like the sensors used with modern fuel injected motorcycles it is in a unit with the pump and senses the pressure of the depth of the fuel. The problem is that it works on a range of resistance from 9 ohms when full to 60 ohms when empty. It works on such low current that any little problem with connections can cause a variation in the resistance "seen" by the gauge. Caerbont told me that for the London Taxi cabs they finished up recommending an earth lead directly back to the battery to resolve issues. I think that in the case of the Morgan this signal is even passed through to th MFU and perhaps even the ECU (as is the coolant temperature) this gives scope for even more points where things can go wrong. For the fuel gauge a tiny resistance of say 10 ohms somewhere along the way would ensure that the guage never reads beyond a little over 3/4 full. This makes a lot of sense. I am sure that the adaptation of the Morgan wiring from the fuel tank into the BMW electronics has caused the wild inaccuracy of my fuel gauge. It wanders about all over the place - even when standing still on a flat surface.
+8 4.8
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