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MG MGB Technical - Petrol gauge reading

I recently became stranded when my roadster ran out of fuel even though the fuel gauge was still showing well over half a tank full. It has always been fairly accurate previously so it caught me out. Obviously I need to sort this out. My question is, how do I know if the problem is from the sender unit or the gauge? Is there a way of testing each unit?
Thanks Trev.
Trevor Harvey

Trevor - Remove the wire from the sender that goes to the gauge and with the ignition on, tough the wire to ground. If the gauge goes to full, then the problem is in the sender. Cheers - Dave
David DuBois

I've got a feeling this may not be that simple. You might consider what has changed recently. Anything in the fuel system change? One overlooked area is the fuel sender ground. Adding a jumper between the sender and the body is a good test. Grounding the wire from the sender is another test, but different gauges react differently. There were at least three different gauges in MGBs.

Try a few of these things and then come back with some more info (such as year of car, and possibly gauge number) if you need more help.
Dave McCann

Hi Trev,

very good article here

www.mgexperience.net/article/fuel-sender-adjust.html

Herb

Herb Adler

Hi again Trev,

Another thought came to mind, I reckon it could be your gauge voltage regulator shorted out. Instead of around 10 volts you are now getting full battery volts to the fuel gauge system, which would cause it to read high.
Measure the output voltage of the regulator, which is situated on the firewall behind the dash, on the driver's side. A little metal box, about 1" x 2". What you should get is 12v then no volts then 12v and so on, as the regulator is a bimetallic switch and the 10 v output is only the average never the instantaneous voltage.

I am on the verge of writing an article on how to convert this regulator to use an electronic regulator. I have attached a photo of the electronics in the old case, before potting with hot melt glue.

Herb

Herb Adler

Herb:
Put me on your mailing list when you get it done.
Thanks
R Murray

Sounds like the sender wiper has got hung up on the resistance wire, which is what happened to mine before it broke, then would suddenly drop below E at around half a tank. Take the green/black wire off the tank sender and the gauge should drop right back, past the E. A regulator stuck at 12v would read high it is true, I doubt it would show half a tank when it is empty, but I must admit I haven't tried it and I have a nearly full tank at the moment so can't check.

One advantage of the electro-mechanical sender over electronic replacements is that the former outputs full battery voltage for a few seconds while it 'warms up' which means the gauge(s) reach their final position that much sooner.
Paul Hunt 2010

Hi,

Paul
Yes it could be the sender hung up, however, I still think that the regulator is it. Consider: when the car is running the battery volts are somewhere around 14 V from the charging device. This is 40% higher than the 10V needed for the gauge and could easily give around half a tank reading, especially as fuel gauges are quite non linear. (a car I've just traded used to do ~400 km on the first half of the tank, by the gauge, and only ~250 on the second half).

I agree that an electronic regulator would get to its final reading slower than the mechanical one, but my driving style doesn't require that I need to know how much fuel is in the tank when I start the car, as I have a pretty good idea from when I last drove it. What I do need to know is when I need more fuel as I am buzzing around, by which time the gauge has well and truly settled down.

Herb
Herb Adler

If it's the regulator that would be instantly recognisable from giving a higher reading when the engine is running, to when you first turn on the ignition for the very reason you say. But before you start the engine, give the gauge a tap occasionally until it really has stabilised, as mechanical stiction and vibration from starting will cause even a correctly regulated gauge to move up a bit.
Paul Hunt 2010

Checked my V8 today. Regulator in circuit (engine running or not) showed mid-way between 1/4 and 1/2. Regulator bypassed engine not running showed just under half. Regulator bypassed engine running and alternator charging showed about a needles-width above half. Gauge left to settle and tapped to make sure in all cases. So quite a bit less than showing half when empty.
Paul Hunt 2010

PS. Off E and F you can usually see the needle pulsing up and down very slightly as the regulator cuts in and out ... if it is working properly.
Paul Hunt 2010

Thanks to all who responded to my original inquiry.
Have sorted it out . It turned out to be the sender unit. The float arm was snagging the windings inside of the little metal box and getting stuck, giving an incorrect reading.
Trev
Trevor Harvey

Herb
In looking at image I see a diode 2 resitors and a capacitor. Can you give a rundown on how these parts connect and values of these parts? Good job
Thanks Mike
mgr mike

Mike,

If you send me your email address I'll send you an article I've written about it.

Herb
Herb Adler

This thread was discussed between 16/04/2010 and 26/05/2010

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