So there used to be a lot of questions on this way back but seems to have died down in recent years, a number 928's have the ability to only read between the 3/4 and full mark when the gas tank is full. Anyway my 87 had the same feature, shorting the wires on the sender proved it wasn't the sender and I have been over every earth contact in the vehicle too.
With my dash out over the Winter I dismantled the cluster and took a look at the gauge, at the back is a resistor, mine looked a little the worse for wear and was supposed to be 100R. Measurement with a DVM showed it to be 110R, this is right at the max tolerance for the part (+/- 10%) so maybe they were delivered this way ... but mine also showed signs of getting hot which wouldn't have helped.
Got hold of some new resistors from Digikey which were of a higher wattage than the original but more importantly were 100R.
When the tank is full now the gauge measure full. Minor detail I know but every little helps.
I'm basing this on my 86 model.
Gauge or gauge circuit connection
Fuel tank vent tube pinch allowing only a 3/4 fuel fill-up at pumps.
There are three wires at the fuel sender connection in the rear of the car. Look under the rear carpet, there is a shield and a plastic round cover. Once removed, you will see the 3 wire connector for the sender. The sender has a resistance of roughly 74 ohms to ground when the fuel level is empty and roughly 0-2 ohms to ground when the tank is full. To check your gauge, pull the connector off the sender, jump the ground wire (brown) to the violet striped wire. Turn on accessories with the key (you don't need to start the car). By jumping those wires, you are sending a close to 0 ohm signal to the gauge. It should read full. If it does, your gauge circuit is probably OK and you may need to look into the possibility of a pinched air relief hose for the tank and the possibility that you are only getting 3/4 of fuel into the car. (I drained my tank when replacing the fuel filter put 5gal in at home and 16.7 in at the pumps. This verified that my tank is capable of holding capacity, my problem is somewhere else, not a pinched hose).
Or a bad sending unit. Fill the car at the pumps, pull the connector and
check the resistance on the pins which fit brown and violet wires at the
sending unit, should be close to 0 ohms when the tank is full.
Or, your problem may be the gauge, harness connectors to the instruments, ground to the steering column area, or interconnect from the sending plug through the central (fuse) board to the dash harness). I can only get 7/8 full off my gauge with the wires jumped. I loose about 1 ohm resistance from the sending plug to the dash harness. I measure about 2 ohms at the instrument harness plug.
I cleaned the edge connectors on the instrument foil where the harness plugs fit. I gained about 1/2 needle width towards full by doing that. Also, my temp gauge reads about 1/2 needle width hotter (closer to reality) than before I cleaned the contacts.
The wires seemed to be clean but I cleaned and sprayed them a lot anyway. I also cleaned up the contacts on the float mech. 2.6 is the lowest I can get. I am still looking to see where it should be by spec.
Oh Well, what that yields is s not-quite-full reading for a while of driving, then normal gauge action down to empty. I can live with that. If I could get the max full resistance any lower, I would, but it's not worth loosing sleep over and certainly not worth replacing the sender, at least for now.
Go to the fuel sender wiring in the hatch area, its on the right side under a thin metal sheet. Pull the connector, then ground the sensor wire, offhand I don't remember the color but refer to the wiring diagrams as it is probably different for your year anyway. I seem to remember there are 3 wires, ground, fuel gauge and low fuel light warning.
If on grounding (the correct wire) the gauge reads 3/4 (or not full) there is an issue in the upstream wiring, if it reads full then there is likely an issue with your fuel sender unit or the breather pipe from the tank may be blocked which would mean you can't fill the tank all the way.
If its the wiring next place to look is the pod, (remove pod) clean all those connection on the back, an eraser works well enough. On the PCB at the back of the pod there are small nuts for the 4 small gauges which hold in a connector. Clean these too. Retest gauge as before. If it now reads full your done.
If not remove the fuel gauge from the pod, you'll find a 100R resistor at the back, wattage of this resistor appears marginal, remove one end of resistor and check resistance with a digital meter. Replace if reading is incorrect.
Of course if at the end of this it still does not read full .... its likely the gauge.
Hope this helps
Posted by Jim Morehouse on 11/22/2005, 10:48 am, in reply to "S4 Fuel guage
Very common problem.. have it on my '82 and on the '88. There is a 220 ohm carbon resistor (memory?) attached to the back of the pod and, IIRC, the problem is that as the resistor ages, the value goes up. I'm not aware of any easy way to do a calibration but you could pull the pod and replace the resistor...make sure you use the proper power rating...you may need to put a few in parallel and see if you can give it a bit more room for heat to dissipate. I think the resistor was a bit under power rated in the original design, which may not have accounted for heat build-up.
One saving grace on my cars is that I know how much I've got left when the 'low fuel' comes on, so I live w it for the time being. If I end up replacing it in the winter, I'll post the results.
There is an equalization hose running from the top of the gas filler to the
middle of the tank. To complete the path the hose is routed through a hole in
the frame. It is common for the hose to become pinched or bent when installing
the tank. Since the hose is blocked it cannot equalize the air in the tank and
when the bottom of the tank is full the rest of the gas runs up the filler and
shuts off the nozzle. Thus the best you get is about 5/8s of a tank.
Check the hose as it goes through the frame. I suspect if you were real patient you could still fill the tank by waiting for gravity to equalize the gas. Jim Bailey says it cannot be done and maybe he is right. I never had the patience to try it.
The gas level for the gauge is determined by one contact on the float running down a wire and changing the resistance. The light is controlled by a second contact - separate circuit - on the sending unit. You can check the functioning of these with an ohm meter. It might be the gauge reads empty but the light is not coming on because the tank is not yet low enough.
I think the manual says there is about a gallon and a half when the light comes on.
If the needle is jumpy then maybe you have a bad connection between the sender and the dash, but my bet is a poor connection on the instrument panel. Since gas sloshes around in the tank, the float is bouncing all over the place. To compensate the gauge is damped. Therefore, it is unlikely an erratic float would cause the gauge to jump around. The connections to the circuit board can become corroded. A good cleaning often fixes these types of problems.
When you turn on the ignition before starting the car the low gas light should come on. In fact, all of the dash lights should come on as an integrity test. It is common for these lights to burn out. Once the gauge unit is apart, the lights unplug and you can get some new ones to replace them. I never priced them since I had some spares. I did find an interesting thing on one bulb. It would glow very faintly, but get super hot. The entire housing had turned black.
BTW - if you pull the instrument panel do yourself a favor and get three higher wattage dash light bulbs. These are the ones at the bottom with the light blue or pink twist out cap. I think stock they are 2 Watts and 4 Watts increases the intensity enough so you can actually see your gauges at night. Take a minute to spay some DeOxIt into the dash light dimmer - too.
Dan the Pod Guy