1967 200 W110 Fuel Gauge Doesn't Work - PeachParts Mercedes-Benz Forum

mr110 · #1 10-08-2014, 12:38 PM

Hello All,

the fuel gauge in my 1967 200 W110 isnt working at all. I asked the previous owner if he had ever had it taken apart or looked at and he said he had but they never fixed it (no idea why). I don't really enjoy driving around using only the trip counter to plot my distance between full and empty. Does anyone know where i should even start looking to try and diagnose this fuel gauge problem?

ScooterABC · #2 10-08-2014, 01:11 PM

I would start with testing the fuel sender.

mr110 · #3 10-08-2014, 01:27 PM

Quote:

Originally Posted by ScooterABC

I would start with testing the fuel sender.

How would i go about doing this?

ScooterABC · #4 10-08-2014, 01:39 PM

Sure. First of all, it is highly unlikely that the gauge is the problem. Fuel sender failures are common.

1) What does the gauge read when you turn on the car? nothing? no movement at all? no low fuel light?

2) In the trunk of your car in the center of the trunk floor there will be a rubber/plastic round disk about 4" in diameter. You need to remove that. Depending on how pliable the disk is it is easy or use a screwdriver. If you mess it up don't worry about it, not that difficult to get a replacement. Besides, it has to come off. Under that you will see a cable with a square plug that is plugged into the top of the fuel sender. The fuel sender is a round cylindrical thing that goes into the tank from the top and has 3 or 4 pins that the plug sits on top of.

3) Is this what you see, or do you not have a plug going to the sender? If you don't have a plug going to the sender then you have to figure out why. Most likely it will be present and plugged in.

4) remove the plug. This is easy to difficult depending on how firmly it is attached. Try rocking it back and forth with your fingers. It needs to go straight up. If necessary, pry a little on each side at the base with a screwdriver.

5) Got the plug removed?

OK, at this point you can do sort of a half-assed test with a piece of wire. take a small piece of stripped wire and shove it into any two of the holes in the plug. Get in the car and turn the ignition to on. Did the gauge do anything? Repeat with every combination of holes and the wire. You should end up with a combination that pegs the needle at full and (if a W110 has a low fuel warning light) a combination that causes the low fuel warning light to come on.

6) Results? If you got action on the gauge then the sender has failed.

7) If the sender has failed - there are two types of senders, a float that looks like a toilet bowl float used on early cars and a cylindrical sender. I don't think a car as late as yours would have the kind that looks like a toilet bowl float. And if it does, they are pretty much impossible to repair anyway.

I'll stop here and wait to hear back before writing about removing, repairing, replacing the fuel sender.

Idle · #5 10-08-2014, 01:49 PM

First locate the fuel sender in the gas tank. Just look on-line for instructions on where to locate it.

Then test it. Again, look on-line for instructions on this.

The sending unit is removed from the top of the gas tank and is a tube looking device. These get gunked up and can be taken apart and cleaned but be very careful when doing this. The wires inside are delicate.

There are really only three things that can go wrong: The sending unit, the gauge itself, and the wires leading to and from these things. You will have to test all of them and the easiest place to start is the fuel sending unit.

The instrument pod of the 110 is a real job to remove. There is bound to be a write-up somewhere on how to do this, but it takes an hour or so.

If you need a new fuel sending unit you can buy them from Pelican. You can also pick up one on Ebay but you would need to insist on a money back listing before you buy one from there. These are actually easy to repair if you know what you are doing. They can self destruct if you don't so go slowly.

ScooterABC · #6 10-08-2014, 02:38 PM

I disagree. There is no risk hazard for him to short the pins in the plug to see if the gauge responds, where as if he removes the sender he is exposing himself to gas fumes which creates some hazard and he will also need a new sender gasket if it isn't the sender or is the sender. I think he should verify that it is the sender as I described before removing the sender. Just my 2 cents. Safety first.

Idle · #7 10-08-2014, 03:59 PM

Quote:

Originally Posted by ScooterABC

I disagree. There is no risk hazard for him to short the pins in the plug to see if the gauge responds, where as if he removes the sender he is exposing himself to gas fumes which creates some hazard and he will also need a new sender gasket if it isn't the sender or is the sender. I think he should verify that it is the sender as I described before removing the sender. Just my 2 cents. Safety first.

Nothing wrong with doing it like that. I just like to rebuild everything in the area as long as I am in there. If you are in there deep enough to test the sender then it is a simple job to pull it and clean it while you are at it.

The best part about this problem is that it is a hard fail. I start to work on things when they first begin to give trouble and that means fully rebuilding each item along the way. I first figured out these fuel sending units when my Porsche sender died about 30 years ago, and once you have cracked the code on rebuilding them it is a pretty simple process.

But with it being a hard fail it is a pretty sure bet these is one problem. Just finding it is the hard part; repairing it will likely be quite easy.

But as long as you are in there... No harm in doing a routine cleaning of everything along the way.

Idle · #8 10-08-2014, 04:04 PM

A word on the float type of sender....

They can be repaired but it takes a special type of repair that is beyond the ability of most home mechanics. There is a grid of printed metal or wound wires that will have been worn down and this will have to be refinished to, as the manuals say, 'attain the proper degree of brilliance'. That's a fancy way of saying all the wires need to work.

If you run into this type the break in the wires will be difficult to see with your nude eyeball so get a magnifying glass and take a look. You will see the worn areas.

After that you will just have to start searching for someone who can rebuild it.

scotty b · #9 10-09-2014, 08:59 PM

How timely a post. I'm having a similar issue. My 74 280c had a bad sending unit. I just bought a brand new, n.o.s. unit and installed it NO difference. Plugged in, the needle pegs full, unplugged it drops to empty. Low level light seems to be functioning just fine as it came on today, put 5 gallons in, and it went right off. Is there any kind of resistor IN the gauge cluster for the fuel ? Oil pressure is also pegged at 45 when running, idle and at speed, however that sender has not been checked.

ScooterABC · #10 10-09-2014, 10:36 PM

Quote:

Originally Posted by scotty b

How timely a post. I'm having a similar issue. My 74 280c had a bad sending unit. I just bought a brand new, n.o.s. unit and installed it NO difference. Plugged in, the needle pegs full, unplugged it drops to empty. Low level light seems to be functioning just fine as it came on today, put 5 gallons in, and it went right off. Is there any kind of resistor IN the gauge cluster for the fuel ?

Hi Scotty B. Scotty A. here. I have some questions:

1) Is this a change from past behavior or has the gauge always worked this way?

2) This sounds very much like a wiring problem to me, particularly if this has always behaved this way. Forgive what might sound like a stupid question, but is there any way you could have the plug plugged into the sender upside-down?

See attached diagram. If you were to use an Ohm meter to test the SENDER removed from the car, this represents the readings you should see on your meter in ohms. The column that says Position - up means you are holding the sender right side up. Down means you are holding the sender upside down. The un-numbered pin shown in the diagram is a plastic alignment pin on the fuel sender that corresponds to a notch on the plug.

Quote:

Originally Posted by scotty b

Oil pressure is also pegged at 45 when running, idle and at speed, however that sender has not been checked.

OK, what does oil pressure read when you turn switch to ON but _not_ start the car? If pegged, oil pressure sender is wonky. If oil pressure rises when you start the car, then I'm again suspicious of wiring. (Does a 114 of this age have mechanical or electronic oil pressure??? Sounds electronic but I would have guessed mechanical).

Is _THIS_ a new thing also?

I'll wait to hear if these are new developments or always-been-this-way...

Scotty A.
Ack! It won't let me attach the diagram because I have previously attached it in another thread. See here:
http://www.peachparts.com/shopforum/vintage-mercedes/351194-fuel-gauge-vs-fuel-sending-unit.html

scotty b · #11 10-10-2014, 06:57 AM

I've only had the car up and running recently, but since I got it running both of these problems have been present. Prior to replacing the old sending unit ( fuel ) the gauge did the same thing as if the sender was grounding out internally. when I removed it and opened it up I found the main wire stretched out and flopping about so I ordered a new unit. Installed it and have the same issue

the oil pressure is dead flat with the key on, as soon as the car is running it goes full bore to 45. I'm fairly certain that sender is bad, it just seems curious that both gauges are doing the same thing and the fuel sender is brand new N.O.S. so the assumption is that it is not an issue. Unless someone in the past changed the wires inside the plug on the fuel sender there is no way to plug it in wrong.

I'm assuming at this point the gauge has a short to ground, or there is an issue in the wiring between the sender and the gauge, but am hoping someone knows of another ( easier to diagnose ) problem

EDIT As of this morning the oil pressure seems to be doing o.k. 25 at idle, as soon as you accelerate it goes to 45 then back down at idle. The car sat for quite a few years and yesterday was the first time I've actually driven it on the road so there may have been sludge in the line that freed up.

Today on start up the fuel gauge was not working at all, I let it sit idling and it stayed on empty. Then I slapped the top of the dash and it jumped right to full. Seems more likely now, that my issue is in the gauge or the wiring in the dash. Oh joy ......

ScooterABC · #12 10-10-2014, 09:55 AM

Quote:

Originally Posted by scotty b

I've only had the car up and running recently, but since I got it running both of these problems have been present. Prior to replacing the old sending unit ( fuel ) the gauge did the same thing as if the sender was grounding out internally. when I removed it and opened it up I found the main wire stretched out and flopping about so I ordered a new unit. Installed it and have the same issue

OK, so you know the old sender was bad because you opened it and you had a wire flopping around. I would remove the new sender and test it with an ohm meter as per the diagram referenced in previous post. Don't be insulted, but is there any way you could have ordered the wrong sender? VDO makes looks of senders that look the same, they are not wired the same.

Quote:

Originally Posted by scotty b

I'm assuming at this point the gauge has a short to ground, or there is an issue in the wiring between the sender and the gauge, but am hoping someone knows of another ( easier to diagnose ) problem

Test the sender first. Actually, for your symptom you don't even have to remove it from the tank (assuming the tank isn't full). Just test the two terminals on the sender (with plug removed) that show the empty-full and see that it is working. It probably isn't the sender but that is the easiest fix.

EDIT

Quote:

Originally Posted by scotty b

As of this morning the oil pressure seems to be doing o.k. 25 at idle, as soon as you accelerate it goes to 45 then back down at idle.

OK, so oil pressure gauge is a non-issue. Good.

EDIT

Quote:

Originally Posted by scotty b

Today on start up the fuel gauge was not working at all, I let it sit idling and it stayed on empty. Then I slapped the top of the dash and it jumped right to full. Seems more likely now, that my issue is in the gauge or the wiring in the dash. Oh joy ......

Or the wiring in the plug. You may need to pull the cluster, which is fairly easy in a W114. It is held in by a rubber friction band that goes around the cluster. Nothing more. If you push the cluster from the back and gently pull from the front rocking it a bit, it will hopefully come free unless someone glued it in. I would check that the wires to the cluster are all attached firmly. If you go down this route, you may want to find another 114 cluster and swap the electrical connections and sort of half-ass install the spare cluster and see if fuel gauge works.

I also _think_ that there is a ground wire in the trunk associated with the sender. Trace the cable from the sender as far as you can and see if there is a ground wire that isn't attached or is poorly attached to the trunk floor. I may be wrong about this - lately I have been spending time in the trunk of a W111 and I don't remember my former W114 trunk too well. The slapping to get it to read is a red herring. That problem will probably go away with use. some things fix themselves over time.

I think you want to be looking for an area where someone has messed with the car. For instance if the cluster is glued in then I would be suspicious of the cluster. If the plug to the sender looks less than perfect I would be suspicious of that.

I wish I had a better answer, 95% of the time it's the sender.

Tomas7 · #13 10-10-2014, 06:06 PM

On my 76 280C it was the connection at the sender in the trunk. I unplugged and took sandpaper rolled it up so that it would fit in the three female prongs and cleaned and then I cleaned the male prongs as well. I then squeezed the female prongs slightly so that they would form a tighter fit, problem solved.

Maybe???????????

Idle · #14 10-10-2014, 09:22 PM

I just pulled the instrument cluster from a 1974 280 since I needed the speedo. It had a mechanical oil gauge so your gauge might be wearing out and it is time for a replacement. But it has a hose that brings oil to the gauge and it works in a mechanical way. There is nothing electrical about the oil gauge on these cars.

When fully warmed up the gauge should show about 1 bar or less at idle. When under full acceleration it should peg out. When you drop way below 1 bar, which is 14.5 psi, keep an eye on things. You would not want it to go below .5 bar.

The sandpaper trick on these cars is one I have used many times. Some of the electrical connections on Mercedes of this and earlier times have silver plated plugs. This is to keep down corrosion. It works, but they will still tarnish after 40 years of use and need a cleaning. The plating is thick so you don't need to worry about sanding it all off; just don't overdo it.

Baxterdown · #15 10-11-2014, 11:04 AM

Hey guys,

This seems like perfect timing to jump in as I'm also having a similar issue. My '72 280SE 4.5 fuel sender unit isn't working properly. When I bought the car it wasn't working at all, so the PO fixed it (I saw him do it) so that it would show the proper amount of fuel in the tank. That part works ok now (sometimes it jumps a bit, but that doesn't bother me).

My issue is that the low-fuel warning light is on most of the time. Once in a while it turns off, but the vast majority of the time is on regardless of how much fuel there is in the car.

Reading ScooterABC's post, should I assume that its an electrical thing in the plug? Or, could it still be related to the sender unit itself. I looked on Fleabay and the units are EXPENSIVE! ($350).

Any advice is greatly appreciated.

Jose