Fuel Gauge Connected Directly To Sender

[Pawel66]

My question and request for help is:

If I connect a fuel gauge right at the sender (fuel gauge supposedly matching the sender), will it show correct reading? Mine is showing full while there is less than half of tank of fuel.

I took the plus and signal from the sender right in the trunk - I wanted to check if the gauge works before getting behind the dash. And it shows wrong reading.

Is there a resistor in the instrument cluster that makes it necessary for gauge to work correctly? Or SLS sold me a wrong gauge?

There is a resistor on the wiring diagram, but there is a resistor in the gauge assembly. Or it is another one in the instrument cluster?

[Pawel66]

I found these remarks, but not commented further:

https://www.sl113.org/forums/index.php?topic=11540.msg77484#msg7748

So I remain with my question: is there a resistor or other component inside the instrument cluster that would affect fuel gauge reading?

Enclosing two more pictures I found on the Forum and ebay. I see connecting plates to fuel gauge, but do not know if there is any component between them and wires coming in.

Again, I see resistor on the wiring diagram. Buy I also see a resistor inside the gauge assembly - I do not know if this is the same resistor as on the diagram or there is another one in the cluster.

[specracer]

Subscribing here. Ive replaced both the sending unit, and gauge, and still have erratic results (gauge jumps all over the place). I would take the gauge I took out, and wire it up in the trunk, and start the diagnosis from there, if your able to confirm that it can be done. Ive lived with this for 2 years, as after replacing everything, and it still didnt work, I had to step away....

[lpeterssen]

Dear Pawel

These are the challenges I like to solve.

There is always a work around to fix these but implies been analytical.

The wiring from the trunk to the instrument cluster is indeed a resistance, as you know electricity drops voltage as it travels trough cables.

In the case of a fuel sender unit and the fuel gauge at the instrument cluster, the thing works like these.

We have a positive T15 feed directly to the gauge.  And on the other hand we have a negative variable signal coming from the fuel sender. The weaker the negative signal is the lower the fuel gauge will mark on the cluster.

There are two values of resistance given by your fuel sender that have to be noted. One with the tank full measured in OHM and another with tank empty also in OHM. The value for a full tank of the resistance should be lower than the value for empty.

Years pass over our wiring harnesses and values vary a lot since many connections are corroded or loose making the resistance values higher than original.

The first approach will be to polish and sand each of the individual connectors involved in the path of ground travel from the fuel sender to the fuel gauge.

Will come soon with another approach. Let me study some before.....

Best regards
Lpeterssen

[ejboyd5]

Take the sender out of the tank, plug it into the connector to the gauge and manually move the float up and down to see how the needle responds. This simple test will demonstrate if the electricals are working properly or if the problem lies with a malfunctioning float.

[Pawel66]

Thank you for your kind attention!

Lpetersen - the topic of increased resistance crossed my mind, but I would have thought that the resistance of wiring between where sender is and the instrument cluster is negligible unless there is some resistor in the instrument cluster, other than the one in gauge assembly itself.

The history is as follows:
1. Some years ago I had the old type of float and gauge - they were working properly and accurately.
2. I wanted to have a system that 280SL had - tube type sender and corresponding gauge.
3. I bought a new sender at MB and installed it.
4. I bought a second hand from a vendor who promised it was from W113.

But I do not think it was a W113 gauge - it showed 1/4 of tank less relatively consistently from full to empty. If filled full, gauge showed 3/4, etc.

I bought W113 late gauge form SLS. Before diving under dash I wanted to check it. I hooked + from fuse box, signal form corresponding pin in the sender and left ground connected to the sender from the harness (brown wire). The tank is 1/3 full. But the hand pegs at 4/4. I see 2 possibilities:
1. Something is wrong with the gauge.
2. The gauge has to be installed in the cluster to show proper value - in this case there needs to be an additional resistor somewhere. This is where I need helping answer, unless I am doing something wrong that I am not aware of.

I did not measure resistance - from what I remember, for the new type of sender it is between 85 Ohm for full and 4 Ohm for empty. I can check it, but I do  not think this would be an issue...

[lpeterssen]

Ok did some research and read some interesting post on these same issue like:

https://www.sl113.org/forums/index.php?topic=4153.0

Apparently for MB W113 250Sl and 280sl the resistance readings at the fuel gauge are as stated on my previous post.

Full = very low resistance close to 5 ohm
1/2 tank is about 100 ohm
1/4 close to 180 ohm.

To check this issue on your particular car you have to do as mentioned on ejboyd5 today.

So my work around would be to run a new ground line between fuel sender unit post where cable BLUE/black is attached and put in that same line a potentiometer in the range 0-200 ohm.  This ground line will run between fuel sender unit to instrument cluster fuel gauge connection at the 12 pin connector below the steering wheel (cable BLUE/BLACK) The objective of this line will be to drop the total resistance read by the fuel gauge by having a resistance controlable line. 

1. So you take out your fuel sending unit and move the float to the full position.   
2. You observe the behavior of your fuel gauge, which I will assume is not marking full as desired.
3. The separated ground line with inline potentiometer installed in the range 0 to 200 ohm will allow us to compensate all the voltage loses on the original circuitry.
4. We place first the potentiometer at the maximum resistance position.
5. We reduce gradually the resistance on the parallel ground line by means of turning the knob on the potentiometer in the direction to which it gives the lower resistance value.
6. We keep turning slowly it and as soon as our fuel gauge indicates fuel there is the sweet point.

We test again in that set value moving the float to the 1/2 level position and see again how our gauge behaves.  Make the necessary adjustments on the potentiometer to have the most accurate measurement possible.

Enjoy your experiment.

[lpeterssen]

Dear Pawel

I think that your problem and mine, because it has happened to me also on one of my mercedes is that fuel gauge never marks full.  So there is much resistance in the path of electricity from the tank fuel sender to the fuel gauge.

In order to fix this we need to find a way to make the gauge read lower resistance values.

One way to check if our gauge is OK will be first to take the gauge out (the full instrument cluster) one on a bench see how it behaves connecting its feed wires directly to a 12v dc source.  The cables are these

12v (+) source is BLACK/blue stripe
12v (-) is BLUE/black stripe.

If we connect the gauge directly to a battery 12.6 volts it should mark FULL.

THEN we on the negative side install a cable with and embedded potentiometer (that is like a volume adjusting knob) in the range 0 to 200 ohm.

We move the knob and see how the gauge moves.

In theory with very low resistance values, let’s say 5 ohm, gauge should mark FULL.

With 100 ohm should mark 1/2 tank and with 180 ohm 1/4 or less.

Let’s do the experiment and then decide how to solve the puzzle.

Best regards
L.peterssen

[Pawel66]

Ok did some research and read some interesting post on these same issue like:

https://www.sl113.org/forums/index.php?topic=4153.0

Apparently for MB W113 250Sl and 280sl the resistance readings at the fuel gauge are as stated on my previous post.

Full = very low resistance close to 5 ohm
1/2 tank is about 100 ohm
1/4 close to 180 ohm.

To check this issue on your particular car you have to do as mentioned on ejboyd5 today.

So my work around would be to run a new ground line between fuel sender unit post where cable BLUE/black is attached and put in that same line a potentiometer in the range 0-200 ohm.  This ground line will run between fuel sender unit to instrument cluster fuel gauge connection at the 12 pin connector below the steering wheel (cable BLUE/BLACK) The objective of this line will be to drop the total resistance read by the fuel gauge by having a resistance controlable line. 

1. So you take out your fuel sending unit and move the float to the full position.   
2. You observe the behavior of your fuel gauge, which I will assume is not marking full as desired.
3. The separated ground line with inline potentiometer installed in the range 0 to 200 ohm will allow us to compensate all the voltage loses on the original circuitry.
4. We place first the potentiometer at the maximum resistance position.
5. We reduce gradually the resistance on the parallel ground line by means of turning the knob on the potentiometer in the direction to which it gives the lower resistance value.
6. We keep turning slowly it and as soon as our fuel gauge indicates fuel there is the sweet point.

We test again in that set value moving the float to the 1/2 level position and see again how our gauge behaves.  Make the necessary adjustments on the potentiometer to have the most accurate measurement possible.

Enjoy your experiment.

Thank you!

But I am afraid this is not the issue I am chasing now...

I saw this post when I was searching the Forum earlier.

I have the Ohm values, thank you, 5, 100, 180 - I can do the check.

I can also check if there is "unwanted" resistance somewhere in the car, simply by measuring it at the pin on the handshake connector.

My question remains: is there an additional resistor in the instrument cluster for fuel gauge? Or the one on the wiring diagram is the one inside the gauge. In other words - can I test the gauge just by connecitng it to the sender in the trunk?

[lpeterssen]

You can test directly to the fuel sender but you will loose the resistance caused by electricity path trough the long wiring harness.  Is not so neglectable since wire gauge is very very thin.

By doing this test directly you will have higher gauge readings since resistance has dropped.

[Pawel66]

Ok did some research and read some interesting post on these same issue like:

https://www.sl113.org/forums/index.php?topic=4153.0

Apparently for MB W113 250Sl and 280sl the resistance readings at the fuel gauge are as stated on my previous post.

Full = very low resistance close to 5 ohm
1/2 tank is about 100 ohm
1/4 close to 180 ohm.

To check this issue on your particular car you have to do as mentioned on ejboyd5 today.

So my work around would be to run a new ground line between fuel sender unit post where cable BLUE/black is attached and put in that same line a potentiometer in the range 0-200 ohm.  This ground line will run between fuel sender unit to instrument cluster fuel gauge connection at the 12 pin connector below the steering wheel (cable BLUE/BLACK) The objective of this line will be to drop the total resistance read by the fuel gauge by having a resistance controlable line. 

1. So you take out your fuel sending unit and move the float to the full position.   
2. You observe the behavior of your fuel gauge, which I will assume is not marking full as desired.
3. The separated ground line with inline potentiometer installed in the range 0 to 200 ohm will allow us to compensate all the voltage loses on the original circuitry.
4. We place first the potentiometer at the maximum resistance position.
5. We reduce gradually the resistance on the parallel ground line by means of turning the knob on the potentiometer in the direction to which it gives the lower resistance value.
6. We keep turning slowly it and as soon as our fuel gauge indicates fuel there is the sweet point.

We test again in that set value moving the float to the 1/2 level position and see again how our gauge behaves.  Make the necessary adjustments on the potentiometer to have the most accurate measurement possible.

Enjoy your experiment.

Yes, the potentiometer would then act as the fuel sender.

Since I do not have any potentiometer at hand, I will check what resistance fuel sender has now. I may also have a handful of resistors, I would need to check their resistance, but I am afraid they are higher value.

Regardless of that: when I connect the gauge that I have from the box form SLS - it reads full whenhooked on the bench (in the trunk) with sender in series between ground and this gauge.

Again, unless there is a resistor in the instrument cluster in that circuit, I think this new gauge is broken.

What you are saying is suggesting there is no such resistor in the instrument cluster and the one showed on wiring diagram is the one inside the gauge.

[Pawel66]

You can test directly to the fuel sender but you will loose the resistance caused by electricity path trough the long wiring harness.  Is not so neglectable since wire gauge is very very thin.

By doing this test directly you will have higher gauge readings since resistance has dropped.

Aha! So you are saying the resistance of the wiring between trunk and instrument cluster is not neglectable? if I have roughly 1/3 or 1/4 of fuel in the tank, the sender resistance should be, say 130 Ohm - that is what is already there. Gauge is reading full. So it is either wrong or it is missing this, say, 125 Ohms. There would have to be 125 Ohm in the wiring - does this make sense? So much? Unless there is some additional resistor there...

[Pawel66]

Let me show perhaps what I am looking for - please take a look at the picture.

Are there two resistors - one in the gauge, one in the instrument cluster?

Or the resistor on the diagram is the one in the gauge?

[lpeterssen]

The resistor on the gauge is the one reflected by the diagram.

So it seems to me that there are to possible solutions......

Remove the resistor on the gauge itself and have there a variable adjustable resistance (potentiometer), or as you say when you connect the gauge directly to fuel sender readings are correct..... so that means that resistance on the wiring is important and is causing the error on the gauge reading.

So a solution  is to run a separated line to run in parallel with actual cable blue/black and have a variable resistance there. The parallel cable should be of a wider cross section let’s say 1.5 mm2 which equals to AWG16.  Remember cable blue/black is 0.75mm2

Un so weiter.....

[lpeterssen]

Dear Pawel

You said:

“
I bought W113 late gauge form SLS. Before diving under dash I wanted to check it. I hooked + from fuse box, signal form corresponding pin in the sender and left ground connected to the sender from the harness (brown wire). The tank is 1/3 full. But the hand pegs at 4/4. I see 2 possibilities:
1. Something is wrong with the gauge.”

The ground side of the fuel gauge sender should be connected to BLUE/black that is the ground coming from the fuel sender float.

If you connect the fuel gauge directly to BROWN GROUND, it will receive a full signal ground and that is why is showing 4/4 full instead of 1/3.

The fuel gauge gets full 12v on the positive post (+)from cable BLACK/blue, and the negative signal is taken from the BLUE/black stripe cable coming from float (fuel sender unit)

[lpeterssen]

Correction on my previous post.

The resistance indicated on your photo on the fuel gauge is ACCORDING to electrical diagram on the POSITIVE SIDE OF THE GAUGE on the black/blue stripe wire.  Is not on the negative side of the gauge coming from the sender.  This makes me conclude that fuel sender needs less than 12v to operate.

So all the comments related to running a separated negative with a variable resistor (potentiometer in range 0-200 ohm) line in parallel to Blue/black stripe cable coming from fuel sender can be maintained.

[Pawel66]

Dear Pawel

You said:

“
I bought W113 late gauge form SLS. Before diving under dash I wanted to check it. I hooked + from fuse box, signal form corresponding pin in the sender and left ground connected to the sender from the harness (brown wire). The tank is 1/3 full. But the hand pegs at 4/4. I see 2 possibilities:
1. Something is wrong with the gauge.”

The ground side of the fuel gauge sender should be connected to BLUE/black that is the ground coming from the fuel sender float.

If you connect the fuel gauge directly to BROWN GROUND, it will receive a full signal ground and that is why is showing 4/4 full instead of 1/3.

The fuel gauge gets full 12v on the positive post (+)from cable BLACK/blue, and the negative signal is taken from the BLUE/black stripe cable coming from float (fuel sender unit)

I think I had it correct:

1. I took the gauge out of the box and took it to the trunk. It had two terminals:
- positive, goes through the resistor to the solenoid inside the gauge
- signal, goes directly to solenoid inside the gauge

2. I removed the plug from sender and wires from the plug; put blue/green (reserve light) and blue/black (signal to gauge in cluster) aside

3. Connected brown (ground) to its pin on sender

4. Took a length of wire and connected signal pin from sender to signal terminal in the gauge (so this is through resistance wire in the sender)

5. Took a length of wire and connected + from fuse box to positive terminal in the gauge

6. (Modified 12.12.2020) Gauge frame was grounded.

The gauge showed full despite 1/3 of fuel in the tank.

I did not connect ground to gauge - I did to pin on sender, then from another pin on sender I took the signal to the gauge.

If there is no other resistor in the cluster and if the resistance between brown and black/blue pins on sender is around 80-90 Ohm, this means the new gauge is shot.

Am I correct?

P.S. I have replaced the attached drawing with the corrected one - with ground for gauge marked (12.12.2020).

[lpeterssen]

Pawel you are right on the connections shown on the scheme.   Either your sender unit is not giving the right resistance values to the gauge or the gauge is shot as you say.

There is still one experiment to do.

Work only with the gauge to find out which resistance values make it work correctly.  Work on a bench, have a tester on hand which can measure resistance, and have a potentiometer to allow you to change resistance values.

Remember that one of the post mentioned on this forum topic said that correct values were:

Full tank = resistance close to zero, around 5 ohm
Medium tank= resistance around 100 ohm
Empty = around 180 ohm.

Enjoy your test and share the conclusions with us.

 Event regards
Lpeterssen

[Pawel66]

Yes, before I read your post, I undertook some actions!

I bought potentiometer 200Ohm. I also bought a handful of resistors 10, 20, 50, 100 and 150 Ohms.

Maybe I can calibrate this thing to work. I will do it on the bench before removing/checking the sender. It is potentially about increasing resistance, maybe it will work.

Thank you for your patience, again, and kind attention!

[Pawel66]

Now I am thinking about the gauge I have in the car currently.

It shows roughly 1/4 less than actual. Resistance is too high by 25%.

Say: for empty it gets 180 Ohm instead of 135. So: what parallel resistor should I add between ground and signal pins to get it to 135 Ohms?

If 1/R1+1/R2 = 1/Rt (where Rt is total resistance of the parallel resistors), we would have:

1/180 + 1/Rx = 1/135

Rx = 1kOhm roughly

For half full:

1/100 + 1/1000 = 1/Rt

Rt = 91 Ohm

For full

1/5 + 1/1000 = 1/Rt

Rt = 5

Does this make any sense? I do not think so. The parallel resistor would have to be variable. For the full tank it becomes negligible, the result is still 5 Ohms that do not work... it would help when empty, half helped with half of tank, no help when full. Would show you true level when close to empty.

If you start form full:

1/5 + 1/x = 1/3.75

x = 15 Ohm

For empty:

1/180 + 1/15 = 1/Rt

Rt = 14 Ohm

Nonsense...

[scoot]

I assume you have tested and eliminated the fuel sender as the problem.  If not, if you have an Ohm meter, here's a little diagram I made many years ago when I was cleaning and selling fuel senders....

[WRe]

Hi,
my 2 cents of knowledge:
- fuel sender and fuel gauge must fit together otherwise fuel showing is wrong
- fuel sender sends resistor values to the gauge depending on a floater and definite resistor wires in the tube
https://www.sl113.org/forums/index.php?topic=31224.msg226551#msg226551
- resistor at the fuel gauge reduces motion of the needle (attenuation)
...WRe

[Pawel66]

Thank you for inputs!

Scoot, the values, e.g. .069 are Ohms?

I thought it was ca 5Ohms in up position and ca 180 Ohms i n down position?

[WRe]

Hi,
did you recognize the VDO specs?
...WRe

[lpeterssen]

Dear Pawel

Repeated your math with a different approach a came to the same conclusion, we need to reduce resistance on the parallel line by 1000 ohm.  So negative resistance.

So I think that best approach is to make actual reading on the field and see which values are getting to the fuel gauge from the fuel sender.  This air assumptions of values is bringing us nowhere.

We need to establish first which resistance values make the fuel gauge you have on hands mark full tank, half tank and empty tank.

Then we need to read which resistance values are the output of your actual fuel sender in the same three positions full tank, medium tank and empty.  From that real data we can start and think if there is a feasible solution or not.

Best regards
L.peterssen