Any instrument techs here?

[BCNZ]

My fuel gauge was reading 3/4 when the tank was full - so I pulled it out and had a closer look at it.
I gave the pointer a gentle nudge and it overcame what seemed like a sticky spot at the 3/4 mark, and I was able to gently move it to the "full" position.

While I had it on the bench, I hooked it up with +12V to the IGN terminal, 0V to the E terminal and a simulated float sender voltage (via an adjustable resistor) to the U terminal (the only one left!).

The gauge moved up and down ok, so I installed it back in the cluster and fitted that back into the car. Switch on and nothing happens.

I took the gauge out again, and swapped the "motor" over for another identical one I had sitting around here. It worked, but it showed 2/3 full when the tank was empty!

Now I have no gauge that works properly. What I need is some electrical information on how these gauges work - what the various terminals do etc.

The car is a 1994 Toyota Caldina Diesel. It uses Yazaki meter equipment.

[tytower]

Older versions used a Rheostat (variable wire wound resistor)
various other systems I have seen

Any chance of a photo?

[global88]

Dont take too much notice of what im posting at the moment untill after you may have found no solution.

Chances are your doing the right thing but...

I had a similar problem twice on 2 diffrent cars.

One car (toyota)the gauge didnt register full. After opening the fuel tank to take the sender unit out i found the float bowl had a pin hole in it and some fuel got in causing it to sit lower in the tank therefore giving a lower fuel reading.

Second car was my GTR on its first track test day. I always went on track with half a tank on each session. This would give me approx 6 laps till the emergency light would flicker occasionaly, sending me back into the pits for more fuel. On one session i noticed that my fuel gauge stopped dropping less than one quarter of a tank so i just kept going in for fuel realising something was wrong with the gauge and not wanting to lean out and destroy the engine.
Got home, pulled out the fuel sender unit in tank, tested, all looked good.
Put my arm into the tank to see if their was any rubbish down the bottom and was surprised to find 5 parts of an old fuel pump and pickup assembly in the bottom.
The last lazy bastard that up rated the fuel pump decieded to leave half of the old assembly in the tank and this jammed the sender unit.

Just food for thought.

[SystemRat]

A lot of fuel gauges use a bimetallic strip with a coil of heating wire around the strip.

The sender unit in the tank is a float with its arm attached to a rheostat which is a fancy name for a coil of resistance wire.

The idea is the fuller the tank the higher the float which gives a lower resistance. The current flows through the coil of heating wire on the bimetallic strip.

More current more heat and the gauge reads higher. Thus when the tank is empty the sender float is low in the tank the resistance is high the gauge does not have much current to heat the strip and it reads empty.

Problems can occur if the coil of resistance wire or wiper arm on the sender wear out which can result in no operation or intermittent operation. I’ve seen gauges where the needle sticks where it leavers off the heated biometric strip.

If that’s the case careful cleaning with contact cleaner and a very light smear of Teflon grease should fix it.

Most of these type of setup have a voltage regulator module on the back of the dash too. The gauges fuel and temp don’t normally run on straight 12 volts there feed via a regulator at something like 8 volts. That’s so variations in battery voltage don’t translate into higher or lower gauge readings.

You should be able to put say 12 volts across the gauge and it should jump to full.

The fuel sender can be measured with an ohm meter if you can get to it and the voltage regulator normally has three terminals input, ground and output so a voltmeter will tell you its status.

[BCNZ]

Thanks guys for all your thoughts and comments.

These late model gauges use something that looks like a small "motor". It's approx 25mm in diameter, and has what looks like hundreds of turns of copper wire in four "humps" - like you'd see inside a permanent magnet DC motor when looking at the armature - only the coils stay stationary and the shaft turns!

I don't think there's anything wrong with the sender, although I have yet to meter that and see.. but the problem at the moment is the gauge itself.

On the circuit board that this "motor" sits on are two resistors - seem to be 1/2 watt and maybe 1 watt respectively. The "motor" has four terminals.

[tytower]

You can't get a photo ?