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MG MGB Technical - Fuel Sender power supply.

I'm pondering a loss of power supply to the fuel tank sender.
When I test the switched power from fuse box terminal 6 (Green wires), there is continuous power; likewise the Green wire power supply entering the voltage stabilizer is continuous; but the Light Green/Green wire from the stabilizer to the fuel gauge shows alternating on/off power at about 50 pulses per minute; and there is no power at all from the gauge to the sender (Green/Black).
Can the electricians here tell me whether the LG/G from the stabilizer should be pulsing? - and since no power is reaching the sender, do I have a faulty gauge, leading to no power reaching the sender?

All advice gratefully received. Yours, John.
J P Hall

John, I have copied this from Paul Hunt's excellent website, which explains how to test your gauge and power supply:

http://www.mgb-stuff.org.uk/electricstext1.htm#fuelgauge

Non- or erratic operation: Briefly connect an earth to the green/black wire on the back of the gauge. Does the gauge pointer move smartly towards to 'Full' (remove the earth as soon as you see the pointer moving)?

No - check for 12v on the light-green/green wire of the gauge. Is it present?

No - check the same wire on the stabiliser. Is 12v present?

No - check the green wire on the stabiliser. Is 12v present?

No - check the green circuit back to the second fuse up in the fuse block.
Yes - looks like the stabiliser is faulty.


Yes - looks like the light-green/green wire from stabiliser to gauge is faulty.

Yes - is it steady or switching on and off 2/3 times per second?

Steady - check the earthing of the metal can of the stabiliser, if that is OK it looks like the stabiliser is faulty (giving high readings) as well as the gauge.
Switching - looks like the gauge is faulty.


Yes - briefly earth the green/black wire at the tank unit. Does the gauge pointer move smartly towards 'Full' (remove the earth as soon as you see the pointer moving)?

No - disconnection in green/black wire back towards the gauge.

Also (from the same website) have a look at the wiring diagram.

http://www.mgb-stuff.org.uk/s_gauge.htm#one

Andy
Andy Robinson

Thanks Andy - I had read in the archives of Paul's suggestion to earth out the green/black at the tank to test the gauge, and it makes sense except that I have no power reaching the sender. I might still do it to see what happens.
From your notes, I'm suspicious of the LG/G from stabilizer to gauge, and will test that tomorrow; I'll also bridge the gauge to see if power then reaches the sender.
Yours, John.
J P Hall

John, The wire out of the stabiliser is supposed to pulse. The stabiliser is basically a make and break circuit which takes the 12+volts from the cars system and reduces it, by switching off and on, to approx 10 volts. If you have output from the stabiliser to the gauge, that's fine. The Green/Black is basically an earth wire going through a variable resistor (the sender). Earthing out that wire should give a full reading on the gauge. If it does the gauge is fine. The sender could be at fault or there is a bad sender earth or bad connections on the green/black. There's a bullet on amongst the bundle under the bonnet on the right side near the bulkhead.
Allan Reeling

Thanks as usual, Allan. I'm working through the Andy/Paul set of checks, and so far I can report that

- Pulsing power is present at the LG/G leaving the stabilizer and also on checking the terminal at the back of the gauge.
- there appears to be no power at the Green/Black wire on the back of the gauge, nor at the sender end of Green/Black.

- So power "disappearing" within the gauge, not completing the circuit?

When I attach an earth wire direct to the Green/Black at the back of the gauge, the needle moves SLOWLY to about 1/4; it does not "move smartly towards full"!
Note: the gauge sill had the central earth wire attached. So why would additional earthing make the needle move?
When my ever-helpful wife stirs from her slumber, I'll follow a couple more checks listed above - eg power at green wire from stabilizer, and earthing the green/Black at the tank to see if the needle moves.
Yours, John.


J P Hall

The earth wire on the back of the gauge is for the illumination.

It does sound like the stabiliser could possibly be faulty.

The fact that the gauge is moving at all is a good sign. To test it further, you could apply 12v directly to the gauge, in place of the LG/G. Then grounding the sender terminal of the gauge (very) briefly should see it move much more quickly.
Dave O'Neill 2

John, The gauge needle is damped, so won't do anything "smartly".
Allan Reeling


"Move smartly ..." was lifted from Paul's site of course.
Just to clarify: when I removed the (lighting) earth from the centre pole at back of gauge, and remove the Green/Black from its terminal on the gauge, then run a wire from the latter terminal to ground, this is when the gauge moves slowly to the 1/4 position.
Obviously this is when there is no connection (that I know of!) from stabilizer or gauge to sender.Just pulsed power from stabilizer to gauge, and earthing out from the other terminal on the gauge.
There is no indication of power at the G/B terminal on the gauge, NOR from the G/B wire at the sender.
I like the idea of a direct 12v feed to the LG/G terminal and a quick earthing-out to see what the needle does - I'll try that tomorrow after work.
Grateful for everyone's thoughts as ever. John.

J P Hall

Check that the stabiliser is mounted correctly. They are position sensitive (they are marked top ) and also need the casing earthed by the fixing screw.
Allan Reeling

B O O M

carefull where you're poking 12v
That sender unit might get very hot very fast
Make sure it's disconnected at the tank if you are intent on melting things

worried
willy
William Revit

I can't believe using 12v on the fuel gauge sender instead of the (nominal) 10v will make much difference in terms of damaging the sender. The only reason for the stabiliser is to try and prevent gauge fluctuations due to changing battery voltage as the battery load alters. I ran the gauge on my Elan both with and without a stabilser and it really made little difference.
Mike Howlett

Ha! Yes Willy, I will test the sender when removed from the tank; I didn't realize the stabilizer (original, not electronic) is position-sensitive, so will be trying both sender and stabilizer for malfunction. I'm still hoping the gauge might still be ok. Hope to report tomorrow. Yours, John.
J P Hall

When removed from the car, Voltage stabilizer showed no power from "I" terminal, pulsing or otherwise. We cleaned the contacts, but still a failure - so new v.s. ordered. Apparently they're all printed circuit now, at least from the usual suppliers in Aus.
Meanwhile, the gauge itself passed the bench test with flying colours, so hopefully this will end up being an easy and cheap ($40) exercise.
Off to the Y Type 70th Birthday bash in south-east Queensland this weekend (thanks Tony Slattery), so will report back upon return.Thanks again for all advice. John.
J P Hall

John
Did you have the stabilizer earthed when you were testing it out of the car
As well as a 12v supply it has to be earthed to work
willy
William Revit

"- Pulsing power is present at the LG/G leaving the stabilizer and also on checking the terminal at the back of the gauge.
- there appears to be no power at the Green/Black wire on the back of the gauge, nor at the sender end of Green/Black."

Surely this indicates the gauge has failed? If you have pulsing 12v on one side but nothing on the other, then there can only be two possible causes - one is that the green/black is shorted to earth, which would result in the gauge reading beyond F, the other is that the electrical circuit inside the gauge is broken.

paulh4

OK- back from Y Type 70th birthday bash.
What you said (8/6) made perfect sense, Paul - so I was a bit surprised to find the gauge working just fine once I fitted the new electronic stabilizer. The gauge reads correctly as I add fuel to the tank, right up to Full. I took the opportunity to shift the Stabilizer down to a more accessible point, for ease of future replacement - So for now, it's "Job Done" as Edd says - thank you all for your ideas, and I hope my experience helps someone else. Yours, John.
J P Hall

Very good - and no smoke, even better
Cheers
willy
William Revit

This thread was discussed between 01/06/2017 and 15/06/2017

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