Morris Minor Owners Club

Hi can anyone tell me what the ohms readings should be for an early sender unit.

Mine read 1.6 Ohms empty , 81.4 Ohms full...

John ;-)

That's about what I'm getting. I've checked the fuel gauge resistances and they're within limits but gauge only shows 1/2 - 3/4 full when sender in full position.

As you've clearly got a multimeter, check the rest of the circuit.
If there a little bit of corrosion/oxidisation at a crimp for example, that will be adding to the resistance in the circuit and thus affecting the reading.
What voltage are you getting at the sender connection?
Is it effectively battery voltage (you said early Sender so I assume you've not got a Voltage Stabiliser setup)?
Has the Speedo been out and the gauge knocked?
How long has it been playing up?
Is this a long term matter that you've got around to looking at, or has it suddenly gone wrong?

Bry wrote: ↑Wed Aug 11, 2021 11:25 pm That's about what I'm getting. I've checked the fuel gauge resistances and they're within limits but gauge only shows 1/2 - 3/4 full when sender in full position.

Maybe it is only 1/2 - 3/4 full?

John ;-)

Sleeper wrote: ↑Thu Aug 12, 2021 11:42 am

Bry wrote: ↑Wed Aug 11, 2021 11:25 pm That's about what I'm getting. I've checked the fuel gauge resistances and they're within limits but gauge only shows 1/2 - 3/4 full when sender in full position.

Maybe it is only 1/2 - 3/4 full?

John ;-)

We've seen sillier things happen here....

I've got 11.9v one side of the fuel gauge and 7.6v on tank side and 7.6v at tank sender unit. Disconnected from earth shows full as it should but when all connected up and operating the float max reading I get shows only just over 12 full. The resistance across the gauge terminals when both wires off is 160 ohms which I think is correct. The tank sender resistance when put in full position shows about 80 ohms. This is the early type so there's no voltage stabiliser fitted.

Interesting.
I'm not going to pull mine apart to check the values at the gauge, but I do know the input should be (presuming you're not running the engine) Battery voltage minus probably 0.1v for the volts drop across the ignition switch. So you might want to check terminal voltage of the battery.
The 7.6v at the sender also seems a little low, by perhaps 0.3/0.4 v, I'd have expected more like 8v.
I just went out and checked at my sender (with ring tag disconnected from Sender) and with a Battery voltage of 12.1v (I've now put it on charge) I had 7.95v.

Have you checked the earth return path from the sender, i.e. is there a good connection between tank & bodywork? A few Ohms here and there can soon add up.

You didn't answer my question about whether it's a new problem or an old one you've just got around to looking at.

This remark confused me....

Disconnected from earth shows full as it should

If the fuel gauge circuit is broken, the gauge will not read, staying in the Below Empty indication, it will not read FULL....


Edited to put my key question in bold....

You DO have the early type sender AND the early type gauge do you? I know you'll get incorrect readings if you mix and match early and late parts

I've found the fault. It is the distance the coils are set from the needle spindle causing the magnetic fields to affect the reading. That was the only thing I hadn't checked because they should never need adjusting once set.

Bry wrote: ↑Sat Aug 14, 2021 9:48 am I've found the fault. It is the distance the coils are set from the needle spindle causing the magnetic fields to affect the reading. That was the only thing I hadn't checked because they should never need adjusting once set.

I'm glad you found it, or have resolved it at least.
Like you say, there is no reason to suspect that in normal circumstances.
Since the induced magnetic field is a product of the coil position and the voltage going through it, I personally would prefer to check and discount other factors before altering the coils.
Your voltage readings seemed a little low to me.
The Coils won't suddenly move without external cause, which leads us back to all the unanswered questions that were posed, including:

Has the Speedo been out and the gauge knocked?
How long has it been playing up?
Is this a long term matter that you've got around to looking at, or has it suddenly gone wrong?


But you're happy, for now at least.
Of course, if it is a voltage issue and all you're done is compensate for it for adjusting the (should never need touching) coils, then should the voltage change in the future, the gauge will now over-read....

Have you tried it with the engine running and the Dynamo providing the voltage, or have you done all this on the Battery....?

The cause was the back of the gauge is slightly out of shape ie not flat so coils not aligned perfectly
Did not want to disturb anything else incase of causing more damage so got the best positioning for a slightly lower reading than normal.
Everything is working fine now, although a replacement will be on the cards in the future if I can find one and funds allow.