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MG MGB Technical - Fuel tank sender

Very frustrated not being able to get the fuel gauge working on a 1969 mgc roadster.
All usual checks have been made and the gauge and voltage stabiliser have been proven to be fine. We have tried three different sender units and we usually always get the same result,...either no reading on the gauge or about 1/4 full indication. With the sender out of the tank we have moved it up and down and the gauge indicates perfectly.
One ray of sunshine; the sender unit has three bayonet connections and we have always used just the top two. My question; should one of our two wires be going to the bottom one and if not, what is the purpose of it?
Thanks very much for any help......
JP Mitchell

JP - Sounds like the sender is not grounded through the tank. One of the three spade lugs on the sender should be attached to the mounting plate, try attaching a jumper wire from that lug to the chassis of the car and see if that makes a difference. Cheers - Dave
D W DuBois

Hello Dave,

Many thanks for your advice. We have tried the grounding issue but will follow your advice and try again.
Kind regards,

John
NE England
JP Mitchell

Hi,


The correct sender should only have 2 connectors.


The loom will have a black, which is the earth and a green/black which is the gauge wire.


Sounds like the sender you have is for a car with a low fuel warning lamp, which no MGB/C ever had.



Look carefully ans see if it's marked W, for warning lamp, ignore that one if it is.
SR Smith 1

As indicated a 69 sender should have both earth and gauge wires, if bridging these two drives the gauge above Full then the problem is definitely the sender. But if manually operating the sender arm through its full travel drives the gauge through its full travel then you are using the correct terminals, but either something is preventing the sender arm rising when in the tank ... or fuel isn't getting to that side of the tank, which seems rather unlikely. Peer in with the sender out and filler tube removed and see if anything is obstructing it.
Paul Hunt


SR and Paul,

Thanks very much gentlemen for your response. Really quite frustrating for such a simple thing.

I will try your respective ideas and report back.

Thanks again,..take care,

John M
Tynemouth, NE England
JP Mitchell

John
If you look closely you will see on one of the outer spades an I casting the plastic,that is ignition feed,centre one is for an earth and the other is not needed unless you have a warning light.
If you touch both wires together and your gauge goes to full then the gauge and wiring are ok.if you plug the wires onto the sender and lift the float it should go to full,if it doesn't bend the stops to allow it to go to full.if after you install it it stops working then it must be hitting something inside the tank
s p brown

John,
I have a new tank and a sender with a fuel pick-up (which I am using as a return Line) and three spade connections. I had to shorten the float arm to 17.5cm overall to stop it catching on the nearest baffle in the tank. The top spade is earth and the centre runs directly off the guage (Sorry s.p.b). The lowest is not used.
HTH Roger
Roger Walker

Roger
So you are using a rubber bumper sender unit which may have the connections a different way round or a different manufacturer,just fit a moss one to my mgb and the connections were as I put but it could only be for the moss one.
Ste
s p brown

As far as I'm aware all MGB senders only had a maximum of two terminals, regardless of whether they were early screw-in, mid-term without pickup pipe, or late with pickup pipe which only had one terminal. If you have a 3-terminal with pickup pipe then it sounds like the wrong item, which is perhaps why it is hitting the baffle. Shortening the arm will reduce the range of movement of the float, meaning it will stop significantly short of full and empty. You may be able to compensate for this by recalibrating the gauge, depending on how much was cut off.
Paul Hunt

Paul
You can't get original metal sender units now.only the white plastic bodied version is available.have a look on Google or moss
s p brown

This is a useful bit of kit..............


http://www.carbuildersolutions.com/uk/electronic-fuel-gauge-matcher



Just used one on an old TVR to match the OE sender to a new type gauge.
SR Smith 1

"You can't get original metal sender units now"

I was aware of the 'modern' ceramic resistor type (AHU1027), but hadn't realised they were the only ones available in place of the originals (ARA966). Most places still show the metal type under the new number.

Whilst they may well have a third terminal they still shouldn't hit the baffle.
Paul Hunt

Gentleman,

Thanks very much for all comments.

I think Roger may have the answer because we have tried a couple of new sender units and each time we get just 1/4 full reading on the gauge. Obviously the float arm is catching on something within the tank.

Thanks very much for your help,.....

John
JP Mitchell

This thread was discussed between 09/01/2015 and 17/01/2015

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