Fuel Gauge Connected Directly To Sender

[Merc_Girl]

Hi Bob!
So when plugged in, the gauge showed some fuel, I filled the car up and no fuel showed!!
How dos one remove the cluster, is it a case of simply prising out, or are there some annoying screws behind dash holding this in place?

[BobH]

I've never needed to remove mine, so i'm not sure, here's a picture of the connector, it may be obvious when you see it, whether there's a fixing screw or you just need to pull it apart.  Looks like the connector isn't actually fixed to the cluster, which may make it easier to access

https://www.sl113.org/wiki/Electrical/InstrumentCluster

Here's another picture

https://www.sl113.org/wiki/Electrical/Rheostat

[Jonny B]

Be careful when navigating around the back of the center gauge cluster. The temperature gauge is a sealed unit and rather fragile. The oil gauge is a direct connection to the engine and can give you a nasty leak or worse.

It is not held in placel

Do a search and you should be able to find information about the removal.

[BobH]

I don't think Katie needs to touch the cluster, which is fortunate, the connector is separate, mounted to the chassis somewhere under the dash

[Merc_Girl]

I’m prepping myself to go in!!!😳

If you hear a lot of swearing from where you are, it’s probably me! 🤣

[Merc_Girl]

Does anyone know why you cannot put in the ‘newer style’ fuel sender to replace the ‘float’ version?
I saw the answer somewhere but can’t find now!
Thank you

[kampala]

I can’t answer your question fully — but I recall reading that the fuel sender is matched to the gauge -  the early senders only work with early gauge and later senders only work with later gauge.  I cannot confirm this first hand, just paraphrasing what I recall reading here. 

Also see JA17 response here:  https://www.sl113.org/forums/index.php?topic=17493.msg121156#msg121156

I bet one of our several knowledgeable techies will know ….

[Merc_Girl]

I can’t answer your question fully — but I recall reading that the fuel sender is matched to the gauge -  the early senders only work with early gauge and later senders only work with later gauge.  I cannot confirm this first hand, just paraphrasing what I recall reading here. 

Also see JA17 response here:  https://www.sl113.org/forums/index.php?topic=17493.msg121156#msg121156

I bet one of our several knowledgeable techies will know ….

Thanks Aslam
Yes, I thought I read it somewhere as well? I wonder if it’s something to do with the resistor as I think the string mentions the gauge has a resistor in it and obviously the fuel sender unit has some sort of variable resistor in it?

[lpeterssen]

In my understanding THE RESISTOR which is installed ON THE POSITIVE side of the fuel gauge is intended only to adjust voltage being used by gauge coil that generates the magnetic field needed to move the indicator needle.

The same gauge coil is used on car/trucks running on 24v and 12v.

So, that resistance there, has nothing to do with the variable one that is on the fuel sender negative pole that feeds the other side of the gauge coil.

I have said many times that problem of Katies car is related to the quality of the wiring going to the fuel sender which is prone to internal oxidation. I have seen that many times in all the wiring harnesses I have rebuilt to mint condition so far.

The ground connection at the trunk left fender should be of optimal quality and that is overpassed all the time.

Second, the blue/green and blue/black cables get damaged due to oxidation. Those cables in many mercedes are from copper wires covered with a kind of aluminium alloy.  The fact is that when you sand them to make possible a new solder on the 4 pole connector at the sender, they break easily. Its better to replace them in full with new lines from the 12 pin connector at LHD footwell to the trunk without SPLICES.  Remember that a very low voltage passes through them.

Do not think that harnesses do not AGE. They do and those conductors made with materials that suffer from aging MUST BE REPLACED.

Not all the wires age the same
Way, depends on the kind of insulation used for the outer jacket, and kind of alloy of which the conductors are made.

Cables made of full copper tend to be in excellent shape after 50 years. Others made with different alloys get oxidized increasing the internal resistance, affecting therefore its performance.

Insulation jackets made of cotton/resin which by the time these cars were produced was the best material for heat shielding, has demonstrated that does not age well, and in many ocasions get’s brittle and disintegrates.

Best regards
L.Peterssen

[lpeterssen]

A final way to discard if the fuel gauge is ok will be to connect it directly to the fuel sender

That can be done either by taking out the fuel sender out of car or by bringing the fuel
Gauge to the trunk and building a provisional wiring setup copying that section of the wiring diagram as shown on the technical section of this forum

That will be:

1. Find a 12v positive source. That will go on one end of the fuel gauge terminals
2. Find an excellent ground connection and feeding that to the fuel
Sender 31 terminal
3. Connecting a good quality wire in the position where the blue/black line is at fuel sender and bringing the other end to the still unconnected terminal at fuel gauge

Then move the fuel sender so that the inner float moves and observing the corresponding movement at the fuel gauge

Best regards
Lp

[Merc_Girl]

Thanks LP

I’m sure the harness will need to be done at some point, but presently I have a girlie week away planned so that is not an option. Sounds like a ‘winter job’.

Apparently the more ‘modern’ sender has been installed, but I remember seeing somewhere that these are not interchangeable. Though it did work, to some extent, I had the the fuel gauge refurbed, the system then failed to work.

Logic would dictate, surely, that one or other (sender unit or fuel gauge) are no longer compatible?

Hence my follow up question as to why the two design of sender units are not compatible? 😁

Katie

[BobH]

Hello Katie, there are several posts that confirm that the new style sender is not compatible with the early gauge, the resistance in the later senders is different from the early senders, so i assume the gauge must be calibrated accordingly, hence they need to match.  Perhaps this has been your problem all along, and your wiring may be ok?  No point checking the loom any further until you're confident that the sender and gauge are compatible

[lpeterssen]

Katie

There is always a workaround. Its just a matter of thinking.

The things that we need to know are:

1. Resistance values read at the new fuel sender with float at full position and at empty position
2. Connect the fuel gauge on one side to a pure 12v source
3. The other side of the fuel gauge at a negative source with AN INLINE POTENTIOMETER or variable resistance that will allow us to determine at which resistance value the gauge marks FULL and at which value the Gauge marks EMPTY.

You will read resistance values with a multimeter connected in parallel at the negative side

Once you have those values. Make a new post and notify me by direct message to THINK OVER A POSSIBLE SOLUTION to make those two actually incompatible devices to talk each other in the same language

Best regards
Lp

Sorry for fast typing but internet is terrible now here in my home address

[rwmastel]

That would be a really nice thing to have documented going forward as the older style senders will presumably get harder and harder to source.  Would be a nice product you could market.

[Merc_Girl]

Hi all, so all fixed after a bit of head scratching. So, sequence of events:

a) I complained that Bessy’s fuel gauge was bouncing all over the place, more so than others had discussed
b) the fuel sender unit was replaced, what I hadn’t realised at the time was that the newer design of sender unit had been installed. Weirdly this did work to some extent (be looking back neither me nor the garage can understand how it worked unless some odd thing was going on with the resistor in the fuel gauge).
c) still wasn’t too happy, although working, with the gauge and sporadic fuel levels. As speedo had to come out for a refurb, decided to have fuel gauge refurbed.
d) with refurb, Bessy obviously remembered that the new design of fuel sender unit and fuel gauge were not compatible, but didn’t decided this straight away as it did work for a short time! Typical lady, wanted some time to make a decision!?!?
e) testing then was made, as we know, fuel sender with float worked with refurb fuel gauge. New design fuel sender now not working!
f) garage concerned that if put the ‘float’ style sender in, liklihood fuel gauge would bounce around.
g) solution. Put a 280SL fuel gauge, which looks the same from driver’s view, into Bessy! So now have the more ‘modern’ set up which all seems to be working well 😁

[rwmastel]

You must be very relieved!  I know you like to drive the car, so having a good working gauge is necessary.  Good job working through it to resolution.

[Merc_Girl]

Thanks Rodd

Maybe helpful to others who have the early pagodas who wish for something a little more ‘helpful’ and not too ‘sensitive’ to latter ‘amendments’. 😁

[lpeterssen]

Dear Katie

Happy  to hear that you came to a final solution with the 280SL later style fuel gauge.

Nevertheless I would like to have my hands on an early style fuel gauge and a modern style fuel sender to find a way to make them work together correctly.

If anyone is willing to send me theirs old style fuel gauge to my postal address in Miami, I am willing to solve the puzzle for all.

Best regards
L.Peterssen
Contact me to l.peterssen@me.com

[acbrock]

Hi, new to the forum.  I have searched and dont see any answers for this so if there are already posts.. Sorry!  I will learn to find stuff as I keep working my car.
I have a 1966 230SL.  I have had it for about 20 years.  It has sat that 7 or 8.  The fuel gage is not working.  It worked last time the car was up and running.  It is just in the empty position with no Reserve light on.
I have pulled some of the parts and it seems like power is good from the battery to the Plug on the Fuel sender.  Everything on the dashboard works except the fuel gage.
My next step was to remove the Fuel sending and see if there were any issues I could see.  I took the plug off and remove the 5 or 6 nuts but it won't come out.  I am assuming it is just sealed in there and I just have to dig under the black metal ring and try and release the seal and then slowly pull it out.   I just wanted to check if that I right before I start trying to pry it out, because I dont see any other connectors.

Is this the best next step?  Also since I am going no gage now.. how many approx miles should this car get?  If I fill the tank and zero out the Trip Distance.  I dont plan to push it to any limit but just curious if it was 200 miles?
Thanks
A

[acbrock]

OMG!  When I hit save and went to check my post all the information was right there above my post.  Not sure how I missed it.  My computer is acting funky still asking me to become a full member even though I have already paid.  I think I am just going clear everything and logout and back in and maybe my searches will be better...

[acbrock]

So reading all this new information that popped up I saw someone state that the Gage when full when unplugged from the Fuel Sender.  So I tried that, and it stayed empty but now playing around and really watching, when I start the car or turn it on the gage jumps a tiny bin but when I unplug it from the Sender and turn it on it does not move.  So I am thinking maybe the actual gage is stuck?  Is there anyway for me to send the correct current thru the plug to the gage to see if I can make it move to full?   When I use my multi meter on the actual fuel sender I am getting no readings.  So I am going to maybe get a new multimeter or watch some videos in case I am using it in correctly.  I was putting on check function and was touching the leads to the 3 poles and was getting no readying at all.

[rwmastel]

acbrock,

Welcome to the Group!

I'm not sure you're problem is in line with the rest of this thread.  Might have been better to start your own with an appropriate title.

If writing a really super long post, I sometimes do it in MS Word or something so I can take my time, walk away, come back, do some research, etc., then paste it in the forum.  This site can timeout your post if you're taking a long time.

I hope the Search feature gets more comfortable for you.  We've been online 20 years, so there's a lot you can lfind and learn on your own before even posting.

Good luck with your gage.

[BobH]

Hello acbrock

I think most of what you need to know is in this post, you should also look in the tech manual for fuel gauge and the wiring diagrams for how the circuit works

There is a +ve on one side of the gauge and low fuel lamp, fed from fuse 5 when the ignition is on, so check that this is present.  You can test at the 12 way connector under the dash, the one that feeds the instrument cluster, so you don't need to remove the cluster or the gauge to test. 

A ground/earth is switched from the sender to the gauge the resistance varying dependent on the position of the float, and the ground is also sent to the low fuel lamp when the float is sitting low in the tank.  You should check for a good earth on the connector to the sender, the ground is picked up locally in the trunk/boot, so check for continuity between the ground and the earth pin on the connector, the plug details are on an earlier post on this thread or in the tech manual

There is no ground on the pins of the sender when the connector is unplugged, so don't test the sender itself, but you can test for resistance between the pins, which will prove if the coil of the sender is intact

There's quite a lot to digest, so carry out these steps first before you remove the sender from the tank

You can easily check your multimeter, just touch the two leads together when set to ohms and you should read 000, no resistance, and you can also check the battery voltage when set to DC volts to prove the meter

If you read OL when testing on ohms with the leads shorted, this indicates open circuit, there's a battery and a fuse inside the meter, so check both if it's not working correctly

Please let us know how you get on

[acbrock]

Hey Rod,
Yea my post did not seem to fit perfectly with the current question.  I was going to post a new question but I cannot find anywhere to Post a NEW Topic.  I could only find a way to reply.  The "Post" at the top of my window is not clickable?  I am wondering if that is because it is not recognising me as a full member?  Going to play around on other computers and browsers...

Bob thanks for the info! I am going to get into it today.  I will post what I find.

Thanks
Aaron

[acbrock]

Ok spent some time with the car today… Side note finally got the front right vent out.  That one was killing me.  I know just needed to force it but did not want to break anything.  So a little WD40 on the pins and finally pulled it out with out break a pin or the little hook clamp!  Wahooo!

But back to the Fuel Gage.  So before I pull the Fuel sending I did some MultiMeter tests.

I removed the Cap and connected the wires. 
With the Ignition OFF

Earth Connector to Earth on the rear light = 0
Earth to Blue/Black = 0
Earth to Blue/Green = 0
Blue/Green to Blue Black = 0

With Ignition ON

Earth to Blue/Black = 1
Earth to Blue/Green = 1
BG to BB = 1

With the Plug not connected to the Fuel Sender
Earth to Blue/Black = 0
Earth to Blue/Green = 1
BG to BB = 1

Does this make sense? If not let me know and I will do more research on my electrical skills.  I know little about a lot but not a lot about electrical things...