Reserve light and adjusting the ohms fuel sending unit? - PeachParts Mercedes-Benz Forum

argus445 · #1 10-05-2007, 07:53 PM

I've looked and searched anyone know how the adjustment procedure for the fuel sending unit and checking/repairing the reserve light system on the sending unit. I took mine apart today had about 2" of muck in bottom of it. Not sure if the reserve light is gonna work or not since it's a 1982 300 D and doesn't do the self lightup i'd like fix it or least know how before i run outa juice..

Ken300D · #2 10-06-2007, 08:02 AM

If you don't get the "self light" test when the ignition is first turned on, then the bulb may be shot.

The reserve light is controlled by simple switch contacts near the bottom. A search here on the forum should give you more information on how to clean them.

My recommendation is to not rely on the reserve light, even when you have it in normal operation. I ran out of fuel one time on a car that reliably lit the reserve light - except for one time.

Ken300D

oldiesel · #3 10-07-2007, 08:30 PM

after you get it cleaned up,connect it to the wires without putting it in the tank,you may have to use a small jumper wire to ground it to the tank.The gauge should read empty and the light should come on. My $.02 Don

Brian Carlton · #4 10-08-2007, 12:40 AM

If you did a good job cleaning the goo inside the sending unit, and don't break any wires on reassembly, the reserve light will probably work for you.

I agree with the test outside the tank. The light should immediately illuminate if you connect power to the sending unit (you'll probably need jumper wires due to the position of the plug).

#5 10-08-2007, 12:47 AM

Quote:

Originally Posted by Ken300D

If you don't get the "self light" test when the ignition is first turned on, then the bulb may be shot.

That's only on the late 82+ models. If he has an early 82 it may not have that feature.

argus445 · #6 10-08-2007, 08:17 AM

Quote:

Originally Posted by ForcedInduction

That's only on the late 82+ models. If he has an early 82 it may not have that feature.

Yes mine is a pre- 8/82 no self test light. Just from memory it had 3 leads i'm assuming one is a hot wire one is a return wire other might be the wire to signal the light? i was curious i haven't found any pictures or info on how it works i looked at it when i had it out other than some dark gunk i didn't see anything wrong with wires inside. I seen someone talking about adjusting a set screw or something to adjust the ohms back to the gauge?? i've did searches haven't found anything

funola · #7 10-08-2007, 09:33 AM

The reserve light self test circuitry is not in the sender. There is no way to adjust the ohms calibration. It is what it is and if it is clean and making good contact and your gauge is ok then it should read correctly as it was designed..

argus445 · #8 10-09-2007, 07:03 AM

Quote:

Originally Posted by funola

The reserve light self test circuitry is not in the sender. There is no way to adjust the ohms calibration. It is what it is and if it is clean and making good contact and your gauge is ok then it should read correctly as it was designed..

Cool beans i must have been reading somewhere else maybe on my audi then about adjusting the sender. So far it seems to be reading fuller than it use too almost 400 miles on 1/4 tank i'm hoping all that gunk was holding it up from working i'll know in couple days.

funola · #9 10-09-2007, 09:44 AM

Here's some notes I took when I worked on my 240D sender.

Fuel sender connector pinout:
T gnd, G sender, W reserve switch

float up 2.4 ohm G to T
float down 56 ohm G to T

float up W to T infinite ohms
float bottomed out W to T zero ohms

argus445 · #10 10-09-2007, 05:39 PM

Quote:

Originally Posted by funola

Here's some notes I took when I worked on my 240D sender.

Fuel sender connector pinout:
T gnd, G sender, W reserve switch

float up 2.4 ohm G to T
float down 56 ohm G to T

float up W to T infinite ohms
float bottomed out W to T zero ohms

so W to T i could jump and should turn on reserve light then?

funola · #11 10-09-2007, 06:58 PM

Quote:

Originally Posted by argus445

so W to T i could jump and should turn on reserve light then?

That's correct.

argus445 · #12 10-10-2007, 05:58 PM

Quote:

Originally Posted by funola

That's correct.

Jumped the T + G i think it was Ground and Reserve sw and it worked so i knew the light was working and with no gas in tank i plugged it back nothing i then put some electric grease in the plug and poof it works i got a idiot light for my fuel gauge