Ye ol' dancing fuel gauge... well, was dancing!

[necromancer]

Hi all,

The fuel gauge on my 1990 300E started acting up this winter... it reads fine until it approaches the 1/2 tank mark then starts oscillating sporatically going from what I would consider the true reading to 1/2 tank again... I initially suspected the fuel level sender but after replacing it and checking the old unit the resistance values seem to be spot on with no hesitation of the shorting bar on the wires...

now with the new sender in the fuel gauge has stripped all pretenses and just stops functioning beyond 1/2 tank. It will reach 1/2 tank then stay there indefinitely... until the reserve light comes on!

I've read some other threads that suggest a fault with a cold solder joint on the actual fuel gauge in the cluster. If I remove the cluster are the solder points in question obvious or will I have to disassemble the gauge cluster to get at them? Can I verify this with a continuity test of the sender leads when the fuel level is below 1/2 tank (and the gauge is presumable not working) or are there parallel circuits that would invalidate this approach? I've also been having problems with my cruise control "cutting out" either during automatic acceleration or de-acceleration... since the cruise control and instrument cluster are on the same 8A fuse (I checked the fuse) could it be a ground loop related to the cruise control cutting out? I think there's a 5 pin connector behind the dash that's known to be faulty in earlier 124s and can cause the cruise control problem.

What's my next step here? (Besides pricing out a reman instrument cluster)?

Thanks,

[neanderthal]

a used one from a dismantler should be your first step

[necromancer]

Originally Posted by neanderthal

a used one from a dismantler should be your first step

Was thinking more along the lines of taking that module out of the instrument cluster and putting the multi-gauge on it to check for breaks in the solder joints.

Does anyone know if there are two separate circuits that detect the fuel level? Seems to me with a resistance range for the fuel level sender of 3 ohms to 87 ohms that's almost 2 orders of 10.

Is there any particular reason, design-wise, that the first 1/2 tank should read fine but the last 1/2 tank results in a dead gauge?

Thanks,

[jmoney6]

I have the same problem under half a tank. I just tell my friends my gas guage dances to the music in the car.

[RedSox14]

Mine boogies too. Moves better than I do. I have not found a solution.

[my260e]

Exact same problem under 1/2 tank. Haven't found a solution yet.