Hello All

This car indicates very erratic gauge readings---for example with well over a quarter tank the reserve light can come on followed by the needle swinging up to 1/3 full, then dropping almost to total zero.

Should these gauges be precise? Can they easily be adjusted?
Guidance regarding how this sort of problem is best approached will be very much appreciated.

thank you

Ed Cumings

The problem is not with the guage but with the fuel sending unit located in the trunk. I replace mine and it does the same thing. With the tank full the guage is fine. However, as soon as it hits 1/2 way mark then it starts to jump back and forth. Replaced the fuel sending unit it and the guage still bounces around. Tried to take it apart but I can't find any points to open it up. Several people have said it here that they took the fuel sender unit apart and cleaned it and it helped but I decided to live it.

I have exactly the same problem as Kahn.
I have not been able to figure it out.
I also replaced the sending unit, but to no avail.

cumined's problem may be different, however. My low fuel light works perfectly.

Best of luck.

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1986 300E 5-Speed 240k mi.

I seem to have the same problem with my 300SEL. With a full tank, needle is fine. At about 1/2 tank, the needle intermittently drops to empty, sits there for a while and jumps up to the correct reading momentarily, then drops down again. Reserve light functions normally.

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Mike Heath
1988 560SL Black/Palomino
1988 300SEL Black Pearl/Burgandy
1984 500SEC Anthracite Grey/Palomino

Exactly as maheath stated. Very odd behavior...

One would naturally suspect the sending unit right away, so I preemptively replaced it, but that had no affect, and even the old one tested fine with a meter.

I was feeling lonely with this problem, but since it's more widespread than imagined, perhaps someone has the solution.

Anyone? Anyone?

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1986 300E 5-Speed 240k mi.

As a followup, I've been trying to eliminate a rough idle problem in my 300SEL. As part of that, I've run a bottle of Techron Fuel System Cleaner and three bottles of Techron Pro-Guard through my gas tank over the last month or so. Two of the three Pro-Guards were on a mostly empty tank. I almost never see the gauge act up anymore.

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Mike Heath
1988 560SL Black/Palomino
1988 300SEL Black Pearl/Burgandy
1984 500SEC Anthracite Grey/Palomino

This type of behavior in a sending switch is indicative of a poor ground in the electrical circuit.
Try checking all ground connections, batt to frame. sending unit ground.

I'm sure Techron will not help in my case, since I have completely drained the tank (to replace fuel pump), and replaced the sender, and the gauge has never worked right since I have owned the car.

Like Inspector says, smells like it just must be a grounding issue, but I have checked both of those grounds.
Perhaps there is yet another ground that we are all overlooking?
Suggestions welcomed!

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1986 300E 5-Speed 240k mi.

Yeah, mine does the same...I've learned to live with it, it's not a big deal, I don't think

I am now starting to have the erratic fuel guage on my '91 300CE and would like to check the grounds you guys have mentioned. Where are those ground points on my car.
Will also try the PRO GUARD, if the grounds are good.
Thanks

I wonder if there are new additives being blended with gasoline which are reacting with the fuel level reader on these older cars. It seems like "all of a sudden" many are having some problem with the gas guage

I know it sounds weird, but check your battery and alternator output. On my 300E, the first sign of a failing alternator was a jumpy fuel guage. When I replaced the brushes, the problem went away.

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Chuck Taylor
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe