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MG MGB Technical - fuel gauge

hey people,

i have a mgbgt 1975 jubelee. got an 18v engine.

my fuel gauge stopped working a week ago until then it was perfect. i bought a new gauge (smiths) as on taking the other apart it fell apart.

i can not get the smiths one to work either, the sender has only one wire to it, so far i have tried earthing the tab on the outside of the sender and nothing has happened.

i am wondering if it is the sender itself?

how do i test this?

any more ideas????

any help would be much appreciated.

thanks for your time.

Matt
matt

How about your temperature gauge Matt, is it working or is it dead also? if the temperature gauge is working, then check the wire from the voltage stabilizer to the fuel gauge. You should see the 10 volts cycle on and off. If both gauges are non functional the check that you have 12 volts coming into the voltage stabilizer - if that is present, change the stabilizer. Cheers - Dave
David DuBois

Matt,

Beyond David's excellent advice, I can only add that the gauge itself almost never fails. If you cannot trace the problem to the voltage stabilizer, or the lead from the stabilizer to the gauge, the problem will be in the sender or possibly the lead from the gauge to the sender. The sender is easy to replace - just do it when the fuel level is low. Jack up the car on the right side only so whatever fuel is in the tank will roll to the other side. Often the float in the sender will perforate with age, fill with fuel, and sink in the tank such that the fuel gauge thinks it's empty all the time. Use good jackstands! I always try, vigorously, to push the car off the stands first before going under. If I can't push the car off, it probably won't fall off.

FWIW,
Allen
Allen Bachelder

Matt
I have been mucking about with my tank as I am fitting a low fuel light.
I found that the resistance across the sender unit, i.e. the resistance from the output terminal to ground shoud be roughly 30 Ohms when full and 250 Ohms when empty.
Regards
Dave
D M Tetlow

UK didn't get an electric temp gauge until 77. Senders (on the tank) before 77 should have the green/black wire that goes to the gauge as well as a black earth wire on the smaller of the two tags. If either of these connections get corroded they can stop the gauge working, and they are very exposed.

With the ignition on if you connect a good earth to the green/black at the sender the gauge should move smartly upwards, only leave it on long enough to see it start to move. If it does, then scratch a clean bit on the sender body and connect the ground to that. If the gauge moves again (assuming you have petrol in the tank!) then the sender earth is bad, it comes from a number plate bolt.

If the gauge moves with the earth on the green/black but not on the sender body the sender is bad - or you have no fuel.

If the gauge doesn't move with the earth on the green/black find where this wire connects to the main harness at a 2-way bullet in the mass by the fusebox - check both sides. If nothing there do the same on the green/black on the back of the gauge. If nothing there use a test-lamp or volt-meter to look for 12v on the light-green/green and green/black (earth disconnected) of the gauge. In normal operation this should be switching off and on again about once per second, taking a few seconds to first switch off when first turning on the ignition. If on the light-green/green but not the green/black the gauge is bad.

If not on the light-green/green locate the voltage stabiliser (possibly screwed to the lower edge of the firewall in front of the driver behind the dash) and test for 12v on its green and light-green/green wires. If on the green but not the other the stabiliser is bad.

If not on the green the circuit back to the fusebox is broken. On a 75 the feed comes from the fusebox to the stabiliser to the tach to the heater fan switch. So if those two are working the wire is OK and it must be the connection at the stabiliser or the stabiliser itself.
Paul Hunt 2

Hi
Don't mean to hijack the thread but can someone direct me to (archive)? or about how to adjust the fuel gague? I remember reading it is easily done by turning a screw? it's on a '70 GT. I have a spare to look at but don't really know how to proceed.
Mike
MK Mike

On that last question: figured it out, thanks
Mike
MK Mike

For anyone else, have a look at http://www.mgb-stuff.org.uk/wn_fuelframe.htm and click on 'Fuel Gauge'.
Paul Hunt 2

This thread was discussed between 25/03/2008 and 29/03/2008

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