Fuel Guage problems

[spoil_kid123]

Need some advise.  My fuel guage is not working properly. When I fill up it reads about 1/2 and as the fuel gets lower the guage reads 3/4 after using the car for some time it will come down to 1/4 so I fill up again not to take any changes.

 

I checked the fuel sending unit all seems well.  But when I disconnected all the wires from the sending unit and turn on the ignition the guage reads just above 1/2.  Are you supposed to get a reading if the wires are all disconnected from the sending unit. please help.

[RicardoM]

Are you supposed to get a reading if the wires are all disconnected from the sending unit. please help.

 

Most people think that the fuel gauge circuit on Felicia is an ammeter in series with a variable resistance and with the battery. That is not true! It is true on older or very cheap cars.

 

 

In fact, I published the schematic of the circuit a while ago in a related topic. Here is a simplified version for Felicia with carburettor.

 

And here is the entire schematic of the fuel gauge circuit. Not so simple, eh?

 

 

spoil_kid,

after this short introduction, let's get back to your issue. Check the following:

• the wires (blue, brown, yellow) are correctly connected to fuel sender (see photo above)
• the copper slider of the sender is making good contact with the resistance from side to side
• the resistance is well fixed and not damaged

If you remember any repair that involved removing the instrument cluster, let me know.

[spoil_kid123]

Thanks for the quick detailed reply.  Will check over the weekend.

[spoil_kid123]

RicardoM.  I checked the wiring on the fuel sender all is good.  What I cannot understand is when all the wires on the fuel sender is disconnected why am I still getting a reading on the fuel guage on the dash.  Can you explain.

[RicardoM]

OK, I will make one more try to explain... so back to science / physics theory

Look at the photo below. I've made this drawing to keep things simple. Let's see how the fuel gauge works.

Inside the gauge there are two opposing windings or coils (L1 and L2). By 'opposing' I mean wound in opposite directions so they produce opposing magnetic fields when a current passes through them. The idea is to have a fixed magnetic field created by L1 and a countering variable magnetic field produced by another current coming through the fuel sender (which is a variable resistor or a rheostat). The resulting magnetic field acts on a rotating magnetic armature (a small cylinder inside the coils) that has the needle of the gauge attached to its axle. The needle moves according to this magnetic "tug of war" inside the can of the gauge.

 

When the current flow through coil L2 is interrupted, the fixed magnetic field "wins" and the needle stays at an intermediary position (around 1/2 tank in your case). I hope that now we're clear, I almost passed out here

 

 

So now you need to diagnose why is the current flow from the fuel sender interrupted.

Here is the list of answers:

• bad contact between the copper slider and the rheostat inside fuel sender
• bad wiring between fuel sender and instrument cluster
• cold solder joints of the yellow connector on the PCB of instrument cluster
• cracked copper routes around mounting pins of the fuel gauge (see photo below)

The German manufacturer of instrument cluster, VDO, was very 'careful' to use as little copper as possible on the PCB. The vibrations of the instrument cluster end up by developing cracks in copper routes, most often around heavy parts that stress the PCB more due to bigger inertia.

 

I repaired many PCBs for Felicia and I had to resolder many joints and in some cases I had to use thin wires instead of bad copper routes.

 

Edited 20 April, 2016 by RicardoM

[spoil_kid123]

Thank You very much for spending your valuable time to explain in detail.

 Will work on this over the weekend and get back to you.

 

Thanks very very much.

[VWD]

one thing I'd suggest( possibly missed from Ricardo's post) is that this PCB contains electronic components, which with all wires attached have protection against any static problems. Wearers of man made clothing can generate huge static charges ,and this can destroy a lot of sensitive components. Then there's another problem with the common soldering iron, which can be leaky and destroy chips etc on contact.

So - step 1- look at body anti static precautions. One simple method, if not near any mains kit is to attach a strap to an earthed source ( metal wrist watch, or a cheap antistatic wrist band).

Step 2- use a good quality soldering iron.

For solder cracks, and track failures, you wil need an eye glass and a meter with something like a pin on the probes. A wire link is the usual cure, but a solder bridge often works.

[spoil_kid123]

Update I checked through the circuit and did find some cold solder.  But after repairing and installing the instrument cluster back into the car turned on the ignition the fuel gauge went all the way to full.  It stays in full wheather I disconnect the wires from the tank or not. Please help 

[RicardoM]

The needle goes to full tank if there is a short circuit between pin 1 of yellow connector and another track.

Measure the PCB carefully. Use a magnifying glass and a bright light to follow the track of pin 1. Perhaps some molten tin shorted two tracks.

 

[spoil_kid123]

thanks for the reply. How difficult is it to connect an external gauge.

[RicardoM]

How difficult is it to connect an external gauge.

Not sure what you mean. A simpler, straightforward, old style gauge without electronics involved?

 

Note: I fully agree that the German designer of fuel quantity indicator really went to town with an unnecessary complicated solution. They didn't compensate well for the non linear shape of the fuel tank, the low fuel warning light comes and goes a few times before staying lit, and it takes 40 seconds (!) of lag to show the true quantity.

[spoil_kid123]

I tried what you surgested last but no luck.  I would like to connect a temporary fuel gauge somewhere on the dash board

 

I have a spare three wire gauge from a 1988 Fiat Fiorino. What wires do I use of the dash.

[RicardoM]

I am not familiar with that fuel gauge.

[ObedP]

 

Note: I fully agree that the German designer of fuel quantity indicator really went to town with an unnecessary complicated solution. They didn't compensate well for the non linear shape of the fuel tank, the low fuel warning light comes and goes a few times before staying lit, and it takes 40 seconds (!) of lag to show the true quantity.

Thank you sir!! I managed to get my fuel gauge back to work. Fixed the fuel sender and the float (it was twisted). Then replaced one blown resistor I had thanks to the guide Ricardo provided me with, and finally I had to take apart the fuel gauge and I discovered it was stuck at the middle of the way. Manually forced it to go to the top and now it works fine, even lighting the yellow light that warns of low fuel level! But as Ricardo just said, it takes a while to show the true quantity of the fuel in the tank.

[ObedP]

Now that I think about it, if someone has messed with the fuel sender and puts it back without proper installation procedures, like turning the fuel sender cap but not preventing the center part not to spin along, the float arm will hit some part of the tank and will be twisted inside, so it will not float, but stay fixed in one position. That was part of my problem.

Edited 15 May, 2016 by ObedP

[RicardoM]

I am glad yoy finally fixed it. Your persistance paid off.

Advice for every Felicia owner having a fuel sender with 3 wires: do NOT swap blindly the wires going to fuel sender hoping that "a miracle" will happen and the fuel gauge will start working. The only miracle will be that a 10 Ohms resistor will blow out in the instrument cluster. In fact if you don't know exactly how the electrical system of your car works, stay out of it. If not, you'll waste time and money or you'll end with a car on fire.

 

About the fuel sender arm: Skoda explained clearly what is the correct geometry of the arm (see photo). Furthermore, the arm myst be parallel with the longitudinal axis of the car (see other photo).

 

 

... the float arm will hit some part of the tank and will be twisted inside.

Incorrect. There is nothing inside fuel tank. No baffles, no drains, no nothing.

 

[ObedP]

Sorry, tried to load a picture, but made a mistake. now the system doesn't allow to delete this post.

Edited 15 May, 2016 by ObedP

[ObedP]

But according to this diagram, there is chances of hitting the wall of the tank with the float arm if it rotates along with the cap while refitting it. That's why Haines manual says that there is risk of bending the float arm.

The float (flange) must stay fixed in the correct position, not moving at all, while refitting the union nut. If not, you will hit the closest wall of the tank and bend the arm. It happened to me. Twice. Then I realized.

Edited 15 May, 2016 by ObedP

[RicardoM]

You can't rotate the flange of the sender too much when mounting the plastic nut that holds it. The fuel hoses are very short and don't allow the rotation. You are either not explaining it right or you are talking about something totally different.

One thing is certain, if the fuel sender arm has the geometry showed in my previous post, it can't hit any part of the tank.

 

[ObedP]

Yeah but as I didn't know, I didn't first attached the hoses. I just started to mount the whole thing by turning it altogether!! That's why I am comenting this just in case someone is making the same mistake. Then I realized I had first to connect the hoses to keep the flange in that fixed position, then start to turn the union nut. Sorry if I didn't explain it clear at first. My english vocabulary fails me often.

[motorbikemitch]

Thank you so much o had issues with my fuel gauge and after reading this it's now fixed however I need to fix my temperature gauge now :L

[Indalsman]

Silly question perhaps, but how will the service hole for tank fitting on Felicia hatchback?

[RicardoM]

The service hole has a pressed cap. There is a lip to help open it using a screwdriver or something similar.

[Indalsman]

Is it under the backseat?

[RicardoM]