6 replies to this topic

[W.Hewitt]

Hi everyone, I'm new here, I recently bought myself a classic mini. It's my 8th. The other 7 were spi/MPI bought new in the 90s. This one is an 89 mini city that was converted to a 60s looking 1275 at some time in the past. It's race tuned and I love it. It has however many issues that I'm working through. Today's issue is that when I got it the fuel guage read full when I topped up the tank but after about a quarter of a tank it was on empty. I bought a new sender unit and replaced the old ( horribly corroded) one. Since then the fuel gauge is reading full constantly. Any help with why this might be is much appreciated. Thanks W

[alpder]

Assuming the new sender is correct for the tank, matches the gauge, is correctly fitted, and hasn't jammed in the "up" position, then the chances are that the wire from the sender is shorted to ground. Probably close to or at the sender, since that's where the wire has been recently manipulated. First test is to unplug the wire from the sender. If the needle goes down, you know it's the sender. If it stays up, you know it's the wire.

Edited by alpder, 08 May 2024 - 06:14 PM.

[W.Hewitt]

Thanks for your response. I tried what you said. The gauge didn't move, when the wires were disconnected . I retried the old sender and it too just went straight to full with only 5 litres max in the tank . With the sender out of the tank , moving the wires produced no signal to the gauge. With the sender in the tank but not tightened up the gauge was kinda of giving a correct reading. Just bizarre to me.

[alpder]

The sender is part of a circuit which (roughly speaking) is a series circuit like this:

12V+ ... voltage regulator ... gauge ... sender ... Earth=12V- (Assuming a neg earth car.)

 

The voltage regulator turns the 11V-14.4V which comes off the battery/alternator into a stable constant voltage. (Actually, the original type don't even do that, but it's a close enough approximation to get how the circuit works.) See below for my ignorance on this, though. Hoping someone else can help on this.

 

The sender just puts a resistance into the circuit - lots of resistance when the tank is empty (so not much current flows and the needle hardly moves up the dial) and very little resistance when the tank is full (so lots of current flows and so the needle goes to full deflection). The sender's body must be connected to earth (i.e. by bolting into the tank firmly, or by temporarily wiring the body to the chassis.) Testing with the sender out of the tank - without it being earthed - will give the same result as unplugging the wire from the sender: it breaks the circuit, no current can flow, so the gauge reads zero. Testing with the sender loosely bolted into the tank will give variable results, as it won't be earthed consistently.

 

At least the following faults can occur which will cause your gauge to go to permanently high reading:

 

Voltage regulator has broke, so that you're getting more than the regulated volts at the gauge. I don't know what the regulated volts should be... if it should be something like 6V then a s/c reg will cause the gauge to over-read (a lot). But if it the reg should normally give 11V or 12V then it's unlikely to be your problem, assuming you're testing with the engine stopped, because that's close to battery volts anyway. Anyone got info on this, please?

 

Sender broke. So maybe it's gone short-circuit, which will cause a "full" read constantly.

 

Wire from gauge to sender shorted to the chassis. Which will cause gauge to read full always, no matter what you do, because the circuit is complete (but is missing the sender's resistance.) Maybe wiggling the wire may break/remake the s/c, causing the needle to move between full and empty.

[W.Hewitt]

Thanks again for your help. I'm getting nowhere fast with it. My new issue is that wipers only worked on slow and one wipe, fast wipe didn't work. So I've unbolted the motor cleaned up all the rusty terminals etc. Put it back together. Now it only works on fast!! Electrical gremlins in every corner 🤷 I'll update is I get it sorted.

[alpder]

My new issue is that wipers only worked on slow and one wipe, fast wipe didn't work.

There's a really good explanation of 2-speed Lucas wipers on an MGA forum: https://mgaguru.com/.../2spd_dr3a2.pdf . Might have all you need to figure out what's going wrong.

[Spider]

When you are connecting the wires to the new Sender, there's usually 2 terminals on them, one being an Earth and the other is an insulated Terminal. There's usually a black wire and a Green / Black wire to connect to the Sender. Check that the Green / Black wire is connected to the Insulated Terminal.