From digest.v6.n224 Sat Feb 15 16:17:51 1997
From: "Brett Anderson" <bretta_at_webspan.net>
Date: Sat, 15 Feb 1997 11:41:57 -0500
Subject: <E36> Fuel sender description

> From: RL <r2l4_at_vitinc.com>
> Date: Fri, 14 Feb 1997 23:35:45 -0500
> Subject: E36 fuel guage reset , why?
>
> I have a 95 E36 M3 and have read many posts
> about the fuel guage causing problems like
> not going back to the full or remaining near
> empty upon fill-up. My car just did this
> today for the first time in 20,000 miles. The
> car was at 1/4 tank and at fill-up it went
> up a small amount and stayed there until I
> disconnected and reconnected the battery. Why
> does this happen and is it a repairable
> problem? Also, after reading through the wiring
> manual I noticed this car has two tank
> sending units why is this?
>
> Thanks in advance.
>
> Robert
> r2l4_at_vitinc.com
>
>
All problems are repairable, if the car is out of warranty, the only problem is if you are willing to invest in diagnostic time. There have been a lot of problems with the fuel senders burning their tracks, although your problem sounds more like a cluster problem. As for why there are 2 senders. The fuel tank is made up of 2 seperate cells, one on each side of the car. The old E30's used a similar design but had a transfer pipe joining the bottom of the 2 cells, in the E36 they opted to do without the transfer pipe and use 2 senders instead. A ground signal is sent from the cluster to the left sender, through it to the right sender, which also has the pump, and back the the cluster. the final signal is the sum of the 2 senders resistance, the more resistance, the fuller the tank.
When you fill the tank, both sides fill as the tank is one at the top, the fuel pump then sucks from the right side of the tank, the return line from the engine bay empties into the left side of the tank. At the base of the fuel pump is a shroud that ensures the pump does not loose prime with a hard corner and a low tank. Inside this shroud is the end of a hose which goes across the top of the tank to the left side, the suction action of the pump creates a siphon with this hose, transferring fuel back to the right side from the left.
When the car has a half tank of gas, the left side will be almost empty, with about 10 Ohms of resistance in the sender unit, and the right side will be full, will about 250 Ohms of resistance. The gauge reads 260 Ohms out of a possible 500 ( or there abouts ) and says half.

Hope this helps those that are interested

Brett Anderson

[...]

Saturday I was near empty on the tank, so I stopped to fill it. I did something I've never done before when filling the tank, I left the key in the ign, in the accessory position. After filling and driving away, I noticed my guage didn't show full, it was still down on E. The range also still said 30 miles on the OBC.

Today it still didn't work, so I brought it to Camelback BMW. They pulled out fuse #21 for 10seconds, then reinstalled it. Presto, the fuel guage now works!

Moral of the story, key belongs in your pocket when fueling the car.

Dale Beuning
'95 M3
'88 325is