Hi, I was wondering if there was a common cause for the reserve light on my fuel gauge to turn on and off and just glow yellow sometimes when the fuel needle is on full, 1/2, 3/4.

The needle works great and doesn’t jump around. So I was thinking the sender is ok. The light comes on for a second or two then turns off. Then it is off for a few minutes and turns on a few seconds. All the while the needle is stable.

When I start the car it doesn’t always turn on. I recall it used to turn on when I turned the key but my memory may be wrong.

Just wondering if anyone had any common places to look before I 1) pull and clean the sender, and 2) trace the circuit.

Perhaps there is a relay in there for startup? Or there is electronics? I assumed the light was controlled by a dumb switch in the gauge. Time to whip out my wiring diagrams.

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79 300TD “Old Smokey” AKA “The Mistake” (SOLD)
82 240D stick shift 335k miles (SOLD)
82 300SD 300k miles
85 300D Turbodiesel 170k miles
97 C280 147k miles

Ok, I started using the search feature (actually Google which lead me back to peach) and I think the reserve light is controlled independently on the sender from the needle. That is my light switch may be gunked up while the needle sender is clean. That may explain why there are two wires and wipers on the sender?

Anyhow, sounds like I need to pull my sender. I was just hoping it was something like wiring but in my experience on these cars the wiring is quite stable.

Edit - ok, yeah I recall from my 240 there being a third terminal on the sender for the light. This thread says so.

http://www.peachparts.com/shopforum/diesel-discussion/231269-diy-w-pix-w124-fuel-sender-disassembly-cleaning-testing.html

The way it appears to work is the third terminal is open till you hit reserve then it closes and the light glows. I must have a short in there on the reserve circuit.

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79 300TD “Old Smokey” AKA “The Mistake” (SOLD)
82 240D stick shift 335k miles (SOLD)
82 300SD 300k miles
85 300D Turbodiesel 170k miles
97 C280 147k miles

The reserve light should glow when you turn the key but don't start the engine, just like the rest of the "idiot" lights as part of the bulb-check procedure. The light is turned on in normal use when the float falls far enough to close a switch in the sending unit. I'd be willing to bet either that switch is full of crap keeping it "stuck" or there's a wiring issue somewhere (chaffed, poor connection, broken solder joint on the cluster, etc).

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Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

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1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]

The shorting bar that closes the low fuel light switch at the bottom of the sender is on the float . If the gauge needle is working correctly, the float is not sitting on the bottom, shorting out the low fuel light switch. So it must be either gunk (carbon?) on the low fuel switch, shorting it out, or fault in the bulb test circuit. I figured how the circuit worked at one time but can't remember details. I think there is a resistor and a diode in the circuit. The resistor lights the bulb at 1/2 brightness.

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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 resistor and diode ground the circuit through D+ when cranking. This gives you some back up bootstrap current in case the charging bulb blows, as well as giving you “bulb verification” at start up. Once the alternator is charging, D+ goes high and no longer serves as a ground. If the alternator fails, D+ can become found and light the bulb. But it wouldn’t be the only bulb lit.

The switch is at the bottom of the sender as mentioned. What I would do is identify which of the three sender wires feeds the reserve light, and just disconnect that. If the problem goes away, it’s in the sender, otherwise it’s a shorted wire.

Thanks guys. I’ll start by checking at the connector.

__________________
79 300TD “Old Smokey” AKA “The Mistake” (SOLD)
82 240D stick shift 335k miles (SOLD)
82 300SD 300k miles
85 300D Turbodiesel 170k miles
97 C280 147k miles