MM Fuel Gauge

[mike.perry]

Does anyone know the function of the two small nuts in slots on the back of the MM fuel gauge?

[ian.mcdougall]

Ithink they adjust the electro magnets in the fuel gauge to bring them closer and further away to the needle to adjust the swing of the needle to either make it more accurate or destroy it forever maybe a company that refurbs gauges would be able to set it up correctly to the sender unit

[mike.perry]

I think that I will concentrate on getting the fuel gauge to work properly. It only reads full.

[fussyoldfart]

They are definitely for adjusting the gauge for accuracy and need to be treated with some respect as they attach to hair-fine wires inside the unit. I had to adjust one on my MM and I found an explanation and instructions on the net. They referred to an MGA but everything worked perfectly. Wish I could just send a copy but you will find them quicker with Google. Try "Fuel Gauge, Smiths, adjust" as your search terms.

Hold on, I found it!

http://mgaguru.com/mgtech/electric/fg_10.htm

TaDaaa!

Darrell

[mike.perry]

Maybe I should just try another gauge

[fussyoldfart]

OK, here's the quick and dirty way. From the rules in that link remember - One resistor = Tank Full. Two resistors in parallel = Tank Half Full. Grounded jumper/no resistance = Tank Empty.

So, with the tank FULL just use the wire from the sender, that's the right resistance for YOUR car. Try small adjustments until the gauge says full. For empty ground the connection and adjust the other end. Half full is an approximation anyway so forget about it. I went out and bought the resistors and then never used them, they are part of the clutter on my bench and will probably be there the day I die.

The thing that slowed me down is that I forgot at first that the case of the gauge is supposed to be grounded at all times. If you have it in your hands don't forget to connect a jumper to a decent ground and always hold the gauge upright when you are checking your adjustments. Loosen the two nuts just enough that you can move the adjustments with your fingers. When the gauge seems to work right then snug everything up so it will stay put. Snug, in this case, is not much more than finger tight.

Oh! before you begin you might try an ohmmeter across the two connections just to be sure there IS continuity. If one of those hair-fine wires is broken there's no fixing it.

Have fun,

Darrell

[mike.perry]

So if the gauge reads full regardless of how much fuel is in the tank, where do I start? Sender float stuck in the tank?

[fussyoldfart]

OK, the sender is the other end of the system. I presume you have the gauge out or accessible at this point, do you? If so, disconnect the sender wire from the gauge and ground that terminal of the gauge. It should now read empty. If that's true you may have to pull the other end of the car apart. First, use your ohmmeter to measure the resistance from the sender wire to ground. It should be something measurable, not open. If you get "open" as a reading check to see where the wire may be disconnected. (My reasoning is that if X ohms = full then very high resistance will also indicate at that end of the gauge.) If you are lucky it will just be a broken wire or perhaps it is disconnected at the tank. At this point you want the tank less than full so you can try a BFRH to thump the top of the sender and see if that moves it. If the gauge is good and the wire is good you need a sender. As I said somewhere else this morning Murphy's law will apply.

Darrell

[mike.perry]

I will try that when I can get the boot (trunk) floor out. At the moment I am using a dipstick and a spare 2 gall can

[fussyoldfart]

In a bygone time I ran a Morris 1000 with a broken fuel gauge but I knew the MPG it would do so I just filled it every 200 miles. I could go into a service station and say "Fill it up, it will take X amount". Actually it was pretty easy because gasoline was 42 cents per gallon and the car always did 42 MPG. At 200 miles I had a reasonable safety margin.

[DAVIDMCCULLOUGH]

Let me know how you get on Mike, mine also reads full all the time and I have checked the sender and tried other gauge heads ( even a new old stock one) without any joy. Hopefully when I fit the new loom it will all work as it should

[mike.perry]

Mine worked until I fitted a new loom!!!!!!

[fussyoldfart]

Mike Perry said "Mine worked until I fitted a new loom!!!!!!" I would say "Check your work before you do anything else."

PS - For those who may read this thread - Later cars had a different gauge that needs a voltage stabilizer in the system. The quick way to know if that's what you have is by watching the gauge as you turn on the ignition. The older ones like the MM spring instantly into place, they're magnetic. The newer ones slowly move from rest to indicate some value, these use bimetal strips and need a voltage control/stabilizer. Adjusting/correcting/calibrating these is a different process. Google is your friend.

Darrell McDonald