MGB & GT Forum

I just bought this '74 MGB which used to belong to a member of this forum. It has a 2.8 V6 Camaro engine and 5 speed transmission. I'm in the process of getting everything up to par. The most recent "issue" is the fuel sender. It pegs at full when grounded and the ohms are all correct. It works fine until I reinstall it into the tank. At that point, it reads empty (with about a half a tank of fuel...) The two wire sender originally had a resistor soldered between them but the dash gauge did not work then, either... Ideas?

Check the fuel tank is earthed

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John
1978 MGB GT UK spec



Edited 1 time(s). Last edit at 2024-04-11 05:46 PM by John37.

It has a separate ground wire which has good contact. I also ran a jumper wire as an additional ground and there was no change in the gauge reading. I'm stumped.

What does the gauge read when the sender is installed?

Slightly above E.

Could the float be being interfered with by the pickup line? I've heard of some units needing to be bent slightly to one side or the other.

I guess I'll have to siphon the tank for the fourth time (!) and check that. Thanks

Did it work before you inserted it into the tank? That is, with the sender wire attached, sender grounded, sender not in the tank, did the sender give you full range readings on the fuel gauge?


GMc

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Wait for me at the gate Tuffy, because without you they're never gonna let me in.

SDCH WTCH-X Twin Oaks Tuff Nutt
2008 - 2022

Yes. The sender appeared to be fully functional out of the tank.

There you go. With sender installed, all you need is sender wire and tank ground. Should work the same.

If it doesn't, verify sender base is grounded. If the sender is isolated from the tank, the tank ground doesn't help. Sender needs ground.



GMc

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Wait for me at the gate Tuffy, because without you they're never gonna let me in.

SDCH WTCH-X Twin Oaks Tuff Nutt
2008 - 2022

Does the float float? If it sinks, that would be consistent w/ your symptoms.

Had similar 'no gauge response to fuel level' problem using an "improved" sender purchased from one of our usual MG suppliers, which is also available from a couple of others.
(Should have bought an OEM style sender, but the "improved" description hooked me)
It is a white plastic unit, that grounds via a wired spade terminal.

The "improved' aspect was nothing more than a solid float, but the Resistance range was far out of line with the OEM 22 to 220. (not mentioned in the part description)

The particular problem with mine, which mirrors your lack of response to fuel level changes, was that the wiper contact arm was exerting too much pressure on the variable Resistance plate.
When checking manually before installation, arm freedom of movement appeared fine, but when in the tank the float buoyancy would not move the wiper arm.

Removed from tank, suspended over a bucket, and added fuel to see float response... float would not follow rise in level.

Had to very gently coax the contact arm to achieve less contact pressure until the bucket test became satisfactory.

Dealing with the erroneously high Resistance range, which would only drive the gauge to half the true tank level, lead me to abandon the 10V stabilizer, and use its 12V feed as my gauge circuit source.
This was done because my problem was too much circuit Resistance, and could not REMOVE Resistance, so had to increase Voltage.

I did this after calculating how much more current the total circuit Resistance would pass at 12V, and found it to be only a couple of 1/100ths of an amp.
The stock gauge, with its 'heater' type winding, now very closely reflects the true gallon to gallon fuel load.
The increase in amperage is so small, I have no concern that it would 'over drive' the heater winding.



Edited 1 time(s). Last edit at 2024-04-12 04:35 AM by Drewski.

Things that come to mind:

1) Do you have the one where the round mounting plate is metal or plastic? One wire or two coming off the sender?

2) Is the float filling up with fuel, thus dropping it to the bottom once installed?

3) Check the tank for continuity between it and the car body........i.e. confirm the tank is grounded.

4) A photo might help.

BH

It has a metal mounting plate with a separate grounding wire. I will do more testing today and return here with the results. Thanks to all....

The float is fine with no leaks.
I ran a jumper wire from the spade terminals at the tank and sat in the front seat with the sender. It has complete and smooth action from Empty to Full. It also does not seem to be anywhere near the feed tube.
Any ideas before I put it back in the tank again?! Thanks.