Fuel gauge

[porschedavid]

Porsche 928 s2
fuel gauge stuck on bottom after fill up.
no live on any of the three wires in sender plug on top of fuel tank

[stepson]

David,
I just lived through this myself.
Plan to buy a new seal for the fuel level sensor prior to doing any work there. It will never seal without a new one.
Loosen the large plastic nut and remove all the connections from the sender unit.
The sender unit will be sealed and will need slight (maybe more than slight) persuasion to release from the fuel tank. Once it is released, remove it very slowly from the tank. Some of the drain holes in the sender unit are very small and it takes a while to drain. Otherwise, you get fuel all over you and the inside of your car.
Be very careful of the wires on the outside of the sender unit. DO NOT BREAK THE WIRES!!
Remove the 6mm nut holding the outside tube to the assembly. (down on the bottom of the assembly)
Does the carriage part of the sender move freely and fully up and down the tube? If not, find what is binding it. It is usually a build up of fuel deposits and gunk on the smaller tube that the carriage rides upon. I used a small scotchbrite pad to clean the small tube until the carriage would move up and down just by tilting the assembly one way then the other.


Once the carriage travels freely, you can reassemble the large outer tube and reinstall the sensor with the new seal. Reseat the sensor by pushing down firmly until it snaps/lock in place then retighten the large plastic nut.


I don't why there is no power to your connector, but check the ground and check all your fuses.

[porschedavid]

Checked fuses and cleaned earths but still no power getting top sender on top of tank

[honerboys]

i think i would find the power problem first - i did the sender unit removal and cleaning years ago and as i recall you can jumper a combination of 2 of the wires in the connector together and the dash gauge will jump to full and a different combination will light the low fuel bulb in the gauge - with no power at the sender it could be the sender is actually working fine - i don't know where the power comes from to the sender - not sure where the wires are routed either - just be very careful working back there with gas and electricity - can be a dangerous combination

[stepson]

Honer, you are, of course, correct.

Porschedavid, did you short the pins of the connector with the key on to see if the gauge pegged?

I am pretty sure that the ground for that unit is the one tucked behind the right rear quarter panel. You might just check it further.

[porschedavid]

Cant short the pins to see if gauge works as there no power on any pins.I took a live from some where else and put live on pins but gauge didn't move.so not sure if its somethink behind the cluster

[JK McDonald]

Hey David - If jumpering the Violet/Black and Brown wires at the fuel level float sensor doesn't send the dash fuel gauge to Full - you may be right in suspecting the problem is at the instrument cluster. You can possibly restore the operation of the fuel gauge by slightly moving it's circuit board edge connector under the dash. The one you want is just above the driver's left knee. Remove the storage shelf under the steering wheel - then the narrow rectangular plastic panel with the variable wiper delay and instrument back light resistors. Lift and lock the steering wheel/instrument pod to it's max height.

Then with a small flash light you can see the bottom edge connector you want to wiggle. Don't pull it completely off it's flexible circuit film contacts initially. If your symptoms change - pull the edge connector off for a careful cleaning. They can be cleaned with "Q" tips and electrical cleaner or I've had alcohol work in a pinch. I chased an inop speedometer signal to the same edge connector...... If you must get at the back of the fuel gauge traces and the oblong plug-in jumper (2nd Photo)- removing the instrument POD isn't too bad of a project.

Rennlist has a couple of great write-ups with photos that may help -
https://rennlist.com/forums/928-forum/426389-instrument-cluster-repair-w-pics-part-ii.html

Good Luck - Michael

[honerboys]

very nice pictures there michael - i did a quick look at wiring diagram (so i'm dizzy from trying to understand that) and it looks like the sender goes to connector T19 and also to the box under the dead pedal??? - don't know where T19 is though - porschedavid.. do all the other gauges work normally?

[honerboys]

ok looked further T19 location is near central electrics - so under passenger floorboard - 14 pin connector - it's the instrument panel /rear wire harness - also i don't know the year of the car so i just used the 88 diagram - or it could be problems up in the cluster as michael stated - i hate going in there - after 35 years everything is so delicate it's easy to cause more issues

[stepson]

Honer,

It's an S2 (84-86 Euro)

[porschedavid]

all other gauges work ok

[JK McDonald]

Hey David - Have you made any progress ? It is not a good idea to jumper an external +12 volts back into the fuel gauge on it's sensing (Violet/Black) wire.

Not sure if you realize that all the instruments are (basically) connected in parallel with (1) a common B+ to the top and (2) ground to the bottom. The difference between each gauge is their calibrated variable resistor as a sensing (3rd) input. Your problem is probably the sensing input from the fuel gauge float is just missing.

The water temp, oil pressure and fuel gauge use a changing sensing resistance to control their needle position. The input signal for the speedometer (differential fed) and tachometer (ignition unit fed) is a repeating pulse that progressively charges a capacitor inside each gauge. A gradual increase in the number of pulses will charge the capacitor to a higher voltage which drives their needle up scale.

Good Luck - Michael