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MG Midget and Sprite Technical - Gauge problems

We are in the stage of debugging the car, and I have two problems to solve, one of which is that the fuel gauge always immediately reads full even though we have only 5 litres of fuel in the tank.

This is an change from the initial problem, which was that the fuel gauge read correctly and would read immediately full when the brake lights were on. This was traced to an incorrect connection at the A pillar, where green/purple and green/black were incorrectly linked to a 4 way connector.

I have carefully checked all the other connections in the circuit and all appear to be correct and in line with the wiring diagram.

The second problem is that the electronic rev counter does exactly nothing, although it is wired correctly according to the diagram. Disconnecting the green cable here has no effect on the fuel gauge reading.

Any suggestions ?
dominic clancy

Car is 1275 positive earth, the fuel gauge is pictured


dominic clancy

You may have a mismatch of gauge and sender.

Early (yours) and late gauges were of different design and the senders worked opposite to each other - i.e. if your gauge reads full when the tank is empty, it should read empty when it is full, if you have a mismatch.

Also, the late-type gauge used a voltage stabiliser, whereas the early one didn't IIRC.
Dave O'Neill 2

It sounds as though the sender unit is stuck do you have an old one you could wire in and just move by hand to check. I cannot help with the rev counter I'm afraid.
Dave
D Pratt

Hi,

That looks like the old type gauge that reads instantly with the ignition, rather than climbing slowly.


That type will read full if open circuit to the sender, and empty when shorted to ground.


What happens if you short the geen/black to earth?
SR Smith 1

will try earthing the green / black tomorrow. I do have an MGA sender (which goes with this type of gauge) so I can try that too.
dominic clancy

OK

I tested the gauge alongside another known good one from my A. Both show the same resistances measured alongside each other, so I am sure the gauge is OK.

Earthing the terminal T (GB wire) does nothing. Attaching the earth to the case makes the needle flip to full immediately. There is enough current flowing that I get a tingle doing it!

I have 12V at the B terminal (green) with ignition on, 0V when off.

I can't measure any circuit on the GB wire so it is not grounding anywhere along its run.

So it looks like a sender problem, but I am going to look at Barney Gaylord's fuel gauge page as it is exactly the same as the A from a circuit point of view.

It's nothing that will cause a fail of the safety test, so it is not critical to get done immediately.

Dominic

dominic clancy

Doing the earth test correctly definitely proves that the gauge is OK, as the needle swings straight back to empty. So its back to dropping the tank and checking the wire to the sender unit (there's only one connection at the A pillar) and then testing the sender. Can anyone confirm what the part number of the correct sender is, or at least what the part numbers for the later senders are so I make sure I get the right one. The car has been built from a large stack of boxes, so there's no guarantee that the previous owner bought the right one.
dominic clancy

the sender units and tanks have different fittings, does your tank take a round unit with a simple locking ring, or a rectangular one with about 8 tiny setscrews to retain it?Do you have access to the online Moss catalogue? as that gives all the different types and change points.
http://www.moss-europe.co.uk/
David Smith

Check that the body of the gauge earthed correctly.
Noel Copping

The body of the gauge is definitely correctly earthed.

I don't yet know what the sender looks like as I haven't dropped the tank yet. Will probably have time to do that on Wednsday evening.

The tachometer is fixed, I just had to reverse the connections for the loop of wire that comes from the coil. Paul Hunt's site was the place I was ponted on the MGB board, and it's very comprehensive on the two types of tachometer.
dominic clancy

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

Lots of info about our gauge in this MG A site!

Flip
Flip Brühl

I am waiting for the delivery of a new gauge sender before dropping the tank.

dominic clancy

http://www.vonmunching.nl/index.php?id=10

He can deliver the rubber gaskets etc. between tank and the sender that is resistant to modern fuel. The standard gaskets are not. It is €12 including post.
Flip


Flip Brühl

I aways use the cork ones, thanks. I already have a few spares.
dominic clancy

So did I but I had to replace them every 1-2 years. The Euro 98 and 95 solved the rubber between the cork. The car began to leak with a full tank and was sniffy. Some kinds of rubber (most of them) solve in modern fuels.
Flip
Flip Brühl

This thread was discussed between 13/07/2014 and 21/07/2014

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