My E32 was terrible when I first got it for trying to judge how much gas was left.
Often,
after about 2 minutes of highway driving, I would see the gauge drop
from 1/4 full to empty (without the warning light) and the on-board
computer range would get screwed-up along with it. I'd heard the sender
unit could often "lose" a "sensor wire", and as the gauge seemed OK
otherwise I was willing to poke around in the sensor to see what I
could find.
I found cleaning my sensor fixed my gauge so it read correctly - and linearly - across the whole scale.
It turns out the E32 sender is based on
* a cylindrical float, riding a vertical rod
* two paralell resistance wires
* a straight on/off contact (at the bottom of the rod) triggered from the float for its "low fuel" warning
* a cylindrical housing, with baffles at the bottom, to reduce float reaction to waves on cornering etc
The
float carries 2 contacts, creating a moving bridge between the sensor
wires - the other ends go to the gauge. My problem seemed to be from
the (very slight) varnish residue on the sensor wires - they're "wiped"
by a pair of very lightly sprung contacts (BMW must have guessed that
this light contact would cause problems, since the contacts are
gold-plated). I cleaned mine with alcohol and the problem cleared up.
Also, be aware that battery condition & load will affect the gauge.
Mine would vary by about two needle widths with all lights on to all
lights off when the car hasn't been used for a week or so.... dirty
plugs/contacts will also give a premature empty reading.
Here's how I cleaned it...
* First, get car on level in well ventilated area
* Remove trunk carpet until you see the tank access cover on the trunk
floor (it's an oval black metal lid with 6-7 screws, placed to the
right of the spare wheel well)
* Remove the bulbs
from BOTH the courtesy lights in the trunk, so there is no spark/flame
hazard. If in doubt, disconnect the battery (it's under the rear seat;
remember to sort out radio security code/ alarm/ airbag issues first!)
* Let's get the sensor & pump assembly out & accessible...
* Remove the access cover - it doesn't go into the tank yet, so don't go mad about fumes yet
* You should now see the access plate entry to the tank, with the fuel out/return lines and the wiring loom
* Disconnect the wiring harness - slide the metal "frame" horizontally
back towards the back of the car - this will "lift" the connector from
the block. Jam the connector out of the way in by the spare wheel
* Now make sure you've got some cheap electrical tape handy
* Remove the two hoses - beware residual petrol pressure (not much on
mine after having run the engine within the last 5mins). Have a rag in
hand ready to soak up the spillage. DO NOT LOSE the circlip clamps down
the hoses!! Give each hose a couple of turns of tape and leave a 6inch
"fly" end and tape this to the floor of the trunk to keep the hose up
and out of the way on the side it came from. DO NOT LET THEM GET MIXED
UP.
* With a 10mm socket undo the 8 nuts holding the access plate to the tank.
* Clean the access plate (with the nuts off, they get dirt held around
them) before you get any dirt in the tank too.
* Pull up the plate the first 1-2 inches firmly to release the cork gasket - mine styed with the plate
*Use two hands from now on
* Lift the plate up and out. Beware the petrol trapped in the sensor
body and the hoses. Take it slowly and you can get most of it to drain
back into the tank. Be careful of the end of the sensor ( the sensor is
a tube about 8" long and aboout 2" diameter, with a bigger "settling
tank" at the bottom) - it's a bit of a squeeze with the hoses to the
pump too.
* Get the sensor/access plate clear, so
that only the hoses and wiring to the pump are left going into
the tank
* Let's remove the pump now.
Get your hand into the tank, and follow the hoses down to the pump.
Follow the pump body down until you feel the horizontal metal flange in
the tank that it sits in, then come back up to the pump mounting collar
and find the two plastic tabs (180 degrees apart) that lock the pump
in. Squeeze them together and then lift the pump out out of the flange.
* Lift the pump out - you can use the hoses (gently) but DON'T use the
wiring. Be careful with the input filter screen at the bottom - it's
fine & plastic.
* Get the whole sensor/pump
assembly out of the trunk. Suggest you wrap the input gauze filter on
the pump in a clean rag to protect it from being accidentally
torn/poked through
* OK, now to clean the sensor...
* Get the small collar nut off the bottom of the sensor tube - don't
lose it. It's the only metal thing on the bottom of the sensor tube.
* Pull the sensor tube off the metal tang it slides onto on the
underside of the access plate. BE CAREFUL OF THE VERY FINE SENSOR WIRES.
* Now you can see the 3 sensor wires - the middle one is just for the
low fuel level contacts, so it doesn't matter if it's "slack". The
other 2 fine ones are the ones you want.
* Check
the 2 fine wires are complete and soldered to their mountings. Mine
were; if one of your wires is broken, it is not normal wire - it's
"resistance wire" what you can get from electronics stores - problem
with that is that resistance wire comes in different "strengths"
(ohms/meter). Try taking a sample of a broken wire into a really good
electronics hobby shop, or try for a replacement sensor.
* Clean your wires carefully
- I cleaned mine with an isopropyl-alcohol computer screen wipe, and
then ran the thickness of the pad/sheet between the wires and the 2
gold contacts on each side of the float (2 per wire, 4 total).
* Use the pad or alcohol or whatever to clean the harness connector
pins as best you can, and the plug too. (With mine I found the best
jewellers' screwdriver that was just undersize and scratched out the
insides of the plug connections too.
Remember,
the total sensor resistance at full-empty is only something like 60-70
ohms, so any bad connection (slight resistance) is going to indicate as
a prematurely empty tank...
* Rinse out
any silt in the sensor body with clean petrol and discard the dregs;
the "lid" on the baffle chamber can be gently popped up enough to clean
in there too
* Reassemble carefully noting the following things....
* Make sure the pump tangs are engaged fully - you don't want a pump
with LIVE ELECTRICAL CONNECTIONS floating loose in a metal tank full of
petrol vapor
* Take care with the hose circlips
* Spray the access plate with a rustproofer if rusting (mine was) but AFTER reconnecting the electrics.
* DO NOT LET THE OUTSIDE OF THE PETROL LINES STAY WET WITH PETROL. The
outers of petrol lines are not usually proof from softening from
petrol, as they shouldn't see any of it - only the insides should.
Don't let them get soft/sticky.