Main.TrailIndexPage | Electrical Systems | Dashboard Instruments | Fuel Indicator

Fuel Indicator or Gauge

This component is part of Dashboard Instruments. It is closely linked to the topic of Fuel.

Definition

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  • part # - start year & end year
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Function

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Maintenance

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  • Symptoms when it faults
  • How to test if it is faulty - what tools to use
  • How to fix / change

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Do any here have experience with the fuel indicator, mine is stuck at the bottom and it doesn't move. I tried to look at the sensor at the back but I did't know how to take it off to clean it. Can I take the sensor off without dismantling the whole fuel deposit? How? when I unscrewed it I wasn't able to pull it up, is any specific tool necessary?

Hans Strom says: the fuel level sensor can be removed out of the fuel tank for cleaning and repairs. Loosen all the small nuts, and pull the unit up. Replace the cork seal when you re-assemble. I enclose some lines by George Murphy with some comments of mine included into it: "in the W113 the float in the tank operates both the needle and the "near empty" light.

The red reserve light in the dash is turned on by mechanical contacts when the float reaches the bottom of the sending unit. The fuel sender unit is designed as an aluminium tube, inside of it a float rides along a metal rod/shaft.

Along this float are also two small resistance wires (actually one wire which forms a "U" in the sending unit) that contact the float. As the float descends, the resistance wire path gets longer and the resistance increases. (Resistance is ~83 W at bottom and 4 W at the top.) This is the transducer that sends the signal to the fuel gauge in the instrument panel. When the float touches the bottom, there is a metal contact that connects two wires together.

The other wire goes to the low fuel light; when it's connected to ground, the light comes on. The rod that the float runs along provides this ground. The fuel gauge sending unit in the tank sometimes gets gunked up in the bottom due to algae or dirt. This can cause both your gauge to read incorrectly and prevent the reserve light from coming on.

Clean it, very carefully. NOTE! Disconnect The Ground Cable From The Battery! A spark around the fuel level sensor opening cold cause a nasty surprise...! Once you get the fuel sender unit loosened - Don't Pull It Out Of The Tank Too Fast! It will be filled to some level with fuel that will squirt out the tiny holes in the bottom. Hold the unit above the hole in the fuel tank until it is drained, then using a rag to catch drips, remove it to your work bench.

Carefully unscrew the round plastic "nut" on the bottom of the unit and remove the nut. Slide off the alloy sleeve, being very careful not to damage the two very fine wires along which the float slides. Clean all parts very carefully, then re-assemble and reinstall.

Tiny plastic "maze" at bottom is there in order to dampen fuel flow (prevent indicator from blinking). If one of the wires is broken near the bottom, re-solder it to its lug - you may have to adjust the bottom fitting upward so the wire can be re-soldered. Installation is the reverse of removal, as they say...

Use fuel-proof RTV Silicon to seal the little round nut so it won't come off. You might want to check the actuation of the little red "reserve" light on your dash, via the electrical plug. Connect the battery, turn on the key and jumper the pins on the connector at the sender unit, to see if the red light by the fuel level needle works.

Early 230 (now I checked it: up to VIN 008953) have a different fuel indicator (111 542 11 04) instead of the later one with the aluminium sleeve (111 542 12 04). The latter one seems to be more reliable. The early one is completely different in that it has an approx. 10 cm long wire arm with a buoy that swims on top of the fuel surface.

You can take it out from the tank in the same way as the newer one, only make sure to be carefully enough not to damage anything. Didn't repair my one so far (was not necessary yet) but to me it seems to be easier than for the later type. Both types are still factory new available with the earlier type being slightly more expensive.

It could be that the tank indicator unit is working but the fuel indicator/sensor in the instrument cluster is nor working. Of course you can disassemble this unit and take the instrument out to check it or to replace it.

Unfortunately only the newer instrument (from VIN 008954 on) which goes with the later fuel indicator is still factory available but not the earlier type. You should consider this if you want to exchange your tank unit.

I've owned two '66 230SLs, and both of them suffered from significant fuel gauge needle fluctuation, often on the order of 1/4 of a tank. It seems to me that the upper point of the fluctuation is the correct one; so, it seems to jump down, then back up. Since both of my cars have done this, I assume it's a common problem?

I've wondered whether it was more likely to be the sender, the wiring or the gauge. BTW, I've run the tank down to the "red light" several times, run for a while, and still seemed (judging from the amount of gas I put in afterwards) to have several gallons of fuel left. This brings up the following question: do the tanks seem to "accept" their full stated capacity (before gas starts pouring out of the filler pipe or the filler nozzle shuts off), and how much of their capacity can they pump out?

There are small plastic floats inside the fuel sending units. Over time they start to leak air out and fuel goes in. When they are half full they are slower to slide up and down the small contact wires, but when they are empty they bounce on top of the fuel surface.

Picture a big boat in stormy waters, now picture a half full one versus an empty one, which one bounces more? Same thing with the fuel sending float. Also if the float is half full it will ride lower in the fuel and give a lower reading on the indicator. Or if it is totally empty it will give a higher reading like in your case. Perhaps if you open up the sending unit you can find a problem within.

My fuel gauge stopped working about a year ago. Does any one have any suggestions as to what I could do to diagnose and fix the problem?

You will have to pull a Sherlock Holmes thing on your car. First you need to find out if it is electrical or mechanical. Electrical by checking the wiring to and from the tank sending unit and the gas guage in the dashboard. Mechanical by checking the movement of the float on the tank sending unit and the needle on gas guage, make sure both move freely.

Most likely it will be a combination, you will need to remove your sending unit in the tank which is underneath the big opaque plastic cover under the trunk floor mat. Remove wire plug and then the 6 or 8 small nuts securing the sending unit to the tank and pull out the 'long cylinder'. On the bottom is a single small nut with a black plastic washer, remove and carefully slip the aluminum cylinder.

Inside you will find a plastic float that runs up and down a shaft and two thin brass or copper wires with contacts on them. One of these might be broken, and you can resolder a new length of wire. Mind you there is gasoline on these parts and you need to use a brake cleaner of sorts to clean out all the gunk and gasoline, let it dry and then you can do your soldering work.


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