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MG Midget and Sprite Technical - Fuel Tank - Gauge always reads full !

Hi gang, scenario is, short trip out at the weekend, with the gauge reading half a tank, then on the way back it reads permanently full.
Mines a 1275 with a new tank from a year or two back. I cannot imagine I didn't change the sender too, but somethings not quite right.
Back of the gauge sees both wires attached. So I guess its drop the tank time, unless anyone knows of a potential cure ??
P Bentley

Id say before dropping the tank ... check the grounds at the tank and your voltage stablizer

V.S. is located under the dash near the center

prop
Prop and the Blackhole Midget

Maybe not the Voltage Stabiliser if other analogue instruments are behaving eg: oil pressure gauge. Because both instruments are essentially voltmeters so rely on stable voltage supply to the sensors to read accurately.

But Earthing very likely to be an issue. I think Spridgets used different senders in different years. Some reading full at high resistance, some at low resistance. (I believe this to be true but I'm ready to be corrected). So the former would read high with a bad earth and the latter if there is a short across the potentiometer that forms the sensor itself.
Greybeard

Here is the information you need:

http://mgaguru.com/mgtech/electric/fg104.htm

it is about MgA in fact the same as a 1275
Flip
st

Hiya,


The oil prtessure gauge is mechanical on all uk market spridgets, and I don't think even the US market ones used the voltage stabiliser.


Unfortunately the MGA is NOT the same as a later 1275 midget. You don't say what year your car is, but a voltage stabilised fuel gauge was introduced Nov 67 right up to the end of production in 79.



If yourr gauge (when it was workin) climbed slowly to indicate the fuel level it is the later type.


This late gauge reads full when the green/blasck is grounded. In other words a bad earth at the tank unit will cause the gauge to read low or empty all the time.


The earlier version will read empty when shorted to grond and full when open circuit.



Try disconnecting the wire in the boot, if it still reads full, the green/black is shorted to ground thro the car or the gauge id at fault. If that test causes the gauge to drop, it's the sender or the wire to it, which means dropping the tank!
SR Smith 1

Thank you Steve, for setting me right on that. Unfortunately I have mislaid my file of wiring diagrams. Also I got sidetracked by my OPG which is electric, but not original.
Sorry for the misinformation.
Greybeard

Depends on the 1275. If the gauge looks the same as the one on MGAGURU.com, then they are indeed the same system.

If it's a later type of gauge, go to http://www.mgb-stuff.org.uk/electricstext1.htm#fuelgauge which covers both types
dominic clancy

Many thanks Gents. . .mine is a Gan 5 1972 to clear that point up. Will take a look at the various suggestions and report back.
I don't have a great understanding of wiring, but will start with the earth issues which seem to be a common suggestion.
P Bentley

Yeah...just pull the rear right wheel and look for some wires comming from the front / top of the tank and follow the wiring to where its bolted to the car....remove the (black ???) Wire and give a good cleaning and rust removeal maybe even a new nut and washer

prop
Prop and the Blackhole Midget

Hi guys,


Why the obsession with a "poor earth"?



A poor earth on this type of system causes the gauge to read low or empty.


The OP says the gauge is reading full all the time, ie the green/black is showing symptoms of being shorted direct to ground.



SR Smith 1

I agree with SR Smith. Disconnect green/black from gauge and measure resistance to earth. If it is 0 ohms with less than a full tank it is shorting to earth somewhere. The later gauge (all 1275?) is different (simpler) than the earlier "wheatstone bridge" type design which has in-built voltage stabilisation.
W Bretherton

The workshop manual has a very clear, easy to follow routine to check whether the problem is with the gauge or the sensor (or indeed, the earth!).

Graeme Williams

Well as I had the ar*e of the car in the air on Friday night I dropped the tank, and re crimped the wire back on the sender unit, just to rule that out. I also checked the wire through the boot floor, and couldn't see any signs of it wearing through. The car has probably only done 500 miles since being back on the road, and the tank, sender and wiring loom was all replaced. so although I dealt with the bits I know better than electrics, i will get someone with a voltmeter over to test the resistance mentioned in an earlier post. With all that in mind I topped the tank up and off I went and cruised around all weekend. . . great to get out and enjoy the attention the car gets once more.
P Bentley

It is also possible your voltage stabiliser has failed in a permanently "on" position. The original stabiliser is a mechanical device which continuously interrupts battery voltage to produce about 10v for the fuel gauge. However it is difficult to measure this with a voltmeter as it is not a regular waveform. If voltage going to gauge is at battery voltage then the stabiliser is stuck. This will produce a high reading on the gauge ( but should still fall as fuel reduces in tank). If this is the case the stabiliser can be replaced with an electronic regulator for about £1 off eBay. It would need to be soldered into the old stabiliser casing. The old stabilisers can possibly be renovated but it would be hit and miss I think and not worth the faffing. But I'm not a true concours person.....

Bill
W Bretherton

Did the gauge read empty with the green/black disconnected at the tank end?


If it did the fault must lie in the sender unit.



If not, try pulling the green/black off the gauge. If it then drops, that wire is shorted to ground thro the car somewhere.



On my 1300 there is a snap connector adjacent to the front edge of the tank, maybe seperate the wire there?
SR Smith 1

This thread was discussed between 08/08/2016 and 17/08/2016

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