Wait... Our tanks are 17 gallons?? I always thought mine was 13. That's all I've been getting into it from an empty (at the gauge) fill up.


Maybe I need to pull my sender again, too. It's never been fully accurate and the gauge will swing quite a bit (within 1/3 tank's worth) while accelerating/turning etc.

Thanks for this. I’m having a similar problem with my 300sd fuel sender/gauge. Goes empty at 1/4 when I manually slide the float to the bottom with my fingers.

I believe this is high resistance so what I need is more resistance or recalibration of my needle.

It makes a ton of sense the system has regulation. Because if it just ran off 12v the gauge would jump when I start the car and the system starts charging. I bet the gain or reference in that little board have drifted over 35 years.

I’ll look into tearing it down.

__________________
79 300TD “Old Smokey” AKA “The Mistake” (SOLD)
82 240D stick shift 335k miles (SOLD)
82 300SD 300k miles
85 300D Turbodiesel 170k miles

W123 instrument clusters have no voltage regulators! Don't tear it down looking for it. Wheatstone bridge!

617 instrument cluster question

__________________
85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

Thanks. Cool post about Wheatstone bridges.

The car I’m talking about is my 300sd. Perhaps it has different gear in the cluster? The speedo is certainly different.

The car is not terribly electronic. I think the most elaborate circuit is the tach amp which is pretty primitive. I was thinking the image in a previous post showed a pot, an ic and some passive components which I interpreted to be a differencing amp with a trimmable reference. With a good op amp all you need is a zener diode to reference off of.

I feel the itch to remove my cluster and have a look. I need to fix my odometer (again) too. A fuel gauge that hangs at 1/4 combined with a trip odometer that reads 20% low is bad.

I cannot get around the fact that stopping at 1/4 means I need to have more resistance on the sender. The usual problem with gumming is too much resistance or sticking of the float. Both I’ve eliminated.

So I’m probably dealing with current limiting via a corroded power contact or fuse, or component drift from aging. Either way a look is in order. I’ll post any schematics I reverse engineer.

Edit-doh! I read your last post on the Wheatstone bridge thread. If a 380sel 1983 used an inductor and a electromagnetic gauge that’s prolly what I’ll find in my car 1982 300sd.

__________________
79 300TD “Old Smokey” AKA “The Mistake” (SOLD)
82 240D stick shift 335k miles (SOLD)
82 300SD 300k miles
85 300D Turbodiesel 170k miles

The OP has a 240D, a W123. Unless your 82 300SD has an identical fuel system and cluster to W123, maybe it's best you post your findings in another thread so as not to make it more confusing?

__________________
85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

The 240D's came with the 18 gallon tanks, 300's came with a 20 gallon one .

I replaced my '82 240D's tank with the 20 gallon one from a 300CD as I drive it a lot and wanted more range .

Yes, from dry I've put 18 gallons into a 240D tank and 20 + into a 300D .

__________________
-Nate
1982 240D 408,XXX miles
Ignorance is the mother of suspicion and fear is the father

I did then what I knew how to do ~ now that I know better I do better

Okay. No need to start another thread for my early W126. I opened it up and took some photos. My W126 Looks like the same non electronic system in the W123. Just an armature wired with a coil and 68 ohm resistor (which was within spec). No fancy solid state stuff.

Do meter armatures drift? I remember zeroing old school volt meters as a kid with a screwdriver.

Where is my zero offset coming from? As in why is empty = 1/4? Don’t know yet.

I think I’m dealing with the same issue.

Edit - I think I fixed my issue. I posted it here in my "I dunno how to read my fuel gauge" thread where I wondered why my needle would never hit R.
How do I read my fuel gauge?

__________________
79 300TD “Old Smokey” AKA “The Mistake” (SOLD)
82 240D stick shift 335k miles (SOLD)
82 300SD 300k miles
85 300D Turbodiesel 170k miles