Talk Morgan Forums Morgan Sports Cars Maintenance & Servicing Fuel gauge anomaly

I'm getting a strange phenomenon with my fuel gauge.

When the engine is running, the gauge reads about 10% lower than when the ignition is on but engine not running.

I noticed it when I filled up, checked the gauge as soon as I switched the ignition on then as I drove out of the petrol station I noticed the gauge reading significantly lower.

I tried the exercise again when I got home after refuelling (about 3 miles) and the same phenomenon again.

I can only think it might have something to do with the voltage (12v with ignition on and 14v with the engine running but I'm not a sparky so I have no real idea.

Any Thoughts?

The fuel gauge in effect measures the resistance on the line to the sensor in the tank. When the tank is full there is very little resistance but as the fuel level lowers the resistance is increased. The fuel gauges are notoriously susceptible to a bad earth between the sensor in the tank and the negative pole of the battery. Naturally this normally passes through the chassis and it may be that you have a bad connection somewhere. I was told at Caerbont Automotive Instruments (Smiths) that they had a lot of problems with the London taxis and in the end recommended a direct earth line from the earth on the sensor to the battery.

IMO, a bad earth is simply ... a bad earth! i.e. it would give a constantly false reading, regardless of whether engine was on or off. So it doesn’t fit your symptoms. Your own idea of variable voltage due to alternator running sounds more feasible BUT the supply to instruments is usually connected through a voltage stabiliser to avoid that very issue. Nowadays, stabilisation is usually part of vehicle’s electronics rather than a separately identifiable component, so if faulty, may be difficult or costly to track down.
One other consideration (& a more likely scenario) is that fuel sensors are usually damped in some way, otherwise every time you accelerate or slow down, the gauge would be very erratic as the fuel sloshed about in the tank. Are you being too quick to test the effects of engine running or not. Try letting everything settle for a good few minutes then try looking with engine running or not, without moving the car.

My fuel gauge on start up always reads full. After about 5 mins it settles back & reads pretty much correctly.

IMHO the fuel gauge on a Morgan is purely an ornament and has no practical use

After living with 3 different Morgan's in 14 years of ownership I can tell you the 1994 Plus 8 and the current 1999 Plus 8 gauges are pretty accurate i.e. within 1/4 of a tank.


The 2009 Sport was a chocolate teapot gauge and various bits came unstuck and fell off - I can only surmise that build quality Q&A has gone downwards.

That's why I reverted to an earlier build and did I mention the V8 rumble well I just don't want to live without it, tried a Roadster but there was a lot missing and a very heavy clutch and they were all very bright colours which I don't like.

Mine on my 2012 plus 4 has two moods. One indicates about 1/8th of a tank more than the other. Each mood lasts a journey or two then it swaps to the other mood. I have learned just to live with it.

I have taken to just sticking £30.00 every time I go out so there is always some in the tank.

I ignore my fuel gauge and just fill her up every 400 miles

The fuel gauge on my +8 is hopeless - just sitting still on level ground, it will wander all over the place. So I use the odometer and refill at 200 miles. As it's been off the road for nearly a month (in all this lovely weather), I am getting better MPG.