Fuel gauge anomaly

[Benny]

Since putting power to my new loom last week, I've been troubleshooting the devices on the car.....most has gone surprisingly easy.

Today I put power to the fuel gauge for the first time, and it went straight to the 'full' position, and it makes no difference how much fuel is in the tank. I wasn't overly surprised to have issues with the fuel gauge, as I've got a bit of a mix of parts. The sending unit in the gas tank appears to be the original 1960 model. The fuel gauge itself was 'donated to the cause' as part of a complete instrument binnacle, and is likely out of a 70's or 80's car.

How do I start troubleshooting this one? Do the early and later cars use a different voltage to the fuel gauge? The sending unit produces a variable resistance, correct? So, should I start by measuring its resistance with the tank empty and full?

Thanks in advance.

[Tim]

Maybe I should wait until someone a bit more certain of the facts comes along, but I'll jump in anyway.

The early sender and the later gauge units are not compatible (this I know). The early gauges didn't use the 10 volt stabiliser (the little tin box attached to the back of the speedo) and have a different resistance coil in the sender.

I'm also under the impression that the sender in the early cars worked in the opposite direction to that of the later cars. So instead of showing high resistance when the tank is full, and low resistance when empty it shows a low resistance when full and high when empty (or the other way round).

Either way, the upshot is that the old senders and the newer gauges aren't compatible. You probably need a sender out of a car of similar age as the gauge.

Having said that I'd expect it to show some movement between a full and empty tank, so you may have another problem as well. Try disconnecting the wire to the sender and shorting it to ground - this should make the gauge do something.

Sorry, that's a bit vague.

Tim

[Benny]

You probably need a sender out of a car of similar age as the gauge.

Thanks for the feedback. I'm not sure a different sender is an option, as the sender in my early tank attaches with 6 screws, and I believe all of the later ones attaches differently, correct?

[Spider]

Tim's suggestions were quite good, however, I'm 99% certain that the gauges still work in the same way in that low resistance at the sender = full tank and high resistance = empty tank. Sorry, I don't know what the full / empty resistance values are for the early cars.

You do have an early tank sender. These are used with the direct / quick acting early gauges and these didn't use a voltage stabiliser. Being quick acting, the gauge will move about a fair bit on bumpy roads. The later (well post 65!) cars on used a bi-metal gauge which was slow acting and so wasn't upset on bumpy roads, but needed a 10 Volt stabiliser.

Disconnecting the Green / black wire from the sender (as Tim suggested) should make the gauge read empty. If is doesn't, then that Green / black wire has a short to earth somewhere. Ah, lots of fun chasing that!

If the gauge does drop to empty, probably the next thing I'd check is if in fact the black wire that is connected to the sender is actually earthed. It does common up on the fuel pump, so when checking, disconnect the White wire off the fuel pump so you don't get a bum steer (back through the ignition coil and the points!). Also, check with your meter that the sender changes resistance when moving the arm from empty to full. Being as old as it is, it could also be jamming, so be worth removing from the tank for a look-see.

[cheleker]

Some research is being done for a good article on Mini fuel senders/gauges. I've seen an early copy. The author posts here and I expect him to chime in. In the meantime, Tim's overview is correct. Not compatible and the senders work in different "directions." You'll need a gauge from a pre-stabilizer car and you'll need to feed 12v to the gauge and sender, bypassing a stabilizer if you have one installed.

[Benny]

Thanks guys. I know the float is moving properly, as I had the tank out recently, and I can see it rising through the filler neck as I add fuel to the tank. I have never checked whether the resistance changes with movement, so I'll do that tonight.

Assuming the sender is performing as it should, could an in-line device be added (resistor?) to get the gauge and sender within the same range, even if the gauge reads backwards?

[mascher]

An early sender with a later gauge will show Full when Empty and when Full will show 1/2 or center on the gauge. It wasn't hard to get used to. Mine has been like that for about 15 years now.

I'm also secure in the knowledge that no one can drive my Mini very far.

Kelley

[monkeyjim]

Funny that , mine showed slightly over half when full too . I also lived with it for 8 years till it came off road for revamp 2 years ago . Must get revamp underway and finished for the track day
Could not tell you what bits were what in mine other than it is a pre 64 new loom but being colourblind I was well chuffed to only have the dicky fuel gauge after I had fitted it and finished the restoration .

Dave

[Benny]

The fuel gauge in my V8-powered Cortina read full when it was empty, and 1/4 tank when it was full....I lived with that for 100,000 miles, so I could definitely live with what Mascher describes. However, when I put 4 gallons in the tank, the gauge didn't react at all.

[Tim]

You can get the situation where the windings on the sender are broken, so the gauge works when the tank is full, but as soon as it drops below the break, it goes straight to empty. Totally useless! - who needs a tank that tells you its full, but goes quiet on the subject when it gets close to empty?

Tim

[Benny]

Ok, here is what I figured out this evening.

With the ignition on, and the fuel gauge at full, I pulled the green wire off the sender, and the gauge returned to empty as it should. Then I measured the resistance across the sender with the tank empty, and it measured 1.6 ohms. I added some fuel, and it measured 20 ohms. Added a bit more, and it was 40 ohms. 5 gallons of fuel gave me about 60 ohms if I remember correctly.

At this point, the fuel gauge was reading about 3/4 full, so the needle HAD moved finally. This got me thinking (which is dangerous when electrics are involved)....if I added an additional in-line resistor of something like 30-40 ohms, could I end up with a gauge that works fairly accurately, but in reverse?

There's an instrument shop in town that I'm going to call tomorrow, and see if they can do anything to the gauge or sender for me. They've converted 4-cyl tachometers to 8-cyl for me in the past, so they might be able to do something creative.

[minimans]

Benny I have a couple of MK1 fuel gauges here at the shop do you want one to put in your binnicle? seems to me to be a lot easier than futtzing around with resisters?

[mini129]

Pre Sept 64 ratio gauges and senders work on 12 volt negative or positive earth.
The resistance range is 3- 83 ohms.
The gauge can and is used in the opposite direction in other cars by connecting the two internal coils the opposite way around.
Post 64 bimetallic gauges work on a regulated 10 volts with a positive earth or negative earth regulator ie you must use the correct one.
The resistance range is 250-19 ohms.
I have formers wound for the fuel senders early or late so the opposing gauge/ sender combination will work. I also have a resistance conversion box made that can be used in line.
A single resistor in series would only mean that the the combination is correct at one point

[Benny]

Benny I have a couple of MK1 fuel gauges here at the shop do you want one to put in your binnicle? seems to me to be a lot easier than futtzing around with resisters?

That definitely sounds like an easier route

Do they fit right into a 70's era speedo?

[cheleker]

Benny, I mentioned in an earlier post that someone working on a fuel gauge/sender article would post shortly. Come to find out he's not a member (yet) of this forum. I've PM'd you a contact email address.

[minimans]

Benny I have a couple of MK1 fuel gauges here at the shop do you want one to put in your binnicle? seems to me to be a lot easier than futtzing around with resisters?

That definitely sounds like an easier route

Do they fit right into a 70's era speedo?

As far as I know it will go straight in I'll check when I get to the shop and let you know......Paul.

Benny do you have a black face or the early silver one? I have both, black face are used and I have one NOS silver MK1 and a used one

[Benny]

Benny do you have a black face or the early silver one? I have both, black face are used and I have one NOS silver MK1 and a used one

I have a black-faced speedo.

[minimans]

OK I'll dig one out, PM your address and I'll post it off to you............Paul....