Fuel level sender resistance

[jlevie]

Anyone happen to know what the empty tank resistance of the level sender should be? I'm seeing 0 for full and 59 ohms with the float at the bottom (and the low warning light on). But the gauge doesn't indicate below about half scale (it does fall to below E with the key off).

The cluster has been pieced together with parts from others, so it is possible that my problem lies in the cluster. But to figure that out I need to determine whether the level sensor is operating properly and for that I need to know what resistance it should have when the tank is empty.

Oh yeah, the car is an 87 325is...

[xLibelle]

i was looking at the same problem a month ago.
technically, its a sensor, not sender.
your car has two of them. one on each side of the tank.
take each out and tilt them back and forth a few times to get them scrubbed and moving again.
plug back in, turn on car and verify the fix.

[jlevie]

Not sure about late production, but early E30's only have one sensor. I've already check to see that the float moves freely over the entire range of travel and gotten the resistance readings with the float at each end of its travel. And with the float out of the tank, but connected, the gauge doesn't go much below half when the float is at the bottom and the fuel warning light on. So either I have a bad sensor or I have problem in the cluster. Figuring out which means knowing the sensor characteristics or swapping in a new sensor. I'm leaning towards the sensor since the OBC range display shows over 100mi when the fuel warning light comes on and I think it should show more like 30-40mi.

[xLibelle]

youre working on your 325i? did you LOOK under the DS tank port???

[jlevie]

An 87 325 does not have a tank port or sensor on the driver's side.

[Jean]

might be helful , it was for me.

from another thread http://www.r3vlimited.com/board/show...ghlight=sender


From - http://www.e30tech.com/forum/viewtopic.php?t=35976
Sounds like the sender's resistance doesn't change gradually with float position. Rather it changes abruptly. The 70 Ohms gives you "empty" and 3 Ohms gives you "full". So, what you're saying then is you're only getting around 25 Ohms when you should be getting 70 Ohms.

It's either the sender or the gauge is screwed up. Go to radioshack and buy some 150 and 100 Ohm resistors (pack of 5). Put the resistors in parallel and turn on the ignition key. See where the needle lands.

5 x 15O Ohms connected in parallel = 30 Ohms
3 x 150 in parallel = 50 Ohms
2 x 150 in parallel = 75 Ohms

5 x 100 = 20 Ohms

Resistor / (# in parallel) = total resistance.

Hook resistors in parallel by twisting the leads on two resistors together. Connect one end to the ground pin on the connector for the sender. Connect the other end to the center pin on the connector. Turn on the ignition key.

The other option is to buy a variable resistor (a pot) and use it instead of the multiple resistor trick.

You should see the following
74 Ohms -> empty
35 Ohms -> at 1/4 + 1/16 tank
24 Ohms -> at 1/2 tank
16 Ohms -> at 1/2 + 1/8 tank
11 Ohms -> 3/4 tank
5 Ohms -> full

[xLibelle]

shell fuels can mess things up too

[jlevie]

UPDATE:

According to the TIS the level sensor should show 3 ohms with the float at the top and 72 ohms with the float at the bottom.

[nando]

bumping an *ancient* thread -

question. Is the resistance value for the late sensors 72 ohms (full) per sensor, or is that after they're measured in series? I'm thinking it's 3-72 in series?

[Jean]

I believe for cars with two sensors the values are higher in the 100+ ohm (when measured in series). The numbers above are just for 1 (thin) sensor that goes inside the pump housing.,

[nando]

Right, but they are wired in series, so the total resistance at the gauge is less than at an individual sensor.

[Jean]

series = more, not less ;)

10ohm + 10ohm in series = 20ohm

thats kind of why the gauge will be OFF if it's not from a matching car.

[nando]

You're right, i had it backwards :)