DirkSx1

This all started with my fuel gauge giving increasingly incorrect readings. Went to Clark's garage and followed all of their recommendations per their well written "Fuel level indication Problem/repairs".

Did the following:
1- Verified that the fuel gauge is working correctly by attaching a variable potentiometer at the fuel sensor connection. Gauge showing appropriate fuel level to resistance supplier
2- Fuel sensor. Checked the resistance and determine it was incorrect. Removed sensor and tried to clean it. No luck
3- Purchased new fuel sensor

a) Before installing the new sensor, I verified that the sensor was giving proper resistance values per recommended specs.
B) Installed new sensor. . Gas gauge say less than 1/2 tank in spite of a full gas tank. Took a resistance reading on the sensor, sensor giving me a 29 Ohms reading
c) Removed sensor, checked movement of float and verified that appropriate resistance values still being generated as the float is moved. All OK
d) With the sensor out of the tank, connected it to the gauge. Gauge showing correct readings as I manually move the float by tilting the sensor
e) Reinstalled sensor, getting same 20 Ohms resistance value in spite of nearly full tank.

OK guys, what am I missing?

Dirk
Vancouver Canada
1989 - 951

PaulD_944S2

Where are you measuring the 29 ohms resistance?
When the sending unit is at full the resistance should be just a few ohms, if I recall.

ElRicardo

I have two 944s, both with swapped fuel level senders and have the same issue. My initial research a couple years back (or more) seemed to indicate that there needs to be some sort of "calibration" to work together properly. But if you do resolve this please post, I would be grateful!

DirkSx1

I am measuring the resistance at the two pins on the fuel sensor.

DirkSx1

I am measuring the resistance at the two pins on the fuel sensor. Resistance for 1985.5 + year vehicles should according to Clark's be 63.2 Ohms when empty decreasing to 2.8 Ohms when tank is full. 21.2 Ohms corresponds to a 1/2 tank.

When it is out of the tank, I get the proper resistances based on the location of the float and when the connector to the fuel gauge is connected, the actual fuel gauge registers the appropriate fuel levels. When it is installed in the fuel tank, the fuel sensor resistance is 20 Ohms which does not correspond to a nearly fuel tank.

Since I posted previously, I got hold of another sensor and it is behaving exactly the same, OK readings out of the tank, incorrect reading when it is in the tank. Both fuel sensors i have tried are giving me the same results. Makes absolutely no sense.

GPA951s

I would verify the wire resistance from tank to plug on gauge. Then check resistance wire to wire.. I thought there was a potentiometer that can be adjusted to “ calibrate” the appropriate level

something changes when it’s in the tank and plugged in. Everything post sensor is where to look.. unless you have very conductive fuel... lol

DirkSx1

Yes there is a potentiometer adjustment on the gauge it self. I have checked the wire from the tank to the gauge and they are all good.

I just cannot account for the fact that 2 fuel sensors are behaving exactly the same. When out of the tank, the measured resistance values correspond to the location of the float and when connected to the plug going to the gauge, the gauge provides the right readings. So it all works until the sensor is place in the actual gas tank. In that case, the measured resistance is 20 Ohms in spite of having a near full gas tank and when plugged into the wiring harness to the actual gauge, the gauge reads a bit less than a half tank of gas which corresponds to the 20 Ohms reading I am getting.

It would seem that the float is stuck but i know it isn't, specially after getting the same results with 2 separate sensors.

The only thing left to try is to empty the fuel tank and then see what happens when I slowly start to fill it up.

Dirk

DirkSx1

Decided to go back to basics, emptied the gas tank completely. Gradually filed it back up and while taking resistance measurements at the fuel sensor contacts. Determined that I was getting the right values all the way up. My conclusion was that there likely was something wrong at the instrument cluster. E.g The gauge was still showing a little over half tank with a known full tank. I then proceeded to take the instruments cluster out. I read somewhere that the connectors can oxidize. I also noticed that my amp meter seemed to be reading low on the gauge, 12 V no matter what. Cleaned the contacts and put dialectic grease on them. I also moved the pot on the gas gauge. I noticed that it seemed to be placed off centre, so simply moved the pot so the slot pointed in the middle to see what would happen. The end result is that the gauge is now showing a full tank with a full tank. My plan is to wait for the the gauge to drop to the 3/4 level and fill the tank to find out how many litres it takes to fill the tank and than do the same thing at the 1/2 tank level. If the litres required match the gas gauge. Basically a 80 litre tank, 3/4 tank = 60 Litres and 1/2 tank equal @ 40 litres. If it all works it means the pot is at the proper position.

Also my reserve light (before pot adjustment) had to have a very low fuel level in order for the light to go on. Am hoping to find out whether that has changed now that the pot setting has been changed.

BTW I suspect that cleaning the contacts also contributed to the more accurate gas gauge reading as the volt meter which previously would not go over 12 volts is now showing a more proper higher value. Will let you know once I confirm the gauge actually registers the right values via the measuring the amount of fuel intake.

Dirk

jeyjey

Had something similar with my oil pressure gauge. I ended up cleaning all the contacts, but just unplugging/replugging was enough to fix it.

RC944

Read with great interest the ongoing saga with the fuel-sending units. I'm trying to clean mine (a VDO in an 86 Turbo). My issue is the gas gauge only goes to 3/4 tank when full. When attempting to clean the sending unit, it seems stuck or attached to something in the tank. I didn't want to use too much force, but does anyone have an idea of how I should proceed?

RC944

Read with great interest the ongoing saga with the fuel-sending units. I'm trying to clean mine (a VDO in an 86 Turbo). My issue is the gas gauge only goes to 3/4 tank when full. When attempting to clean the sending unit, it seems stuck or attached to something in the tank. I didn't want to use too much force, but does anyone have an idea of how I should proceed? thx.

931guru

Remove the sender from the tank. Reconnect it electrically. Turn the sender upside down and the gauge should read full. Turn the gauge right side up and the gauge should read empty. The gauge should change smoothly.

RC944

Thanks for the reply. Right now I can't pull the sending unit from the tank. It comes out 2-3 inches, then acts like it is caught or connected to something in the tank. I don't know whether I should just force it out.

931guru

It sits in a well inside the plastic tank. If the tank has encountered some damage in the past 40 years, the well may be deformed. Forcing it may damage the sender, and it may be the cause of your interesting gauge readings. I would leave it alone and drive on the top of the tank until I wanted to force the isssue (but have $$$ ready.

RC944

Thank you. Sound advice.