Morris Minor Owners Club

1968 1000 Traveller. I have just replaced a blown instrument light bulb. On replacing the speedo in the panel, the fuel gauge reads empty (with a full tank). If I remove the speedo again, the gauge reads normally and in addition, the instrument lights come on despite the light switch being in the off position. Anyone have any thoughts please?

It sounds like the gauge terminals are hitting something as the speedo is positioned, push any nearby wires away from where the back of the speedo sits. Check for loose wires or broken insulation. Sorry it’s a guessing game, not being there !

Crystal Ball time, with a fair bit of logical thought process..

On refitting the Speedo, you've clearly disturbing, i.e., breaking a connection at the Voltage Stabiliser, that's why the Fuel Gauge isn't working.
However, Instrument Lights coming on, well that connection to the Voltage Stabiliser is apparently shorting to the Instrument Lights.

So you'll just have to conduct a thorough examination of all the wiring and connections in that area; you'll find something eventually.

Only you know the condition of your car's wiring, the older and dilapidated it is, the greater the chance of these things happening.
Assuming it's the original loom, it's 57 years old, long beyond it's intended lifespan.

I have checked for anything which may be contacting the back of the gauge and giving a spurious earth. My understanding is that the speedo needs to get it's earth from contact with the dash when it is pushed into place, but everything works as it should when the speedo is out of the dash, so it must be getting an earth some other way?

Do any of your speedo lamps ( turn,ignition , oil ,battery etc. ) have fittings with earth attached?

John ;-)

The oil pressure lamp sender earths through the engine block.
All the lamps including the two speedo illumination lamps work with the speedo out of the dash.

Speedo drive inner?

John ;-)

I disconnected the Speedo drive. Everything still works. Pulled everything electrical out of the back of the Speedo, just leaving the B & I connections to the voltage stabiliser and the green/black from the tank sender. Fuel gauge still works. Shorting the Speedo body to the panel or the two lugs for the Speedo retaining bolts, fuel gauge stops working.

Looks like you're just left with the voltage stabiliser, which should need an earth to work? Next B and I ?

John ;-)

Looks like you're just left with the voltage stabiliser, which should need an earth to work? Next B and I ?

John ;-)

I can’t see how it is the voltage stabiliser, as the gauge works correctly when the Speedo is out of the panel.

The instrument lights on the speedo have a live feed via the little switch under the dash and the earth is provided by the speedo fixings. If the panel lights are still on with the speedo out of the dash and the panel light switch in the off position then there is obviously something amiss. The little switch under the dash has screw terminals that just make contact with bare wire. Has a wire come adrift? The feed to the switch is a red wire and the output to the panel lights is a white/red wire.

Stephen

Kittyhawk wrote: ↑Tue Jul 01, 2025 6:19 pm I can’t see how it is the voltage stabiliser, as the gauge works correctly when the Speedo is out of the panel.

Looks like we're searching for a stray earth on the speedo when not in the dash..... By a process of elimination,you only have the B and I wires left ?

John ;-)

The instrument lights are now behaving normally.

But not the fuel gauge. I think the only thing left is to try a new voltage stabiliser.

Kittyhawk wrote: ↑Wed Jul 02, 2025 5:52 am But not the fuel gauge. I think the only thing left is to try a new voltage stabiliser.

The original voltage stabilisers are electromechanical and are not polarity sensitive. Replacements are polarity sensitive so make sure you order the right one. Before buying a new part, check the fuel tank sender earth. This is often the issue and it may be coincidental that it occurred when you changed a bulb. Also if you have a multimeter, check the output voltage of the instrument voltage stabiliser. It should be 10 volts or close to that. B terminal connects to battery, I terminal connects to instruments.

Stephen

But surely the sender must be earthing correctly, as the fuel gauge reads correctly when the speedo is out of the dash?

Kittyhawk wrote: ↑Wed Jul 02, 2025 9:58 am But surely the sender must be earthing correctly, as the fuel gauge reads correctly when the speedo is out of the dash?

When the speedo is out of the dash are you sure it’s not just earthing against the metal dashboard?

Out of the dash, as in held in my hand with no contact with anything metal and all other electrical connections removed except for 12V feed to stabiliser B terminal and green/black from tank to fuel gauge.

Kittyhawk wrote: ↑Wed Jul 02, 2025 10:33 am Out of the dash, as in held in my hand with no contact with anything metal and all other electrical connections removed except for 12V feed to stabiliser B terminal and green/black from tank to fuel gauge.

I don’t understand how the gauge can work at all without a connection from “I” to the gauge.